But none of that explains what COVID-19 really is.

What is a coronavirus? How does it work? What does it do to the body? Let’s take a look.

What are Coronaviruses?

Coronaviruses are a large family of viruses. They’re called “corona” (latin for “crown”) viruses because of the spiky projections on their surface.

Coronaviruses can infect birds, mammals, and sometimes humans. They’re a very common type of virus.

In fact, if you’ve ever had a common cold (and who hasn’t?), there’s a good chance that you’ve been infected by a coronavirus before. About 20% of common colds are caused by coronaviruses.

However, this isn’t the first time this family of viruses has caused major problems. If you remember the SARS pandemic of 2002-2003 or the MERS outbreak of 2015, those were caused by coronaviruses, too.

In fact, there are so many similarities between COVID-19 and the SARS coronavirus that it’s technically called SARS-CoV-2. You won’t hear anyone refer to it that way on the street, but it just goes to show how closely related these two viruses are.

Thankfully, though, the mortality rate of COVID-19 is much lower than the mortality rate of either SARS or MERS. If those coronaviruses had spread as widely as COVID-19, the death toll might have been 2-5x higher.

That’s the good news.

The bad news is that while COVID-19 might not be as deadly as some of the other coronaviruses out there, it’s a heck of a lot deadlier than the common cold…and just as contagious.

This unique blend of traits that have made COVID-19 particularly hard to control. As a result, this novel coronavirus isn’t just affecting a country or small region—it’s affecting the world.

How COVID-19 Works

Coronaviruses are lipid-enveloped viruses. That means each individual virus is like a little self-delivering package surrounded by the same sort of membrane that protects our own cells.

Now, if you remember your high school biology, viruses aren’t technically “alive”. They reproduce, but they can’t do it on their own—they have to hijack living cells to replicate.

To watch the process in action, check out the video below.

To put it simply, the spiky projections—or “crown”—on the outside of the coronavirus are proteins that respond to specific proteins on the outside of human cells.

You can think of it like a lock and a key. When the “key” on the outside of the virus (called a “spy protein”) finds the right “lock” on the outside of a cell (called an ACE2 receptor), it clicks into place.

Once the virus locks into place, it merges with the host cell, releasing its genetic material (called RNA) into the host cell. The RNA then takes over the cell and forces it to produce new coronaviruses.

Eventually, it all proves to be too much for the host cell and it dies, releasing all of the new coronaviruses it just manufactured. From there, all of those new viruses are free to infect new cells and start the whole process over.

How COVID-19 Affects Your Lungs

Since you usually get COVID-19 by inhaling airborne droplets of water with the virus in them (released when an infected person breathes, coughs, or sneezes), the virus is designed to go after your lungs. It is a respiratory disease after all.

COVID-19 vs Your Lungs

The ACE2 receptors that COVID-19 targets seem to primarily show up on goblet cells (which produce mucus) and ciliated cells (which clear away dirt, much and…you guessed it, viruses) in the lungs.

Both of these types of cells are important parts of how your lungs protect themselves. For your lungs to work properly, they need to stay moist. That’s what your goblet cells are for.

Your lungs also need to stay clean, which is why you have ciliated cells to move stuff out of the lungs and into the throat where it is swallowed and destroyed by your stomach acid.

When these types of cells don’t do their jobs, it creates an ideal environment for COVID-19. The cells in the lungs start to dry out—making them more vulnerable to invasion—and the viruses released after a host cell dies don’t get cleared away by the cilia.

Instead, the virus has plenty of time and opportunity to infect new cells and continue the cycle of destruction.

Your Immune System vs Your Lungs

To make matters worse, once your body discovers that your lungs are infected, it goes on the warpath. However, it takes time to develop and replicate white blood cells that can specifically target COVID-19.

So, your body does what it can in the meantime. Unfortunately, it doesn’t have antibodies to tell it which cells are infected and which ones are healthy, so it just attacks everything in the area.

You can think about it like this. Antibodies are like a covert ops expert lighting up a building for a laser-guided missile. In this scenario, you can blow up the target building and leave the ones next to it standing.

But, to do that, you need a covert ops expert on the ground.

For your body, that expert is your antibodies. They light up infected cells and tell your immune system “kill this now!”

Without that covert ops expert, though, you’re shooting blind. You just have to bomb everything and hope that you hit your target.

This is what your body does to your lungs early on. It can’t distinguish healthy cells from infected ones, so it bombs everything in the area and hopes for the best.

Unfortunately, since COVID-19 is already doing a number on your goblet and ciliated cells, there isn’t a good way to clear all of resulting debris away. Your lungs get congested and you may end up with pneumonia.

If your body can’t get things under control, COVID-19 and your immune system end up punching holes in your lungs. When this happens, you develop respiratory failure and may, sadly, die.

COVID-19 Symptoms

Now that we’ve talked about how COVID-19 affects the body, the symptoms of COVID-19 should make a lot of sense.

Generally speaking, the most common symptoms will be:

• Fever (the result of your immune system ramping up)
• Shortness of breath (due to all of the junk in your lungs)
• Cough (from body trying to force all of the junk in your lungs out)

If you’re under 60 years old and aren’t at high-risk for severe illness (no lung disease, serious heart conditions, asthma, severe obesity, diabetes, kidney failure, liver disease, pregnant, etc and not immunocompromised or in a nursing home) and develop these symptoms, you’ll probably be just fine.

However, if things get really bad, you may develop the following symptoms (call your doctor if you develop any of the following):

• Trouble breathing
• Persistent pain or pressure in the chest
• Bluish lips or face

All of these symptoms are due to your lungs filling up with fluid. As a result, you can’t get enough air.

In addition, you may also experience other symptoms due to an exaggerated immune response, like unusual confusion, grogginess or increased pain sensitivity. Up to half of COVID-19 patients experience nausea, vomiting, and/or diarrhea, so that’s a possibility, too.

By the way, as a quick aside on fever, you can actually have a lot of variation in your body temperature based on time of day, where and how you are measuring, and what your personal baseline temperature is.

So, if you have a temperature of 99 degrees Farenheit, you don’t necessarily have a fever. If it pops above 100.4 degrees, though, it’s definitely something to pay attention to.

If you have manageable symptoms and you’re worried that you might have COVID-19, call your doctor—don’t go into the office without talking to a healthcare professional first. Showing up unannounced is a surefire way to spread it to other people.

If you’re experiencing more severe symptoms, call the hospital. As with visiting the doctor’s office, it’s best to give them warning in advance. That way, hospital staff can protect themselves and their patients.

All in all, COVID-19 isn’t something most people need to be scared of.

Our bodies have handled this sort of respiratory illness before. They know what they’re doing. They’ll beat this virus and many viruses to come. You may notice some symptoms if you catch COVID-19, but you’ll probably be fine.

The Problem with COVID-19

So why is COVID-19 such a big deal? Our bodies beat rhinoviruses, coronaviruses and influenza viruses every yea. Why are we so worked up about this one?

Well, the simple fact of the matter is that while you may get this coronavirus and do just fine, this virus could be a death sentence for many people—and you might just be surprised by who it claims.

For example, I’m a fit, physically active guy in my mid-thirties. Most people would look at me and think, “He’s definitely safe.”

However, I’m actually at high-risk for severe illness. I’m immunosuppressed and have an immune system disorder. Casual exposure to COVID-19 could kill me.

But no one would ever expect that.

To make matters worse, you can have COVID-19 and not even realize it. You may infect, hospitalize and possibly kill people—all without even realizing that you’re a carrier for the disease.

We’ll cover all of the potential implications of this in a future article, but it makes things pretty dangerous for people at high-risk for severe illness—and it’s part of the reason why we are seeing such rapid spread of the disease here in the USA.

Like SARS—its parent virus—catching COVID-19 can have lifelong consequences. So, if we aren’t collectively careful, our negligence can put people in the hospital…or worse.

Conclusion

All of this information is why doctors, businesses, and governments worldwide are taking COVID-19 so seriously.

As a virus, COVID-19 is incredibly good at its job. It’s highly contagious, survives for prolonged periods on surfaces, and can be transmitted to a new host before the original host is even aware that they have the virus.

Couple all that literal virality with a disease process that can maim or kill the host, and is it any wonder that we have a global pandemic on our hands?

Now that the USA has more reported cases of COVID-19 than any other country in the world, this isn’t a disease we can take lightly. COVID-19 is here to stay and, if we aren’t wise about how we respond, the effects of this novel Coronavirus on our communities, economy, and nation will be profound.

The post Understanding COVID-19: How It Works appeared first on Aden Andrus.

Mechanism of action

Alcohols are believed to exert their antimicrobial effects primarily by denaturing and coagulating proteins, leading to disruption of cellular metabolism and membrane lysis.^2,6-10 The attraction between the carbon tails of alcohols and hydrophobic protein residues enables alcohols to insert themselves into proteins, where the polar presence of the alcoholic oxygen atom weakens lipophilic interactions between non-polar residues and increases the internal affinity of the structure for water. This decreases the stability of the 3-dimensional protein structure and causes enzymes to fall apart at physiologically relevant temperatures.^11-16

Alcohols appear to permeabilize the cytoplasmic membrane through a similar mechanism.¹⁷ Although this mechanism of action has not been intensively studied, the microbicidal effects of cytoplasmic disruption and protein denaturation are validated by reports linking cell death with loss of protein viability.^18,19 Ultimately, alcohol exposure destroys proteins and disorganizes the cell membrane, causing catastrophic damage that rapidly kills a variety of microbes.

Efficacy

Not surprisingly, the devastating protein denaturation induced by alcohol exposure renders ethanol and isopropanol highly effective antiseptic agents. In vitro, alcohols rapidly eliminate a wide variety of microorganisms and this activity has been shown to extend to in vivo tests as well, where alcohols are generally recognized as the most effective form of hand antisepsis.² While they offer no protection against post-application contamination, alcohols have been shown to reduce bacterial counts on inanimate surfaces as well, providing solid rationale for their use as general healthcare disinfectants and antiseptics.

Unfortunately, while studies indicate that alcohols potently reduce microbial contamination, they have not been shown to have an independently superior effect on infection rates in either the community or healthcare setting. However, alcohols are easy to use, can be placed directly at point-of-care sites, and are equal to or better than other antiseptics at reducing microbial load, making them excellent options for effective, accessible antisepsis and disinfection.

In vitro spectrum

Ethyl and isopropyl alcohol are rapidly microbicidal against vegetative bacteria, fungi, and most viruses.^2,6,7-9,20 This potency is highly concentration-dependent, with antimicrobial effectiveness beginning at concentrations of 30% and maximum efficacy typically achieved at concentrations of 60-95% ethanol and 70-91.3% isopropanol, though decreases in activity towards the higher end of these ranges have been observed.^2,5-9,21 Activity is also enhanced as temperature increases from 10° C to 40° C.²² Against Gram-positive and Gram-negative bacteria, the 15-30 second in vitro reduction factors (RFs) of alcohols antiseptics typically range from 4-8 log₁₀ colony-forming units (CFUs).^23-30 Seventy-percent ethanol is highly effective against mycobacteria—including Mycobacterium tuberculosis—after 30 seconds of exposure and eliminates fungi within 15-20 minutes.^23,31-34 High alcohol concentrations are active against enveloped and non-enveloped viruses, including human immunodeficiency virus (HIV), influenza A, SARS-coronavirus, and hepatitis B/C, though hepatitis A, poliovirus, and MS2 bacteriophage appear to be relatively insusceptible to alcoholic inactivation.^{2,23,30,35-47} Despite this broad antimicrobial spectrum, alcohols are not considered high-level disinfectants due to their lack of relevant sporicidal activity.^7,48-54

In vivo results

In addition to their potent in vitro efficacy, alcohols also demonstrate significant antimicrobial activity in vivo. Since hands are the most significant target for reducing nosocomial infection, dozens of studies have explored the effects of alcohols on both transient and normal bacterial hand flora.⁶ In these studies, ethanol and isopropanol are associated with multilog RFs at a variety of concentrations and exposure times. Alcohols are also effective as disinfectants on a wide variety of surfaces, though specific RFs have not been extensively reported for inanimate surfaces.⁷  Unfortunately, this antiseptic and disinfecting efficacy is highly dependent on exposure time, and since reported in-use application times are typically far shorter than those used in most in vivo studies, log₁₀ RFs seen in clinical practice may not reflect those observed in vivo.⁶ However, while the clinical relevance of most study times is debatable, alcohols are generally superior to other biocides in vivo and are widely considered the standard of reference for antimicrobial efficacy.

Antisepsis

The antiseptic effects of ethyl and isopropyl alcohol are well-established in the literature. After a comprehensive literature review, Kampf et al (2004) estimated that ethanol and isopropanol reduce transient bacterial flora by 2.6-4.5 and 4-6.81 log₁₀, respectively, and resident flora by 1.5-2.4 log₁₀.² An independent analysis of more than 30 studies and over 2,000 data points generally corroborates these ranges (Tables 1 and 2), which appear to be accurate regardless of the method used to contaminate hands.^55-57 Alcohols are also effective against viruses in vivo, and are associated with RFs of 0.9-4.64 log₁₀ at 10-30 second exposure times.^58-62 Although the results of in vivo trials are not entirely consistent, the bacterial and viral RFs of alcohols are generally superior to the RFs of other antiseptics, leading the CDC to recommend alcohols as the preferred agents for general hand antisepsis.^6,63

Like most biocides, the efficacy of alcohols is highly dependent on exposure time. Unfortunately, clinical handwashing duration is typically less than 10 seconds, and most in vivo studies evaluate the efficacy of alcohols at antisepsis times ranging from 30-60 seconds.⁶ To address this discrepancy, Sickbert-Bennett et al evaluated the effects of 61% ethanol on transient Serratia marcescens after 10 seconds of exposure.⁶⁴ Unlike the 2-4 log₁₀ RFs seen at 30-60 seconds of antisepsis, 10 seconds of 61% ethanol use was associated with a 1.55 log₁₀ RF in bacterial load and actually performed worse than 4% chlorhexidine (1.89 log₁₀ RF), 1% triclosan (1.9 log₁₀ RF), plain soap (1.87 log₁₀ RF) and tap water (2 log₁₀ RF). Similarly, another trial reported a 2.15 log₁₀ RF for 70% isopropanol on Escherichia coli at 10 seconds of exposure, although isopropyl alcohol outperformed chlorhexidine (1.96 log₁₀ RF), plain soap (0.5 log₁₀ RF) and tap water (1 log₁₀ RF).⁶¹

These findings indicate that alcoholic antisepsis may not provide the same superior antisepsis observed in most in vivo tests under in-use application times. In contrast, the results of a study by Dharan et al suggest that 15-30 seconds of exposure to alcohols may be associated with even greater effects on transient bacteria (5.03-7.18 log₁₀ RF) than previously described.⁶⁵ However, due to differences in the contamination, disinfection and recovery techniques employed by Dharan and colleagues, these results were excluded from the mean RF value calculations in the following table. Inclusion of this study increases the mean RFs of ethanol and isopropanol to 5.74 (range 2.7-6.8) and 5.66 (range 5.03-6.05) log₁₀ RF, respectively, at 15 seconds application time and 5.37 (range 1.97-6.88) and 4.43 (range 2.9-6.81) log₁₀ RF, respectively, after 30 seconds. Although this study paints alcoholic antisepsis in an even more favorable light, the aberrant nature of the reported RFs also highlights the risk of under or overestimation of in vivo efficacy inherent in current testing methodology.

Further exposing the intrinsic variability of in vivo tests, several studies report unusually low RFs associated with alcoholic antisepsis. In one trial, 61% ethanol was completely ineffective (-0.2 to 0 log₁₀ RF) in eliminating the spores of a Bacillus anthracis surrogate, though this result is not surprising since alcohols lack sporicidal activity in vitro.⁶⁶ The results of two other studies comparing ethanol with a surfactant, allantoin and benzalkonium chloride (SAB) antiseptic are more difficult to explain. After a 2-minute application time on hands artificially contaminated with Serratia marcescens, both SAB antiseptic and 62% ethanol were associated with a 2.6-2.8 log₁₀ RF, while 62% ethanol gel was associated with a 2.1 log₁₀ RF.^67,68 Since ethanol and ethanol gel are associated with a 3-4 log₁₀ reduction of transient bacteria after 60 seconds of exposure, these results seem largely incongruous with established literature.^4,69 The authors fail to address this inconsistency in their article, calling the quality of their study into serious question. Aside from these few studies, however, alcohols perform quite well in most trials and the weight of evidence seems to favor their use as antiseptics at 30 second exposure times.⁷⁰

In addition to their proven antiseptic efficacy, alcohols have significant disinfecting activity. Alcohols are highly flammable and are not recommended for disinfection of large environmental surfaces; however, they can be safely used to disinfect a variety of inanimate objects.^62,103,104 The efficacy of alcoholic disinfection been examined on a variety of medical devices as well as a slew of more routine objects, where ethanol and isopropanol generally demonstrate excellent antimicrobial activity against bacteria and fungi.^62,105 Unfortunately, alcohols have poor activity against sporulated bacteria and some viruses, leading to the recommendation that ethanol and isopropanol only be used to disinfect noncritical healthcare objects.¹⁰⁴

Medical devices are one of several target healthcare vectors for alcohol-based disinfection. In several trials, 70% alcohol outperformed iodine and quaternary ammonium compounds (QACs) and was equivalent or superior to phenolics and formaldehyde for disinfecting oral thermometers, reducing contamination rates by 79.6-97%.^106-108 In contrast, coliform bacteria persisted on 71.2% of contaminated rectal thermometers wiped down with 95% ethanol and soap.¹⁰⁹ Alcohols have also been tested for use in the disinfection of endoscopes, which have been linked to more infection outbreaks than any other medical instrument.^104,110-115 Following 5 minutes of immersion in benzalkonium chloride, immersion of endoscopes in 96% alcohol for 5 minutes is associated with complete elimination of vegetative bacteria.¹¹⁶ However, due to the poor activity of alcohols against spores, alcoholic disinfection of endoscopes is limited to rinsing after high-level disinfection.^114,117,118

Further studies have evaluated the efficacy of alcohols for disinfection of intravascular transducers and ophthalmologic tonometers, both of which have been implicated in nosocomial infection outbreaks.^119,120 For transducers, wiping with 70% alcohol sterilizes 98% of the blood pressure monitors heads and is equivalent to ethylene oxide in terms of positive culture risk and patient outcomes.^121,122 Alcohols also eliminate over 3 log₁₀ of type 1 human immunodeficiency virus (HIV), coronavirus 229E, adenovirus type 5, and parainfluenzavirus type 3 from tonometers and other non-porous surfaces.^123,124 However, 70% isopropanol is ineffective at eliminating adenovirus 8 (0.47-1.07 log₁₀ RF)—a common cause of epidemic keratoconjunctivitis—from inanimate objects and is not recommended for disinfection of tonometers or other ophthalmologic devices.^120,125,126

Alcohols have also been evaluated as disinfectants for a variety of noncritical healthcare objects.  In a study of hospital staff pagers, swabbing with 70% ethanol was associated with a 1.22 log₁₀ RF in bacterial contamination.¹²⁷ Similarly, wiping ultrasound probe heads with 70% ethanol-moistened paper reduced bacterial load by 2.31 log₁₀ CFUs.¹²⁸ Other studies have reported that 89.3% of contaminated scissors and 100% of contaminated stethoscopes swabbed with 70% ethanol are sterile after disinfection.^129,130 Wipe down with 70% alcohol has also been shown to reduce overall herpes simplex virus recovery from CPR manikin heads by 84%, completely eliminating the virus from all but the least accessible test sites.¹³¹ Although limitations in scope of effect prevent alcohols from being considered high-level disinfectants, these studies indicate an important role for alcohols in the disinfection of a wide variety of potential fomites.

Persistent activity

One often-mentioned weakness of alcoholic antisepsis is a lack of persistent activity. Defined by the CDC as “prolonged or extended antimicrobial activity that prevents or inhibits the proliferation or survival of microorganisms after application of the product,” persistent activity is facilitated by the presence of residual biocide on a surface after the initial application.⁶ This persistence is characterized by either continued suppression of surviving bacteria or activity against post-application inoculation. Alcohols are highly volatile and quickly evaporate under normal conditions, making them ideal for rapid antisepsis or disinfection; however, they leave no residue and provide no protective long-term antimicrobial effect. Despite this lack of residual presence, alcohols mildly suppress the regrowth of resident hand flora, but they offer no defense against exogenous recontamination. Unfortunately, since clean skin is quickly reinoculated by contact with contaminated surfaces, the antiseptic effects of alcohols are short-lived under most conditions.

Suppression of normal flora

While antisepsis substantially reduces the number of viable bacteria on the hand, it does not leave skin sterile. Even without external contamination, normal flora regrowth leads to recovery of bacterial load within hours of antisepsis. Bacterial regrowth is particularly important under surgical conditions, where contamination from skin surrounding the surgical site or glove perforation can lead to contamination of the surgical site and subsequent infection. To prevent this, the FDA has mandated that surgical scrubs must maintain hand flora below baseline for at least 6 hours after application.⁵ Alcohols meet this criterion through two mechanisms: 1) they have a mild post-antiseptic effect in vivo, and 2) they produce a decrease in bacterial load that cannot be overcome within 6 hours. Additionally, chlorhexidine gluconate (CHG) is sometimes added to ethyl or isopropyl alcohol in an effort to increase the longevity of their effects on hand flora, though the superiority of this combination is debatable. However, even in the absence of chlorhexidine, alcohols appear to independently inhibit bacterial regrowth long after their initial application.

Resident hand flora recovers by 0.49 log₁₀ CFUs (weighted mean, range -0.29 to 1.3) within 3 hours of a 180-second application of an alcoholic surgical scrub.^84,88,93 Further, a trial of the long-term effects of 61% ethanol reported that despite an initial 2.22 log₁₀ RF, bacterial counts returned to baseline levels within 8 hours.¹⁰¹ In contrast, a trial by Mulberry et al found that 61% ethanol use was associated with a 1.1 log₁₀ immediate RF in general hand flora and a 1.4 log₁₀ RF three hours after antisepsis. Six hours after exposure, however, bacterial counts were only 0.5 log₁₀ lower than baseline.¹⁰⁰ Similarly, another study reported an increased RF (2.77 vs 2.48 log₁₀) on gloved hands 3 hours after application of an 85% ethanol gel.⁸⁸ This post-antiseptic effect may be due to sublethal damage to surviving bacteria that can be overcome on a culture medium, but results in further bacterial death on skin, a hypothesis that may also explain the slow initial regrowth rates observed in these studies.¹³²

Chlorhexidine gluconate, a biguanide antiseptic with very low volatility, is frequently added to alcohol-based products in an effort to improve both immediate and long-term RFs. Problematically, while 93% of applied chlorhexidine remains on the skin for 5 hours, studies of alcoholic chlorhexidine at a 3 minute application time indicate that resident hand flora recovers by approximately 0.83 log₁₀ CFUs within 3 hours of exposure and by 1.6 log₁₀ after 6 hours.^84,94,133 In 1 study, Mulberry et al reported initial, 3-hour and 6-hour RFs of 2.5 log₁₀, 2.9 log_10, and 2.5 log₁₀, respectively, against general hand flora; however, it should be noted that many chlorhexidine studies suffer from inadequate validation of utilized neutralizing agents, potentially causing a 2 log₁₀ overestimation of antiseptic efficacy.^{4,100,134-137} Regardless of neutralization validation, alcoholic chlorhexidine appears to lack any advantage over plain alcohol in terms of immediate effect and is often associated with a more rapid recovery of bacterial load, seriously undermining arguments favoring the use of this antiseptic combination.

Alcohols, alone or in combination with chlorhexidine, have also been shown to suppress general flora on other clinically relevant skin sites. Since baseline microbial loads for these sites are several log₁₀ lower than typical hand counts, alcoholic antisepsis results in near-complete bacterial elimination, making recovery more difficult and thus amplifying the long-term effects of alcohols. In 4 trials, inguinal or abdominal antisepsis with 70% isopropanol led to a mean 2.78 log₁₀ initial RF, 2.58 log₁₀ 6-hour RF and a 2.09 log₁₀ 24-hour RF. Addition of 2% chlorhexidine increased these values to 2.87 log₁₀ initial RF, 2.76 log₁₀ 6-hour RF and 2.89 log₁₀ 24-hour RF.^138-140 By comparison, using 89.5% N-propanol on the arms or back caused a mean 2.1 log₁₀ initial RF and 0.35 log₁₀ 72-hour RF, whereas general skin flora remained 1.27 log₁₀ lower than baseline at 72 hours when N-propanol was supplemented with 2% chlorhexidine.¹⁴¹ In contrast, while 60% isopropanol initially reduced inguinal carriage of Proteus mirabilis by 2.15 log₁₀, 4% chlorhexidine had no meaningful effect and bacterial load for both agents returned to baseline within 4 hours.¹⁴² The combined results of these trials indicate that alcohols have a temporary suppressive effect on normal flora that may be mildly improved in combination with chlorhexidine.

Contamination prevention

Unfortunately, while alcohols appear to exert suppressive effects on surviving flora, the long-term effects of alcohols are also countermanded by exogenous contamination. Studies of fingertip recontamination after hand antisepsis report that healthcare staff acquire an average of 16-20 CFUs/minute during routine patient care.^143,144 Assuming an average fingertip surface area of 1 cm², the recontamination rate for the palmar surface of a whole hand may exceed 150 CFUs/minute, an estimate that is supported by reports that nurses may acquire from 100-6000+ CFUs during brief, ostensibly “clean” activities such as lifting, touching or taking a pulse from an infected patient.^145,146 Baseline hand counts range from 3.9 x 10⁴ to 4.6 x 10⁶ total bacteria and should be reduced to 10-400 bacteria by alcoholic antisepsis.^2,147 Using these values, regrowth times for general flora and a linear contamination rate of 150 CFUs/minute, hand flora will recover by 0.74-2.19 log₁₀ CFUs within 10 minutes and reach 3.9 x 10⁴ bacteria approximately 90 minutes after antisepsis.⁹⁴

This rapid recontamination rate highlights the primary weakness of alcohol-based antisepsis: a lack of protective effect. Lowbury et al reported greater post-application inhibition of an inoculum of 5.1 x 10³ Staphylococcus aureus after plain soap handwashing (0.12 log₁₀ RF) than after use of 70% ethanol (0.03 log₁₀ RF). Inoculation with 78,500 bacteria resulted in a 0.25 log₁₀ RF for plain soap and a -0.01 log₁₀ RF for ethanol.⁸² Similarly, Herruzo et al described a mean post-application RF of 0.3 log₁₀ for 60% isopropyl alcohol against inoculums of Escherichia coli, Pseudomonas aeruginosa and Staphylococcus aureus. A combined alcohol-quaternary ammonium compound-chlorhexidine product, however, had a mean 3.9 log₁₀ residual RF.⁷⁸  Alcoholic chlorhexidine was also associated with a 4 log₁₀ post-application RF in another study that showed no residual activity for 60% isopropanol, although both trials failed to sufficiently validate their neutralizing agents.¹⁴⁸ These studies demonstrate the limitations of alcoholic antisepsis and indicate the potential benefits to supplementation with a less volatile antimicrobial.

Clinical trials

Despite their lack of protective effect, in vivo studies generally indicate that alcohols are superior to most other antiseptics. In an effort to link this efficacy to clinical outcomes, alcohols have been studied in a variety of trials evaluating their effects on hand contamination and infection rates in surgical, general healthcare and community settings.⁶ Although many of these studies suffer from limitations in design, size and scope, they generally fail to directly link alcoholic antisepsis to the ultimate goal of hand hygiene: reduced infection.¹⁴⁹ Instead, the evidence indicates that under general use conditions, alcohols may reduce infection rates chiefly by improving handwashing compliance. Unlike soaps, alcohols are effective without water and can be strategically placed to facilitate point-of-care antisepsis.  As a result, while the superiority of alcoholic antisepsis is unproven, alcohols increase ease of use and, by extension, handwashing compliance. This explanation helps to explain the conflicting results of efficacy trials and offers rationale for the continued use of alcohols despite disappointing clinical outcomes.

Surgical scrubbing

While numerous trials verify the in vivo effects of isopropanol and ethanol on perioperative bacterial counts, there is a lack of evidence to prove that alcoholic antisepsis reduces infection rates to a greater extent than other surgical scrubs. Since conducting a placebo-controlled trial on the effects of alcohols on nosocomial infection rate is ethically unreasonable, most studies compare antiseptic agents for effects on either microbial load or infection rate.¹⁵⁰ However, in a test of antiseptic activity on murine surgical sites inoculated with highly virulent pneumococcal bacteria, ethanol and isopropanol decreased mortality from 100% to less than 15%.¹⁵¹ Unfortunately, clinical results are far less conclusive in humans. Although alcoholic antisepsis appears to provide superior perioperative reductions in hand contamination, the long-term effects of alcohols on either baseline bacterial load or surgical site infection rates appear to be equivalent to other antiseptic agents.

As preoperative antiseptics, the clinical effects of alcoholic antisepsis on bacterial load appear to be generally superior to most antiseptic agents. In one study, rubbing with 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate for 3 minutes reduced hand colonization by 2.05 log₁₀ CFUs immediately and maintained a 1.66 log₁₀ RF through the end of a 15-60 minute surgery, as compared to a 1.58 log₁₀ immediate and 1.12 log₁₀ post-operative RF using 7.5% povidone-iodine (P = 0.02-0.04).¹⁵² A separate study reported a 2.4 log₁₀ initial and 1.8 log₁₀ post-surgery RF for the same alcohol-preservative combination, whereas 4% chlorhexidine was associated with a mere 1.3 log₁₀ initial and 0.9 log₁₀ post-operative RF (P < 0.001).⁷⁷ In another study, alcohols demonstrated better post-operative dampening of bacterial regrowth than 7.5% povidone-iodine or 4% chlorhexidine (4.7 vs 5.2 log₁₀ CFUs, P = 0.017) at surgery times > 3 hours.¹⁵³ These results show that despite a lack of persistent presence on the skin, alcohols are clinically equivalent or superior to other surgical scrubs in terms of perioperative bacterial counts.

In spite of their superior effects on hand counts, alcohols do not offer a clear benefit over other antiseptic agents in terms of baseline bacterial load or the incidence of surgical-site infections. Three weeks of antisepsis using 1% chlorhexidine in 61% ethanol has been shown to reduce prescrub flora by 0.54 log₁₀ CFUs; however, use of 4% chlorhexidine also reduced baseline counts by 0.51 log₁₀ CFUs (P = 0.08).¹⁵⁴ In another hospital study, changing to a 3-minute, 70% ethanol-based rub slightly increased the incidence of surgical site infection (1.33% vs < 1%).¹⁵⁵ The clearest data stems from a 16-month clinical trial comparing the 30-day incidence of surgical site infection (SSI) after antisepsis with 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate or either 4% povidone-iodine or 4% chlorhexidine. In this crossover study, infection occurred after 53 of 2,135 surgeries (2.44%) when hands were sanitized using the alcohol-preservative combination and after 55 of 2,252 surgeries (2.48%) using standard surgical scrubs.¹⁵⁶ The incidence of SSIs was statistically equivalent for alcohols, povidone-iodine and chlorhexidine (P < 0.01). Based on these results, there appears to be poor correlation between reduction of hand flora and decreases in infections. Alcohols are more effective on bacterial load than other antiseptics; however, that efficacy does not directly translate into fewer surgical infections.

Hygienic hand antisepsis

Alcohols have also been widely studied as general antiseptics in both the healthcare and community setting. As general healthcare antiseptics, the effects of alcohols on bacterial load and nosocomial infection rates are similar to those reported for their use as surgical antiseptics. Hand antisepsis with alcohol-based products is associated with lower baseline bacterial load than reported for plain soap; however, alcohols have not been shown to independently reduce hospital-acquired infections.¹⁴³ Several clinical trials have also shown benefit to implementation of an alcohol-based hand hygiene program, but these results appear to be largely due to increased handwashing compliance rather than improved antiseptic efficacy. Similarly, alcohols also appear to reduce community-acquired infections by facilitating quick, accessible hand sanitization, although the quality of most community-based studies is debatable and comparison studies between alcohols and other antiseptics are lacking.¹⁵⁷ Overall, while alcohols reduce hand contamination to a greater extent than plain soap, their impact on healthcare and community infection rates is mild and appears to derive mainly from their beneficial effects on handwashing compliance.

Contamination

In terms of bacterial elimination, alcoholic antisepsis has repeatedly demonstrated superiority over plain soap, although proof that alcohols are more efficacious than other antiseptics is limited. Alcohol is more effective than soap at ridding hands of Gram-negative bacteria, and has been shown to prevent transfer of Gram-negative bacteria from contaminated hands to sterile catheters in 10/12 experiments, while plain soap was effective in only 1/12 tests.^146,158 Prior to training in antiseptic technique, glycerol in 70% isopropanol gel and plain soap were associated with respective 0.52 log₁₀ and 0.05 log₁₀ RFs, which increased to 0.68 log₁₀ and 0.4 log₁₀ RFs, respectively, with technique standardization.¹⁵⁹ In a Russian neonatal intensive care unit (ICU), use of a 79% ethanol/0.1% quaternary ammonium compound hand rub instead of plain soap reduced the Klebsiella pneumoniae colonization rate from 21.5 colonized patients/1,000 patient days to 3.2 patients/1,000 patient days. The colonization rate for other pathogens—especially Candida albicans—was also reduced, albeit to a lesser extent.¹⁶⁰ In other studies, alcoholic antisepsis reduced pathogen isolation from 68% to 25% on hands with artificial fingernails and substantially reduced transient organism recovery on the hands of nurses wearing rings.^161,162 These results validate the current preference for alcoholic antisepsis over handwashing with plain soap.⁶

In addition to the preceding studies, 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate has proven superior to plain soap antisepsis in multiple European hand contamination studies. In 4 trials, this combined alcohol-preservative product was associated with a 0.93-1.7 log₁₀ RF on fingertips, which was significantly greater than the 0.3-0.74 log₁₀ RF decrease in contamination observed with soap (P ≤ 0.0003).^163-166 Another trial actually reported a 0.12 log₁₀ increase in bacterial load for plain soap versus a 0.34 log₁₀ decrease for the alcohol-antiseptic, while a study of infection control professionals using 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate reported a mean RF of 2 log₁₀ (range 0-3.85 log₁₀).^167,168 For both soap and the alcohol-based product, however, bacterial counts appear to return to baseline within 10-30 minutes of antisepsis.¹⁶³ Although N-propanol is not approved for use in the United States, these well-designed trials are included here as further evidence of the superiority of alcohol-based antisepsis on bacterial load.

Despite their generally accepted superiority over plain soap, alcohols do not offer a clear clinical advantage over other antiseptics. For example, a 4-week study reported no effect on baseline nurse hand flora for either 60% isopropanol or 4% chlorhexidine.¹⁶⁹ Alcohols are also non-superior to other antiseptics for reducing false-positive rates during blood culture sampling and are statistically equivalent to other antiseptics in interrupting the transfer of bacteria from contaminated hands to sterile fabric.^170,171 Furthermore, 1 study reported a 1.4 log₁₀ fingertip RF following use of 45% isopropanol/30% N-propanol/0.2% mecetronium etisulfate, whereas handwashing with 10% povidone-iodine or 4% chlorhexidine resulted in a 1.13-1.2 log₁₀ RF. This difference was statistically irrelevant.¹⁶⁴ In contrast, a trial of hand antisepsis during patient care reported a significantly greater (P = 0.012) median reduction in bacterial load after a 30-second application of this alcohol-antiseptic than after use of 4% chlorhexidine (0.77 log₁₀ vs 0.49 log₁₀ RF).¹⁷² Although alcohols do not appear to offer a definitive advantage over other antiseptics, it should be noted that there is a conspicuous lack of quality trials comparing the effects of various agents on hand contamination. Based on the available data, however, alcoholic antisepsis appears to be superior to plain soap handwashing, but its place in the hierarchy of antiseptic efficacy has yet to be established.

Healthcare infection rates

Similar to their effects on hand contamination, there is a lack of clear evidence that alcohols independently reduce infection rates to a greater extent than other forms of hand antisepsis. While many studies show an infection-rate benefit to alcohol-based antisepsis, the results of these studies are complicated by a simultaneous increase in handwashing compliance. Since a 12-40% improvement in handwashing compliance—independent of changes in antiseptic regimen—has been shown to decrease nosocomial infections by 41-45%, the direct effects of alcoholic antisepsis on infection rates are difficult to extricate from an increase in compliance.^173,174 A 5-year trial reported a 57% reduction in cases of methicillin-resistant Staphylococcus aureus (MRSA) bacteremia 3 years after initiating a hand hygiene program utilizing alcoholic chlorhexidine (P = 0.01).¹⁷⁵ However, hand hygiene compliance doubled (21% vs 42%) during the first 12 months of the study. Improved compliance (56% to 95%) also confounded the results of another trial, where conversion from 2% chlorhexidine to an alcohol foam reduced the rate of Clostridium difficile colitis from 4.96 infections/10,000 patient days to 3.98 infections/10,000 patient days (P = 0.0036).¹⁷⁶ While these studies demonstrate that alcoholic antisepsis can positively affect infection rate, this benefit may be due to improved handwashing compliance rather than superior effects on bacterial load.

The hypothesis that increased compliance is the primary factor driving decreased infection rate is further supported by the results of trials with stable handwashing compliance. In a 2-year crossover trial of 2,826 infants in 2 neonatal ICUs, alcoholic antisepsis was associated with 12.1 infections/1,000 patient days. Handwashing with 2% chlorhexidine was associated with 9.5 infections/1,000 patient days. However, after adjusting for birth weight, study site, surgery and follow-up time, both biocides were statistically equivalent.¹⁷⁷ Similarly, an 8-month crossover trial of 1,894 patients in 3 ICUs reported 38 infections/1,000 patient days when 4% chlorhexidine was used and 50.7 infections/1,000 patient days with the use of 60% isopropanol. This difference, however, was statistically nonsignificant and did not affect length of stay or mortality.¹⁷⁸ During a 1-year interventional study with stable handwashing compliance, MRSA infections decreased by less than 5% and vancomycin-resistant Enterococcus (VRE) infections actually increased by 12-13% despite a doubling in use of an alcohol foam product.¹⁷⁹ On the other hand, 2  trials reported no difference in hospital-acquired infection rate between alcoholic antisepsis and plain soap handwashing despite near-doubled compliance.^158,180 These results indicate that compliance may be a better, albeit imperfect, independent predictor of nosocomial infections than use of alcohol-based products.

Internationally, the increased handwashing facilitated by alcohol-based antiseptics has been shown to dramatically reduce nosocomial infections under conditions where soap-and-water handwashing is impractical. Nosocomial infections fell from 13.1% to 2.1% in a Vietnamese trial after implementation of an alcohol-centered handwashing program as handwashing compliance increased from approximately 0% to 43-71%.¹⁸¹ In another Vietnamese trial, SSIs decreased from 8.3% to 3.8% with use of a 70% isopropanol/0.5% chlorhexidine antiseptic, whereas SSIs increased from 7.2% to 9.2% in the control group. Although compliance rates were not reported, baseline hand antisepsis rates were likely low, since only 1 sink was available in the 60-bed surgical ward.¹⁸² By comparison, during an 8-year Norwegian trial of 27,284 patients, the prevalence of nosocomial infections only fell from 7.9% to 7.1% while use of an ethanol antiseptic increased 7-fold (P = 0.04).¹⁸³ Taking these findings into account, it seems likely that alcohols reduce nosocomial infections primarily by increasing the speed and accessibility of antisepsis, an effect that becomes more apparent when handwashing compliance is low.

Community infection rates

Although less research has been conducted regarding the effects of alcoholic antisepsis on infection rates in the community than in healthcare, available evidence indicates that regular use of alcohols may decrease illness. As in the healthcare environment, this benefit appears to stem largely from increased handwashing compliance. The effects of alcoholic antisepsis on community-acquired infections have been studied most in elementary schools, where transmission of infectious pathogens increases student and parental absenteeism from school and work.¹⁸⁴ Despite repeated instruction and encouragement, handwashing compliance is remarkably poor among school-age children. For instance, only 54% of middle- and high-school students wash their hands after using the restroom, 37% wash for > 5 seconds and a mere 18% use soap.¹⁸⁵ By facilitating improved handwashing compliance among students, alcoholic antisepsis has the potential to substantially reduce scholastic infection rates. For example, in a study of 290 kindergarten-to-3rd grade students in 5 schools, classrooms equipped with a bottle of 62% ethanol reported 1.05 illness-related absences/100 student days, whereas classes without hand sanitizer had only 2.08 absences/100 student days.¹⁸⁶ Similarly, a controlled study of 138 students reported a 28% reduction in absences due to illness after installation of an alcohol dispenser.¹⁸⁷

Despite the positive finding of these trials, increased access to alcohol-based hand sanitizers is not conclusively associated with decreased illness.¹⁵⁷ The strongest evidence comes from a trial incorporating 6,080 students from 4 states, 5 school districts, and 18 schools. In this trial, classroom use of 62% ethanol reduced year-long student absences by 19.8% and teacher absenteeism by 10.1%. While these overall results were statistically significant, analysis of individual school districts showed that only 3 of 5 districts actually reached a statistically relevant reduction in student absenteeism. One district actually reported a nonsignificant 3.75% increase in absences with use of the alcohol product.¹⁸⁸ Similarly underwhelming results have also been reported in several other trials. In a study of 253 students, classes with alcohol dispensers averaged 3.45 absences/100 student days, while control classes averaged 3.75 absences/100 student days.¹⁸⁹ A study of 360 second-to-third grade students reported no difference in absenteeism between plain soap and alcoholic antisepsis.¹⁹⁰ Further, while an 8-week study of 285 students reported a statistically significant 8% decrease in school absences due to gastrointestinal illness with use of 70% ethanol, it also reported a nonsignificant 2% increase in respiratory illness for the test group.¹⁹¹ Based on these results, alcoholic antisepsis does not appear to be the cure for the common cold; however, increases in hand hygiene facilitated by alcohol use may help reduce community infection rates.¹⁹²

Hindering factors

A variety of factors may help to explain the lack of correlation between the in vitro, in vivo and clinical efficacy of alcohol-based antisepsis. As mentioned previously, in-use application times are typically too short to eliminate hand flora to a greater extent than plain soap handwashing.^6,64 Applied volume and inoculum size are also important considerations, since 3 mL of alcohol has been shown to cause a 0.27-1.28 log₁₀ greater RF than 1 mL of alcohol and increasing bacterial load from 10³ to 10⁶ CFUs/fingertip reduces alcoholic RFs by 0.24-1.83 log₁₀.^72,73,91 Furthermore, practical application technique rarely resembles the regimented procedures used during in vivo tests. One recent study reported that only 31% of tested healthcare workers properly applied an alcohol hand rub, resulting in a mere 1.4 log₁₀ RF. After hand rub training, antibacterial efficacy rose to 2.2 log₁₀ RF.¹⁹³ Other personnel-related factors include artificial fingernails or ring wearing, which have been shown to significantly decrease the effectiveness of alcoholic antisepsis.^161,162 Overall, these factors seriously undermine the efficacy of alcohols and may limit their clinical effectiveness.

In addition to personnel-related factors, alcohols have several inherent limitations. Alcohols affect microbes by disrupting proteins and are inhibited by extraneous protein, leading to recommendations against their use on visibly dirty surfaces.^{6-9,20,32,33,103} Product formulation may also reduce efficacy, since alcohol gels appear to be about 1-2 log₁₀ less effective in vivo than their rinse counterparts (see Table 1).^{64,76,77,87,166,194} Furthermore, dried bacteria are less susceptible to alcohols, potentially enabling desiccated microbes to better survive surface disinfection.^21,49 Infection outbreaks have been linked to alcohol solutions contaminated with Bulkholderia cepacia—which has an unusual ability to metabolize alcohols—and sporulated Bacillus cereus.^195,196 Lastly, while not related to clinical efficacy, the flammability of alcohols also limits their use as disinfectants.¹⁰³ At concentrations ≥ 70%, ethyl and isopropyl alcohol have flash points lower than 21° C and are considered “easily flammable;” however, the reported incidence of alcohol-related fires is extremely low.^6,70,197 Failure to account for these limitations may also help to explain the disconnection between in vivo and clinical results.

Biofilms

Another hindering factor of alcoholic antisepsis and disinfection is poor efficacy against biofilms. Biofilms form readily on almost any surface and are associated with an estimated 65% of nosocomial infections, making them a significant consideration in infection prevention.^198-200 Unfortunately, an in vitro study reported that 1 minute of exposure to 70% alcohol reduced the number of viable biofilm Pseudomonas aeruginosa and Staphylococcus aureus by a mere 0.59-0.96 log₁₀ and 0.96-1.22 log₁₀ CFUs, respectively. After a full hour, ethanol decreased both strains by 1.4-2 log₁₀ CFUs and isopropyl alcohol reduced Staphylococcus aureus by only 1.9 log₁₀ CFUs. However, 70% isopropanol was relatively effective against Pseudomonas aeruginosa, causing a > 5 log₁₀ RF after 30 minutes of exposure.²⁰¹ Isopropanol also appears to be efficacious against Staphylococcus epidermidis biofilms, killing 5.3 log₁₀ bacteria within 30 seconds.²⁰² In contrast, 70% ethanol only reduces sessile Salmonella Typhyimurium by about 2 log₁₀ CFUs after 1 and 5 minutes of exposure.²⁰³ Furthermore, fungal biofilms also demonstrate decreased susceptibility to ethanol.²⁰⁴ Given the ubiquitous nature of biofilms, the generally poor efficacy of alcohols against sessile microorganisms may seriously undermine the clinical impact of these disinfectants.

Toxicity

While the clinical efficacy of alcohols is somewhat limited by personnel-related factors, the presence of organic material and biofilms, toxicity does not appear to be a significant hindrance to their use. Unfortunately, despite their excellent safety profile, alcohols suffer from the popular belief that they damage skin.²⁰⁵ This stigma is likely derived from the burning sensation experienced when alcohols are applied to pre-damaged skin.²⁰⁶ Alcohols do solubilize sebum and lipids in the epidermis; however, the majority of studies indicate that alcohol-based antisepsis is less drying and better tolerated than plain soap or other antiseptics.^{23,67,154,167,169,177,207} Additionally, alcohols can be combined with a variety of emollients to reduce skin irritation and most healthcare workers consider them to be acceptable for routine use.^{155,163,166,208,209} Since the Environmental Protection Agency (EPA) considers ethanol to be practically non-toxic to both humans and animals, alcohols appear to be environmentally and clinically safe antimicrobial agents.²¹⁰

Conclusion

After more than a century of scrutiny and research, alcohols remain among the safest, most effective biocides available and are used for a remarkably wide variety of disinfecting and antiseptic purposes. Given their potent, largely irresistible mechanism of action, broad antimicrobial spectrum and superior in vivo efficacy, alcohols appear to deserve their status as the preferred antiseptic agent. Unfortunately, in spite their many strengths, ethanol and isopropanol have not been shown to independently reduce infection rates. Rather, they appear to mitigate infection primarily by improving handwashing compliance. Since the greatest weakness of alcohol-based antisepsis and disinfection appears to be a lack of residual presence, efforts to further reduce community and nosocomial infection rates should focus on providing inter-application antimicrobial activity. Until a product with protective, persistent activity reaches the market, however, alcohols will likely remain the most effective and accessible antimicrobial agent available.

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205. Boyce JM. Using alcohol for hand antisepsis: dispelling old myths. Infect Control Hosp Epidemiol. 2000 Jul;21(7):438-40.
206. Löffler H, Kampf G. Hand disinfection: how irritant are alcohols? J Hosp Infect. 2008;70(Suppl 1):44-8.
207. Boyce JM, Kelliher S, Vallande N. Skin irritation and dryness associated with two hand-hygiene regimens: soap-and-water hand washing versus hand antisepsis with an alcoholic hand gel. Infect Control Hosp Epidemiol. 2000 Jul;21:442-48.
208. Pereira LJ, Lee GM, Wade KJ. An evaluation of five protocols for surgical handwashing in relation to skin condition and microbial counts. J Hosp Infect. 1997;36:49-65.
209. Larson EL. Hygiene of the skin: when is clean too clean? Emerg Infect Dis. 2001 Mar-Apr;7(2):225-8.
210. Prevention, Pesticides and Toxic Substances. Reregistration Eligibility Decision (RED): Aliphatic Alcohols. EPA-738-R-95-013. Washington, DC: United States Environmental Protection Agency. 1995 Apr. 238 p.

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Mechanism of action

Quaternary ammonium compounds are characterized by a positively-charged nitrogen atom bound to four alkyl groups and are believed to exert their effects on cell membranes. The cationic benzalkonium chloride is attracted to negatively-charged phospholipid headgroups in the cytoplasmic membrane, allowing it to adsorb to and disorganize the lipid bilayer. Loss of membrane integrity leads to leakage of cytoplasmic contents and loss of cellular viability.^1-3,7-11  The affinity of QACs for phospholipids is an inverse correlate of alkyl chain length.^11-14 In Gram-negative bacteria, benzalkonium chloride is believed to cause transient damage to the outer membrane that promotes diffusion into the periplasmic space and corresponding access to the cytoplasmic membrane.^3,11

Efficacy

The use of benzalkonium chloride as an antiseptic is not well supported. QACs have high activity against Gram-positive bacteria and enveloped viruses, limited activity against Gram-negative bacteria; inconsistent activity against mycobacteria, fungi, and non-enveloped viruses; and no activity against bacterial spores. Moreover, their effects on bacteria tend to be bacteriostatic rather than bactericidal. In handwashing tests, benzalkonium chloride exhibits no residual antimicrobial activity after the initial application and is typically associated with extremely low reductions in bacterial load.^2,15 The combination of poor efficacy results and a long history of infections due to contamination of benzalkonium chloride antiseptic solutions has led to widespread recommendations against the use of benzalkonium chloride for skin antisepsis.

In vitro spectrum

Quaternary ammonium compounds are effective against bacteria and lipid-enveloped viruses under laboratory conditions (in vitro).^1-3,16 Benzalkonium chloride is generally considered a bacteriostatic, rather than bactericidal agent and has an effective minimum inhibitory concentration (MIC) of 0.5 µg/mL against Gram-positive bacteria and an MIC of 50-250 µg/mL against Gram-negative bacteria, though these values are often several times higher in resistant organisms.^3,17 Benzalkonium chloride is mildly tuberculostatic against mycobacteria and may have some activity against fungi.^17-19 Viral assays show mixed effect for benzalkonium chloride, which has good efficacy against enveloped viruses such as human immunodeficiency virus (HIV) and herpes simplex virus (HSV) and variable effectiveness against non-enveloped viruses (eg, coxsackievirus, coronavirus).^17,20,21 Overall, QACs appear to be most effective against Gram-positive bacteria and enveloped viruses.

In vivo results

Despite the demonstrated activity of QACs in vitro, the antimicrobial effects of benzalkonium chloride on skin (in vivo) are fairly unimpressive. This disparity in efficacy was highlighted in a study by Maillard et al evaluating the effects of 1% benzalkonium chloride against Staphylcoccus aureus, Escherichia coli and Pseudomonas aeruginosa in solution, on glass and on freshly excised skin.²²  In solution and on glass, application of benzalkonium chloride resulted in a >4 log₁₀ reduction in bacterial load within 30 seconds. On the other hand, when used on inoculated skin, benzalkonium chloride had no effect on S. aureus or P. aeruginosa after 10 minutes of exposure (~1 log₁₀ RF). Results at 10 minutes with E. coli were too variable to be considered reliable, but at 1 minute of exposure, benzalkonium chloride was associated with only a 1 log₁₀ reduction.

Due to repeated infection outbreaks connected to antiseptic solutions contaminated with resistant bacteria (see below), QACs have been seldom used for hand antisepsis in the healthcare setting during the past 15-20 years.¹ As a result, there is a marked paucity of research regarding the use of these agents as antiseptics, though the available reports generally corroborate the results of Maillard et al.  In an early handwashing test (1971), 0.1% benzalkonium chloride was associated with a 0.27 log₁₀ reduction in normal flora after a 2-minute application time.¹⁵ In contrast, a novel surfactant, allantoin, 0.13% benzalkonium chloride (SAB) antiseptic formulation was reported in 2 studies to have similar efficacy to 62% ethanol after a 45-second wash (2.6-2.8 log₁₀ RF) of hands artificially contaminated with Serratia marcescens.  After repeated washings, the QAC-based product demonstrated clear superiority to ethanol.^23,24 In both of these studies, however, the RFs reported for ethanol were highly inconsistent with the RFs typically reported in many similar trials, a point the authors failed to mention or address.^25-27 The aberrant nature of these results calls the validity of both studies into serious question, particularly in light of multiple other articles showing benzalkonium chloride to be grossly inferior to alcohol as an antiseptic agent.^22,28,29

The most compelling evidence against the use of benzalkonium chloride as an antiseptic was reported in 2005. Unlike the previously mentioned studies, this trial was designed to mimic typical exposure times.  In observations of healthcare workers, antiseptics are typically applied for less than 10 seconds.¹ Since the majority of in vivo trials typically utilize exposure times of 30 seconds or longer, Sickbert-Bennett et al conducted an experiment evaluating the effects of various antiseptics on hands artificially contaminated with Serratia marcescens and MS2 bacteriophage after 10 seconds of application.  Of the 12 agents evaluated (including nonantimicrobial soap and tap water controls), 0.4% benzalkonium chloride had the worst efficacy. Against S. marcescens, benzalkonium chloride exhibited a 0.25 log₁₀ RF after 1 wash and a 0.01 log₁₀ RF after 10 washes (vs 2.0 and 1.68 log₁₀ RFs, respectively, for tap water). Against the MS2 virus, benzalkonium chloride use was associated with a 0.23 log₁₀ RF after 1 wash and a -0.46 log₁₀ RF after 10 washes.²⁹  These dismal results make a clear case against the use of benzalkonium chloride as an antiseptic.

Resistance

Further undermining the case for the use of benzalkonium chloride as an antiseptic, bacterial resistance to QACs is well documented and has been repeatedly implicated as a cause of infection.³⁰ As noted earlier, even the most susceptible Gram-negative bacteria are unaffected by concentrations of benzalkonium chloride that kill their Gram-positive counterparts.^3,17 Many species of Gram-negative bacteria (eg, Bulkholderia cepacia, Pseudomonas aeruginosa, Xanthamonas maltophilia) exhibit a particularly high intrinsic resistance to benzalkonium chloride that enables them to survive in antiseptic solutions.³¹  This has significant clinical consequences. Since 1958, medical literature has been rife with reports of infection outbreaks due to the use of benzalkonium chloride as an antiseptic. References 31-42 contain a sampling of these reports. To complicate matters more, increasing QAC resistance has now been observed in a wide variety of bacteria and linked to simultaneous development of antibiotic resistance.^43-45 The high incidence of infection associated with QAC antisepsis has led to repeated recommendations from the CDC and independent researchers that benzalkonium chloride not be used as an antiseptic, particularly in the healthcare setting.^{1,4,16,31,46-48}

Hindering factors

In addition to innate or inducible resistance, bacterial survival may also be aided by the presence of substances that limit the activity of benzalkonium chloride. QACs are inactivated by anionic detergents, soaps and organic matter such as protein.^1,16,47-49 This may help to explain why benzalkonium chloride is associated with such poor in vivo results. Depending on the material, up to 70% of benzalkonium chloride solution may adsorb to fabric, dramatically decreasing the concentration of available agent.^49,50 Use in the presence of these hindering factors may result in partial or complete loss of antimicrobial activity.

Clinical Studies

Not unexpectedly, clinical trials involving benzalkonium chloride in the healthcare setting are almost nonexistent. Recently, however, SAB antiseptic was evaluated in the domestic setting in 2 trials.^51,52 Conducted by the same investigators, these trials examined the effects of SAB antiseptic on elementary school absenteeism in children. Children in the SAB group were instructed to use hand sanitizer whenever they entered the classroom, before eating, after sneezing or coughing and after using the restroom. Children in the control continued handwashing with plain soap without supervision or monitoring. During the trials, children using SAB antiseptic were absent 30-40% less often than children in the control groups. However, despite the positive results of these studies, a recent review of the use of hand sanitizers to prevent illness-related absenteeism deemed the studies “of low quality and methodologically weak” and determined that there was insufficient evidence available to conclude that hand sanitizers played a meaningful role in reducing illness-related absenteeism.⁵³ Interestingly, other trials included in the review utilized alcohol-based antiseptics and reported equivalent or better results than the SAB trials.

Toxicity

While the efficacy profile may be low, benzalkonium chloride has a very low incidence of skin irritation at typical concentrations, especially when compared with other antiseptics.^23,49,54 In the environment, benzalkonium chloride has a half-life of 13 days, making accumulation unlikely.⁵⁵ QACs typically adsorb to soil, where they have been shown to inhibit microbial flora, though the macroscopic consequences of this exposure are currently unknown.^55,56 On the other hand, benzalkonium chloride has been found in hospital effluent at concentrations that are highly toxic to fish and very highly toxic to invertebrates.^45,55,57 The overall environmental impact of these findings remains uncertain, however, and the Environmental Protection Agency generally considers benzalkonium chloride exposure to be a low risk to the environment.⁵⁵

Conclusion

Rationale for the use of benzalkonium chloride as a hand antiseptic seems generally unfounded. With the exception of a few questionable studies exploring the effects of a novel SAB antiseptic formulation, the available literature indicates that antisepsis using benzalkonium chloride may be associated more with infection than disinfection. In vitro and in vivo, quaternary ammonium compounds demonstrate mixed antimicrobial activity, a difficulty that quickly becomes critically relevant in the healthcare setting. Given its poor efficacy profile and clinical track record, antisepsis with benzalkonium chloride appears to be true “QAC” medicine.

References

1. Boyce JM, Pittet D; Healthcare Infection Control Practices Advisory Committee; HICPAC/SHEA/APIC/IDSA Hand Hygiene Task Force. Guideline for Hand Hygiene in Health-Care Settings. Recommendations of the Healthcare Infection Control Practices Advisory Committee and the HIPAC/SHEA/APIC/IDSA Hygiene Task Force. Am J Infect Control. 2002 Dec;30(8):S1-46.
2. Rotter ML. Hand washing and hand disinfection. In: Mayhall CG, editor. Hospital Epidemiology and Infection Control, 3rd ed. Philadelphia (PA): Lippincott Williams & Wilkins. 2004:1727-46.
3. McDonnell G, Russell DA. Antiseptics and disinfectants: activity, action, and resistance. Clin Microbiol Rev. 1999 Jan;12(1):147-79.
4. Moll D. Re: Citizen Petition to Request FDA Find Benzalkonium Chloride (0.11%-0.13%) is Generally Recognized as Safe or Effective as Defined by the Tentative Final Monograph for Health-Care Antiseptic Products. Rockville (MD): Food and Drug Administration. 2000 Aug 6. 7 p.
5. Dyer DL. Re: Proposed Rule; Reopening of the Administrative Record for Topical Antimicrobial Drug Products for Over-the-Counter Human Use; Health-Care Antiseptic Drug Products. Rockville (MD): 2003 Oct 28. 10 p. Available from http://www.fda.gov/ohrms/DOCKETS/dailys/04/jan04/010704/75n-183h-c000089-01-vol200.pdf.
6. Food and Drug Administration. Tentative final monograph for healthcare antiseptic products: proposed rule. Federal Register. 1994;59:31441-52.
7. Hugo WB, Frier M. Mode of action of the antibacterial compound dequalinium acetate. Appl Microbiol. 1969 Jan;17(1):118-27.
8. Salton MRJ. Lytic agents, cell permeability and monolayer penetration [abstract]. J Gen Physiol. 1968 Jul 1;52(1):227-52.
9. Maillard JY. Bacterial target sites for bactericidal action. J Appl Microbiol Symp Suppl. 2002;92:16-27S.
10. Gilbert P, Moore LE. Cationic antiseptics: diversity of action under a common epithet. J Appl Microbiol. 2005;99(4):703-15.
11. Denyer SP. Mechanisms of action of antibacterial biocides. Int Biodeterior Biodegrad. 1995;36:227-45.
12. Topical antiseptics and antibiotics. Med Lett Drugs Ther. 1977 Oct 7;19(20):83-4.
13. Gilbert P, Al-Taae A. Antimicrobial activity of some alkyl trimethylammonium bromides. Lett Appl Microbiol. 1985;1:101-4.
14. Brown MRW, Tomlinson E. Sensitivity of Pseudomonas aeruginosa envelope mutants to alkylbenzylammonium chlorides. J Pharm Sci. 1979;68:146-9.
15. Lilly HA, Lowbury EJL. Disinfection of the skin: an assessment of some new preparations. Br Med J. 1971 Sep 18;3:674-6.
16. Fraise AP. Choosing disinfectants. J Hosp Infect. 1999;43:255-64.
17. Damani NN. Manual of Infection Control Procedures, 2nd ed. London: Greenwich Medical Media Limited. 2003. 333 p.
18. Tiwari TSP, Ray B, Jost KC Jr, Rathod MK, Zhang Y, Brown-Elliott BA, Hendricks K, Wallace RJ Jr. Forty years of disinfectant failure: outbreak of postinjection Mycobacterium abcessus infection caused by contamination of benzalkonium chloride. Clin Infect Dis. 2003 Apr 15;36(8):954-62.
19. Russell AD, Chopra I. Understanding Antibacterial Action and Resistance, 2nd ed. Hertfordshire, England: Ellis Horwood. 1996. 292 p.
20. Wood A, Payne D. The action of three antiseptics/disinfectants against enveloped and non-enveloped viruses. J Hosp Infect. 1998;38:283-5.
21. Rabenau HF, Kampf G, Cinatl J, Doerr HW. Efficacy of various disinfectants against SARS coronavirus. J Hosp Infect. 2005 Oct;61(2):107-11.
22. Maillard JY, Messager S, Veillon R. Antimicrobial efficacy of biocides tested on skin using an ex-vivo test. J Hosp Infect. 1998;40:313-23.
23. Dyer DL, Gerenraich KB, Wadhams PS. Testing a new alcohol-free hand sanitizer to combat infection. AORN J. 1998 Aug;68:239-51.
24. Moadab A, Rupley KF, Wadhams P. Effectiveness of a nonrinse, alcohol-free antiseptic handwash. J Am Podiatr Med Assoc. 2001 Jun;91(6):288-93.
25. Kampf G, Kramer A. Epidemiologic background of hand hygiene and evaluation of the most important agents for scrubs and rubs. Clin Microbiol Rev. 2004 Oct;17(4):863-93.
26. Kampf G, Rudolf M, Labadie JC, Barrett S. Spectrum of antimicrobial activity and user acceptability of the hand disinfectant agent Sterillium Gel. J Hosp Infect. 2002 Oct;52(2):141-7.
27. Herruzo R, Vizcaino MJ, Herruzo I. In vitro-in vivo sequence studies as a method of selecting the most efficacious alcohol-based solution for hygienic hand disinfection. Clin Microbial Infect. 2010;16:518-23.
28. Hayes RA, Trick WE, Vernon MO, Nathan C, Peterson BJ, Segreti J, Pur SL, Schmitt BA, Rice TW, Welbel SF, Weinstein RA. Comparison of three hand hygiene (HH) methods in a surgical care unit [Abstract K-1337]. Presented at the 41st Interscience Conference on Antimicrobial Agents and Chemotherapy. Chicago (IL): American Society for Microbiology. 2001.
29. Sickbert-Bennett EE, Weber DJ, Gergen-Teague MF, Sobsey MD, Samsa GP, Rutala WA. Comparative efficacy of hand hygiene agents in the reduction of bacteria and viruses. Am J Infect Control. 2005;33:67-77.
30. Sanford JP. Disinfectants that don’t. Ann Intern Med. 1970 Feb;72(2):282-3.
31. Oie S, Kamiya A. Microbial contamination of antiseptics and disinfectants. Am J Infect Control. 1996;24:389-95.
32. Bacteria in antiseptic solutions. Br Med J. 1958;2:436.
33. Malizia WF, Gangarosa EJ, Goley AF. Benzalkonium chloride as a source of infection. N Engl J Med. 1960;263:800-2.
34. Lee JC, Fialkow PJ. Benzalkonium chloride—source of hospital infection with gram-negative bacteria. JAMA. 1961 Sep 9;177:144-6.
35. Cragg J, Andrews AV. Bacterial contamination of disinfectant. Br Med J. 1969;3:57.
36. Hardy PC, Ederer GM, Matsen JM. Contamination of commercially packaged urinary catheter kits with the pseudomonad EO-1. N Engl J Med. 1970;282:33-5.
37. Kaslow RA, Mackel DC, Mallison GF. Nosocomial pseudobacteria: positive blood cultures due to contaminated benzalkonium antiseptic. JAMA. 1976 Nov 22;236(21):2407-9.
38. Frank MJ, Schaffner W. Contaminated aqueous benzalkonium chloride: an unnecessary hospital infection hazard. JAMA. 1976 Nov 22;236(21):2418-9.
39. Sautter RL, Mattman LH, Legaspi RC. Serratia marcescens meningitis associated with a contaminated benzalkonium chloride solution. Infect Control. 1984;5:223-5.
40. Lee CS, Lee HB, Cho YG, Park JH, Lee HS. Hospital-acquired Bulkholderia cepacia infection related to contaminated benzalkonium chloride. 2008 Mar;68(3):280-2.
41. Tiwari TSP, Ray B, Jost KC Jr, Rathod MK, Zhang Y, Brown-Elliott BA, Hendricks K, Wallace RJ Jr. Forty years of disinfectant failure: outbreak of postinjection Mycobacterium abcessus infection caused by contamination of benzalkonium chloride. Clin Infect Dis. 2003 Apr 15;36(8):954-62.
42. Serikawa T, Kobayashi S, Tamura T, Uchiyama M, Tsukada H, Takakuwa K, Tanaka K, Ito M. Pseudo outbreak of Burkholderia cepacia in vaginal cultures and intervention by hospital infection control team. J Hosp Infect. 2010 Jul;75(3):242-3.
43. Chaplin CE. Bacterial resistance to quaternary ammonium disinfectants. J Bacteriol. 1952 Apr;63(4):453-8.
44. Holah JT, Taylor JH, Dawson DJ, Hall KE. Biocide use in the food industry and the disinfectant resistance of persistent strains of Listeria monocytogenes and Escherichia coli. J Appl Microbiol Symp Suppl. 2002;92:111-120S.
45. Hegstad K, Langsrud S, Lunestad BT, Scheie AA, Sunde M, Yazdankhah SP. Does the wide use of quaternary ammonium compounds enhance the selection and spread of antimicrobial resistance and thus threaten our health? Microb Drug Res. 2010;16(2):91-9.
46. Dixon RE, Kaslow RA, Mackel DC et al. Aqueous quaternary ammonium antiseptics and disinfectants. Use and misuse. JAMA. 1976 Nov 22;236(21):2415-7.
47. Donowitz LG. Benzalkonium chloride is still in use. Infect Control Hosp Epidemiol. 1991 Mar;12(3):186-7.
48. Hussey HH. Benzalkonium chloride: failures as an antiseptic. JAMA. 1976 Nov 22;236(21):2433.
49. Bradley C. Physical and chemical disinfection. In: Ayliffe GAJ, Fraise AP, Geddes AM, Mitchell K editors. Control of Hospital Infection: A Practical Handbook, 4th ed. New York: Oxford University Press Inc. 2000:75-91.
50. Bloß R, Meyer S, Kampf G. Adsorption of active ingredients of surface disinfectants depends on the type of fabric used for surface treatment. J Hosp Infect. 2010 May;75(1):56-61.
51. Dyer DL, Shinder A, Shinder F. Alcohol-free instant hand sanitizer reduces elementary school illness absenteeism. Fam Med. 2000 Oct;32(9):633-8.
52. White CG, Shinder FS, Shinder AL, Dyer DL. Reduction of illness absenteeism in elementary schools using an alcohol-free hand sanitizer. J Sch Nurs. 2001 Oct;17(5):268-65.
53. Meadows E, Le Saux N. A systematic review of the effectiveness of antimicrobial rinse-free hand sanitizers for prevention of illness-related absenteeism in elementary school children. BMC Public Health. 2004 Nov 1;4:50.
54. Müller G, Kramer A. Biocompatibility index of antiseptic agents by parallel assessment of antimicrobial activity and cellular cytotoxicity. J Antimicrob Chemother. 2008;61:1281-7.
55. Prevention, Pesticides and Toxic Substances. Reregistration Eligibility Decision for Alkyl Dimethyl Benzyl Ammonium Chloride (ADBAC). EPA739-R-06-009. Washington, DC: United States Environmental Protection Agency. 2006 Aug. 126 p.
56. Sarkar B, Megharaj M, Xi Y, Krishnamurti GS, Naidu R. Sorption of quaternary ammonium compounds in soils: implications to the soil microbial activities. J Hazard Mater. 2010 Dec 15;184(1-3):448-56.
57. Kümmerer K, Eitel A, Braun U, Hubner P, Daschner F, Mascart G, Milrandri M, Reinthaler F, Verhoef J. Analysis of benzalkonium chloride in the effluent from European hospitals by solid-phase extraction and high-performance liquid chromatography with postcolumn ion-pairing and fluorescence detection. J Chromatogr. 1997;774:281-6.
58. Savage PB, Genberg C, Jones RN et al. Activity of a novel cationic steroid antimicrobial (CSA-13) targeting aerobic and facultative Gram-negative bacilli, Gram-positive anaerobes and select agent surrogate strains. Abstracts of the Forty-fifth Interscience Conference on Antimicrobial Agents and Chemotherapy, 2005. Washington, DC: American Society for Microbiology. Abstract F-1231.
59. Isham N, Ghannoum MA. Determination of Antifungal Activity of Ceragenix Discovery Compounds against Dermatophytes, Aspergillus fumigatus, and Candida species, as Measured by Minimum Inhibitory Concentration (MIC). Center for Medical Mycology. Denver (CO): Ceragenix Pharmaceuticals, Inc. 2005. 17 p.
60. Lara D, Feng Y, Bader J et al. Anti-trypanosomatid activity of ceragenins. J Parasitol. 2010 Jun;96(3):638-42.
61. Howell MD, Streib JE, Kim BE, Lesley LJ, Dunlap AP, Dianliang G, Feng Y, Savage PB, Leung DYM. Ceragenins: a class of antiviral compounds to treat orthopox infections. J Invest Dermatol. 2009 Nov;129(11):2668-75.
62. Lai XZ, Feng Y, Pollard J, Chin JN, Rybak MJ, Bucki R, Epand RF, Epand RM, Savage PB. Ceragenins: cholic acid-based mimics of antimicrobial peptides. Acc Chem Res. 2008 Oct;41(10):1233-40.

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A membrane-active agent, Chlorhexidine is primarily used to reduce the number of bacteria found on the skin, and is recommended for both surgical hand and general skin antisepsis due to its persistent presence on the skin.³ Despite these generally accepted roles, actual evidence of benefit is conflicting and continued use of chlorhexidine seems largely unjustified.

Mechanism of action

Chlorhexidine is believed to exert its antimicrobial effects through three mechanisms. The cationic nature of chlorhexidine enables it to specifically target the negatively charged bacterial envelope. At low concentrations, chlorhexidine disrupts the lipid bilayer, inducing leakage of ionic potassium, inorganic phosphates, amino acids, and other cytoplasmic material.^3-7 This effect is primarily bacteriostatic in nature and neutralization or removal of chlorhexidine may allow the cell to recover.⁴ As the concentration of chlorhexidine increases to bactericidal levels, membrane leakage decreases as the biguanide causes nucleic acids and proteins to precipitate and coagulate.^3-7 Additionally, high concentrations of chlorhexidine are thought to inhibit membrane-bound adenosine triphosphatase (ATPase), thereby reducing the cell’s ability to produce useable energy.^4,5 Cytoplasmic coagulation and inhibition of ATPase result in cell death.⁴

Efficacy

The in vitro antimicrobial effects of chlorhexidine exposure have been repeatedly demonstrated; however, concerns regarding insufficient neutralization and conflicting in vivo results have cast serious doubt on the practical value of this agent. Chlorhexidine acts far more slowly than alcohol-based antiseptics and appears to exert an immediate antimicrobial effect similar to non-antimicrobial soap, leaving its clinical value highly debatable. Despite the demonstrated superiority of alcohol-based antiseptics both in vivo and in clinical trials, chlorhexidine has remained in use due to its purportedly persistent antimicrobial activity following application. However, in vivo studies indicate similar long-term reductions in normal flora for both alcohol and chlorhexidine. In light of the available literature, the rationale for chlorhexidine-based antisepsis appears largely unfounded.

In vitro spectrum

Chlorhexidine is effective against most Gram-positive and Gram-negative bacteria, has some activity against enveloped viruses and fungi, but is not sporicidal and lacks substantial activity against mycobacteria and nonenveloped viruses.^3-9 Gram-positive and Gram-negative bacteria are typically susceptible to the bacteriostatic effects of chlorhexidine at minimum inhibitory concentrations (MICs) of 1 µg/mL and 2-2.5 µg/mL, respectively.^5,7 Bactericidal effects are generally observed at concentrations ≥ 20 µg/mL.^5,7 In a test of 1,165 bacterial isolates, the majority of strains were inhibited by 12.5 µg/mL of chlorhexidine and 100 µg/mL was sufficient to inhibit all evaluated strains.⁴ Since typical in-use concentrations range from 0.5-4% (5,000-40,000 µg/mL), clinical levels of exposure should exceed most reported MICs by several orders of magnitude.^3,4

In vivo results

Despite the broad antimicrobial spectrum of chlorhexidine, in-use efficacy is at best marginally superior to plain soap. For example, a 15-second handwash with chlorhexidine showed no advantage over plain soap in reducing normal hand flora. Chlorhexidine did not demonstrate superiority until 15 washes had been performed, and 5 consecutive days of 15 hand washes resulted in a mean log₁₀ reduction factor (RF) of 2.04 (vs. 0.59 for plain soap).¹⁰ In another study, fingertip-contamination with methicillin-resistant Staphylococcus aureus (MRSA) showed equivalent log₁₀ RFs for chlorhexidine 4% and plain soap (1.91 vs. 1.96) following inoculation with 10³ bacteria. Following inoculation with 10⁶ bacteria, chlorhexidine was inferior to plain soap (1.37 vs. 1.77).¹¹  Chlorhexidine was also inferior to plain soap in eliminating 10⁶ Clostridium difficile (47% spores) from gloved hands (1.3 vs. 1.7) and 10⁶ Bacillus anthracis surrogates (1.4 vs. 2.2).^12,13 After a comprehensive literature review, Kampf et al (2004) estimated that chlorhexidine reduces transient flora by 2.1-3 log₁₀ and resident flora by 0.35-1.75 log₁₀ (vs. 0.5-3 and ≤ 0.4, respectively, for plain soap).⁷

Trends in the testing timeline provide further perspective. The vast majority of studies demonstrating the superiority of chlorhexidine over plain soap occurred around the time the biguanide was approved by the FDA, while most trials conducted in recent years have shown the two to be roughly equivalent. Assuming a lack of research bias, this can be largely attributed to differences in application time. Many early studies assessed the efficacy of chlorhexidine after 2 minutes of application. Since changes in time of exposure have shown to increase the RF of chlorhexidine from 1 log₁₀ at 30 seconds to 1.74 log₁₀ at 2 minutes and handwashing duration is typically 10 seconds or less, the results of these early studies cannot be assumed to apply to normal practice.^3,14 In contrast to their predecessors, more recent studies utilizing relevant exposure times show a general lack of effect. These differences in methodology and results further undermine in vivo efficacy claims, particularly in settings that mimic clinical practice.

The Neutralization Controversy

Further complicating the efficacy debate, recent reevaluation of neutralizing agents used in chlorhexidine testing has cast considerable doubt on the validity of many in vitro and in vivo trials.²⁸ Since residual chlorhexidine persists on surfaces after initial application, incomplete neutralization of chlorhexidine results in continued bacteriostatic or bactericidal effect in the sampling fluid, diluents and/or agar. This reportedly leads to a 2 log₁₀ overestimation of reduction factor in vitro and in vivo.^29-32 The discovery that commonly used neutralizing agent combinations (eg, 3.5% Tween 80, 0.5% lecithin, and 0.5% sodium thiosulfate) failed to inactivate chlorhexidine has generated significant concern regarding the dependability of the majority of chlorhexidine studies.^28,32-34

The relevance of these issues is particularly evident when considering the in vivo chlorhexidine studies referenced by the CDC.³ While all of these studies utilized some sort of neutralizing agent, many failed to validate their neutralization process. The absence of demonstrable chlorhexidine neutralization renders the results of these trials unreliable. Since trials conducted without validation of neutralization can be assumed to err on the side of efficacy overestimation, it is reasonable to presume that trials which failed to show benefit in the absence of proven neutralizing agents would have failed to show benefit in their presence as well. Using this logic, of the dependable in vivo studies referenced by the CDC guideline, there is only a 5:3 superiority-to-inferiority ratio when comparing chlorhexidine with plain soap. Notably, the CDC guideline does not reference two articles with negative efficacy results authored prior to release of the guideline.^12,28 Taking these trials into account, there is a 5:5 superiority-to-inferiority ratio of validated chlorhexidine vs. soap studies conducted prior to compilation of the CDC guidelines.

Clinical studies

Not surprisingly, the clinical benefits of chlorhexidine use are controversial.^7,35 Positive efficacy studies are few and generally limited by poor compliance, inconsistent results, or comparison solely against povidone-iodine. One study showed similar efficacy between 2% chlorhexidine and 61% ethanol in a neonatal intensive care unit (ICU) with poor handwashing compliance over a 22-month period.³⁶ Similarly, another ICU study showed improvement in total nosocomial infection rate with chlorhexidine vs. alcohol use; however, this was not associated with difference in length of stay, mortality, or total number of infected patients.³⁷ Chlorhexidine 4% also demonstrated better general antibacterial activity than triclosan 1% (0.27 vs. 0.07 log₁₀ RF) during a 16-month longitudinal study, though chlorhexidine was ineffective against MRSA.³⁸ Additionally, multiple clinical trials have demonstrated the superiority of chlorhexidine over povidone-iodine; however, both biocides appear to be less effective than alcohol-based antiseptics.^4,39-41

In contrast to these studies, chlorhexidine use has been frequently associated with a lack of clinical benefit. Despite its purported persistent antimicrobial activity, chlorhexidine was unable to maintain resident bacterial counts below baseline values during operations lasting ≥ 3 hours (4.5 preoperatively vs. 5.2 postoperatively).⁴² Frequent handwashing with chlorhexidine in a neonatal ICU was associated with a doubling in baseline bacterial colonization over the course of a month.⁴³ Marena et al reported that chlorhexidine was significantly less effective than plain soap in reducing hand contamination in a 4-month crossover study of two surgical wards.⁴⁴ The results of these and other studies leave the clinical value of chlorhexidine use highly debatable.^45,46

Persistent activity

In spite of mediocre efficacy and dubious clinical benefit, chlorhexidine is still used in a variety of roles thanks to its non-volatility and ability to adsorb to surfaces. Five hours after application, 93% of radio-labeled chlorhexidine remains present on uncovered skin, which correlates well with claims of persistent antimicrobial effect for 5-6 hours. Twenty-four hours after application of a 5% tincture, concentrations of 12.2 µg/mL (~0.25%) remain on the skin.⁴ Chlorhexidine appears to adsorb less readily to inorganic surfaces—approximately 1% binds to glass.⁴⁷ Overall, these results indicate the presence of residual chlorhexidine greatly outlasts the original application time, though the clinical relevance of this non-volatility has never been demonstrated.⁷

The CDC defines persistent activity as “prolonged or extended antimicrobial activity that prevents or inhibits the proliferation or survival of microorganisms after application of the product.”³  Multiple trials conducted in an effort to demonstrate the persistent activity of chlorhexidine have reported mixed results. These results are complicated by the synergistic effects of alcohol on residual chlorhexidine activity as well as a lack of neutralization validation. Combination with alcohol appears to enhance the antimicrobial effects of residual chlorhexidine, making it difficult to quantify the independent activity of either agent.⁴⁸ Additionally, while the need for proper, verifiable neutralization appears to add weight to residual antimicrobial activity arguments, insufficient neutralization makes it difficult to tell whether changes in microbial count are due to the residual presence of chlorhexidine on the skin or the presence of chlorhexidine in the growth medium after sampling. This concept is corroborated by the results of an “ex-vivo” study, where the presence of chlorhexidine in solution resulted in a 3-4 log₁₀ greater reduction of pathogenic bacteria than observed when chlorhexidine was applied to skin samples.⁴⁹

Suppression of Normal Flora

The CDC handwashing guideline cites eight studies as proof that chlorhexidine exhibits persistent antimicrobial activity.³ On close inspection, however, the majority of these articles provide little evidence to support this claim.²⁸ One “study” is actually one of the first review articles to point out the need for proper chlorhexidine neutralization, arguing that the reported persistence of chlorhexidine may be due to its action in sampling fluids.²⁹ A second article reports only the immediate effects of chlorhexidine.¹⁸ A third claims to demonstrate the superiority of 0.5% chlorhexidine over 70% isopropyl alcohol in reducing bacterial counts on the hands. However, despite multiple daily washings, chlorhexidine use was associated with increased baseline colonization increased (5.76 to 5.92 log₁₀ CFUs) and 6-hour regrowth that was far more rapid than that observed with alcohol (see below).⁵⁰

Other articles cited by the CDC are less contrary, but still inconclusive. In one, both chlorhexidine and alcohol—alone and in combination—were associated with an approximately 1 log₁₀ larger reduction in normal flora after 3 hours of glove use than after initial administration, but there was no significant difference in RF between agents at any point.¹⁶ Another study reported slower normal flora recovery with 4% chlorhexidine than with a 70% ethyl alcohol and 0.5% chlorhexidine combination, however, overall bacterial reduction was greater with the combination product regardless of number of washings or the length of time after antisepsis.²² Lastly, Pereira et al compared various strengths of chlorhexidine and combinations with alcohol. Though all tested groups showed a greater reduction in CFUs 2 hours after initial administration than at administration, post-exposure CFU counts were higher following a second washing, suggesting swift bacterial recovery between treatments.⁵¹

Many other trials have been conducted in an effort to quantify the persistent activity of chlorhexidine. The majority of these have found chlorhexidine alone or in combination with alcohol to be approximately equal to or slightly inferior to alcohol-based hand-rubs in suppressing the normal flora.^16,23,53-58 Faoagali et al reported a mild persistent effect on the first day of administration, though this disappeared after 5 days of twice-daily antisepsis.²⁵ Results from another study indicated that delay of 72-hour regrowth of the normal flora on the upper arms and back was greater for the combination of chlorhexidine and n-propanol than for n-propanol alone by approximately 1 log₁₀. Baseline CFU counts, however, were far lower than those typically observed on hands (~2.5 vs. ~ 6 log₁₀), and delay of regrowth may have been due in part to virtual elimination of the natural flora during initial antisepsis.⁵⁹ These results, along with those previously discussed, indicate that while chlorhexidine may mildly suppress the normal flora over time, the effects are controversial and of limited clinical relevance.

Contamination prevention

Bacterial contamination after antisepsis can also be used to assess the persistent activity of a biocide. Studies evaluating the ability of chlorhexidine to prevent new bacterial adherence and survival are limited, though their results are favorable. In 3 separate studies, Lowbury et al inoculated hands immediately following antisepsis with 20-50 µL suspensions of Staphylococcus aureus or Escherichia coli. The skin was allowed to dry for 2 minutes, rinsed and dried on a sterile towel, and then sampled 1 hour later.^15,16,60 The quantity of bacteria administered was not reported in two of these trials, which reported only the number of viable bacteria per mL after sampling for chlorhexidine and plain soap. Both trials showed a substantially greater decrease in surviving bacteria following chlorhexidine use.^15,60 Initial inoculum counts were included in the third trial, which compared 0.5% chlorhexidine in 95% ethyl alcohol with 70% ethyl alcohol.  Alcoholic chlorhexidine demonstrated a much greater ability to prevent artificial S. aureus contamination than alcohol alone (3.47 vs -0.01) after inoculation with 4.7 log₁₀ of bacteria. Intriguingly, while Lowbury also tested both biocides against E. coli, they did not report these results due to an unexpected residual effect in the alcohol group.¹⁶ Unfortunately, prolonged application time and immediate administration of a bacterial suspension make the results difficult to extrapolate to clinical practice.

Resistance

Given the already low efficacy profile of chlorhexidine, increasing reports of bacterial resistance are particularly concerning. Most Gram-positive and many Gram-negative bacteria lack significant resistance to chlorhexidine, though increases in MIC have been noted in some strains of MRSA, Escherichia coli O157, Salmonella enterica and Acinetobacter baumannii.^62-68 In contrast, effective resistance to 0.5% chlorhexidine is often observed in clinical strains of Providencia stuartii (83.3%), Pseudomonas spp. (45.7%), Proteus spp. (38.7%) and Klebsiella spp. (1.8%).⁶² While the link between biocide resistance and antibiotic resistance has not yet been completely elucidated, many strains with significant chlorhexidine resistance also exhibit broad antibiotic resistance.^62-64,69-71 Fortunately, exposure to residual chlorhexidine does not seem to engender meaningful resistance.^47,72,73  However, regardless of reported planktonic MICs—most of which are irrelevant at in-use concentrations—chlorhexidine resistance and subsequent pathogenicity appear to be primarily due to the formation of biofilms.

Biofilms

Bacterial biofilms have high chlorhexidine resistance that allows constituent members to survive in concentrations 100s-1000s of times greater than the MICs reported for their planktonic counterparts.^74-78 Biofilms form readily on almost any surface and are associated with an estimated 65% of nosocomial infections.^77-79 Bacteria exhibit varying propensities for biofilm formation, which correspondingly affects their resistance to chlorhexidine. For example, in vitro biofilm studies demonstrated that 1% chlorhexidine eliminated 1.3 log₁₀ of S. aureus and 1.6 log₁₀ of Salmonella spp. after 5 minutes of exposure and only 0.22 log₁₀ of P. aeruginosa after 30 minutes of exposure time.^80,81 Clearly, biofilm-conferred resistance permits microorganisms to survive for prolonged periods in the presence of chlorhexidine.

Clinically speaking, biofilm formation not only enables biocide survival on fomites and environmental surfaces but may also directly facilitate infection outbreaks through contaminated chlorhexidine solutions. ⁸² Serratia marcescens biofilms are able to survive in chlorhexidine for years and can be cultured from solutions containing 2% chlorhexidine.⁸³ Contamination of chlorhexidine solutions by Serratia marcescens and Pseudomonas aeruginosa has been repeatedly reported and linked to outbreaks of infection.^68,82,83 Ironically, biofilm contamination is such an omnipresent infection risk that it is recommended that stock solutions of chlorhexidine be periodically disinfected with alcohol.^68,82

Hindering factors

Putting clinical efficacy aside, a variety of substances undermine the killing ability of chlorhexidine. As a cationic molecule, chlorhexidine is inactivated natural soaps or other anionic compounds, non-ionic surfactants, cork, and hand creams containing anionic emulsifying agents.^2-8,84 The activity of chlorhexidine is reduced in the presence of organic material (eg, blood, sputum), though it remains more far more effective than other cationic compounds.^2,3,8,85 At concentrations ≥ 0.05%, chlorhexidine precipitates from solution as an insoluble salt when combined with borates, bicarbonates, carbonates, citrates, nitrates, phosphates, sulfates, and most dyes.³ Additionally, chlorhexidine may also be neutralized by hard water.³

Toxicity

Chlorhexidine use is typically well-tolerated and has a good safety record, though reports of skin irritation with chlorhexidine use range as high as 90% and can lead to poor use rates.^{3,4,8,9,55,86} Since chlorhexidine is not absorbed through the skin, systemic exposure following topical administration is extremely unlikely and has not been reported.^4,87,88 There are 5 documented cases of acute toxicity following ingestion of high doses of chlorhexidine that manifested as gastrointestinal erosion, gastritis, and/or acute liver toxicity.⁴ Ocular exposure to preparations containing > 1% chlorhexidine can cause conjunctivitis and/or severe corneal damage and care should be taken to avoid touching the eyes after application.^3,4,89 Sensorineural deafness has been reported in patients who received 0.05% chlorhexidine in 70% alcohol for perioperative disinfection of the ear. Additionally, allergic reactions can occur, particularly when chlorhexidine is used in the genital area or on neonates, and this hypersensitivity has lead to non-fatal anaphylaxis in several cases.⁴ However, considering the widespread, decades-long use of chlorhexidine, the incidence of toxic effects is remarkably low and chlorhexidine appears to be safe for general use.

Environmental effects

While chlorhexidine has an excellent topical safety profile, widespread use of the biguanide has raised concerns regarding the role of chlorhexidine in the environment.⁹⁰ Chlorhexidine accumulates in the environment and may reach concentrations as high as 10.3 µg/mL (0.001%) in domestic wastewater.^90,91 Concentrations of 10 µg/mL have been shown to have effects on algal and cyanobacterial biofilm, with bacterial biofilms beginning to change at concentrations of 100 µg/mL. The ultimate effects of this perturbation are unknown, but since biofilms form the foundation of the food chain, changes at this level may have broad consequences. Additionally, microbes appear to lack the ability to render chlorhexidine inert, which may result in progressive environmental accumulation.⁹⁰ Since the EPA has determined that chlorhexidine is not only toxic to microbes but also slightly toxic to birds, moderately to highly toxic to fish and very highly toxic to invertebrates, continued use at current rates may lead to rising levels of environmental exposure with far-reaching ecological effects.^89-91

Uses

Orally, chlorhexidine gluconate is used to reduce flora in the mouth for a variety of therapeutic and prophylactic purposes. Chlorhexidine binds strongly to negatively charged oral surfaces (eg, hydroxyapatite on tooth enamel, pellicle on the tooth surface), and is reported to exert an inhibitory effect on oral flora anywhere from 2 days to 12 weeks.^92,93 The true residual oral activity of chlorhexidine is somewhat debatable; however, since biofilms of resident oral bacteria such as Streptococcus mutans demonstrate marked resistance to 0.2% chlorhexidine and may not be accounted for during the spit tests used to determine persistent activity.⁹⁴ Despite these conflicting reports, chlorhexidine is used clinically to treat gingivitis (0.12%) and periodontitis (biodegradable subgingival pellets). Additionally, chlorhexidine mouthwash is used to prevent dental carries and plaque when normal oral hygiene is impossible, to reduce the incidence of mucositis and other oral complications in immunocompromised patients, and to decrease the risk of nosocomial respiratory tract infections in critically ill patients. Chlorhexidine appears to be highly effective in preventing respiratory tract infections, but efficacy data for other prophylactic oral uses is poor.⁹²

Alternative formulations may offer ways to circumvent some of the limitations of chlorhexidine. Catheters impregnated with chlorhexidine and silver sulfadiazine have been shown to reduce catheter-related bloodstream infections by approximately 60%, though efficacy has been somewhat inconsistent.^95-97 Additionally, while still approved in the United States, these catheters have been discontinued in Japan due to severe anaphylactic reactions associated with use.^97-99 Another formulation currently under development utilizes chlorhexidine-loaded nanocapsules to achieve a slow-release, sustained antimicrobial effect. Nanochlorex® appears to have a similar immediate effect to 62% propanol on resident bacteria and a dramatically superior effect (5.5 log₁₀ vs 1 log₁₀) on the survival of transient Staphylococcus epidermidis. However, this persistent activity disappears after 4 hourly inoculations with 1 mL of 10⁷ bacteria.^100,101 These preparations may provide more clinical benefit than currently available forms.

Conclusion

Chlorhexidine’s persistent presence in the market appears to be as unwarranted as claims of chlorhexidine’s persistent antimicrobial activity. While the membrane permeabilizing abilities of chlorhexidine give it a good in vitro profile, in vivo tests of immediate effect demonstrate a mild, equivocal superiority to plain soap, particularly at in-use application times. The vaunted residual presence of chlorhexidine also appears to be largely irrelevant, since numerous studies have shown an overall equivalence or inferiority to alcohol in long-term reduction of normal flora. Chlorhexidine may have some role in preventing exogenous recontamination, but this is undermined by poor clinical efficacy data, microbial resistance, limited effect against biofilms, skin dehydration and environmental accumulation. Despite this dearth of proven benefit, since the biguanide appears to provide the best persistent activity of currently available agents, chlorhexidine will likely remain in general use until the advent of a practical alternative.

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