Benzalkonium chloride: A hand sanitizer monograph

The antimicrobial effects of quaternary ammonium compounds (QACs) such as benzalkonium chloride were first described in the 1900s.¹ Under laboratory conditions, benzalkonium chloride demonstrates decent activity against some bacteria, fungi and enveloped viruses; however, practical results are far less favorable.^2,3 In light of the poor efficacy data available and despite numerous petitions to categorize the QAC as “safe and effective” for use in healthcare antiseptics, the FDA has not granted this status to benzalkonium chloride.^4-6 This decision appears to be justified since, at present, there appears to be little evidence to suggest that benzalkonium chloride has practical value as an antiseptic.

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

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