Is antibacterial soap just a marketing gimmick?

This week the Food and Drug Administration (FDA) issued "a proposed rule to require manufacturers of antibacterial hand soaps and body washes to demonstrate that their products are safe for long-term daily use and more effective than plain soap and water in preventing illness and the spread of certain infections. Under the proposal, if companies do not demonstrate such safety and effectiveness, these products would need to be reformulated or relabeled to remain on the market." This statement is consistent with past FDA advisory panels who have questioned the effectiveness of antibacterial soap at preventing illnesses any better than regular soap and water. So what is going on? Is antibacterial soap just a marketing gimmick or is there any substance behind the claims of soap manufacturers?

As context, I will mention several underlying themes that are relevant to this discussion. First is the importance of using soap especially when washing your hands to reduce the possibility of transmitting bacterial or viral-borne illnesses. Second is the fear that many people possess of man-made (synthetic) chemicals i.e. chemophobia. Third is the concern over the rise of antibiotic-resistant bacterial strains that can cause infections in humans that are not treatable by modern medicine i.e. antibiotics.

Soap is the salt of a fatty acid (e.g. sodium stearate) which can act as a surfactant (reducing water tension) and detergent (dissolving hydrophobic, i.e. not water-soluble, compounds like oils and fats). These properties enable soap to help water "wash away" things on the surface of your hands including bacteria that may be encased in oily grime. In addition, all living cells and many viruses contain a hydrophobic membrane that protects the cell interior from the external environment. Soap is able to dissolve this membrane and thus kill bacteria (and viruses).

Antibacterial soaps add specific chemical compounds to regular soap to enhance the killing of bacteria. These compounds include triclosan and triclocarban. I will focus on the former which is more prevalent and better understood.

Triclosan is a phenyl ether compound that possesses both bacteriostatic (prevents bacterial growth) and bactericidal (kills bacteria) properties. Its mechanism of action was not know until recently, and there is still some debate. At low concentrations (<0.01%), it is an inhibitor of the enzyme enoyl reductase which is involved in fatty acid synthesis which is necessary for bacterial growth. At higher concentrations (0.1 to 1%), which is the concentration used in soap products, triclosan can act in a non-specific fashion presumably by disrupting the bacterial membrane due to its hydrophobic composition in a similar manner as regular soap. This dual mode of action gives triclosan properties of both a biocidal (low potency, non-specific targets) and an antibiotic (high potency, specific target).

One source of controversy is the discrepancy between studies in the lab versus clinical studies in the field with people using soap in real life. Studies in the lab show that triclosan can be a potent inhibitor of bacterial growth in standard assays measuring growth in laboratory media in the presence of the chemical. However, in these lab assays, soap itself can be a potent inhibitor too, raising the question of the efficacy of triclosan above and beyond that of soap.

Outside of the lab, triclosan has not demonstrated significant effectiveness. Levy and colleagues cite 4 clinical studies (3 in Pakistan and one in the U.S.) in which one group used antibacterial soap and another group used regular soap as part of their daily household routine, and then the investigators measured infectious illness symptoms such as cough, fever, and diarrhea. There were no significant differences between the two groups.

As comparison, it should be noted that numerous studies have shown that washing hands with soap cuts diarrheal disease, respiratory infection, and parasitic infections by 50% or more compared to washing hands with water alone.

Triclosan is not completely ineffective, however. An intermediate test between the clinical field studies and the lab assays is bringing people into a controlled setting where they repeatedly wash their hands and then the bacteria levels are directly measured on their hands. In these experiments, triclosan did indeed reduce the bacterial content on the hands, but the most significant differences were observed for high concentrations (2%) of triclosan or when the hands were washed repeatedly and extensively. Indeed in hospitals, showering or bathing in 2% triclosan is used to decontaminate patients exposed to staph infection (MRSA).

What are the dangers of triclosan? First from a standard toxicological standpoint, triclosan is non-toxic to humans, which is evident since nearly all of us have used antibacterial soap products without any adverse consequences. Second, laboratory studies have shown that it is possible that high levels of triclosan can give rise to cross-resistance to other antibiotics. In other words, bacteria can become resistant to triclosan through mutations, and some of these mutants will be resistant to other antibiotics. This has to do with the fact that triclosan can act both as a biocide and antibiotic (see above). Importantly, however, this antibiotic resistance has not been observed outside of the lab. There is no difference in the population of antibiotic-resistant bacteria in households that use antibacterial soap versus those who use regular soap. Third, while there is evidence that triclosan can act as an endocrine disruptor in certain animals, there is no evidence that it is an endocrine disrupter in humans.

In summary, although triclosan (the active ingredient of many antibacterial soaps) possesses potent antibacterial activity in the lab, there is no evidence of its efficacy in clinical studies monitoring typical soap use. In addition, there is no clinical evidence that triclosan promotes antibiotic resistance or is an endocrine disruptor in humans. More studies are needed, which is the recommendation of the proposed rules change by the FDA, but at this time antibacterial soap products appear to be no more effective or harmful than regular soap products.

Figure 1. The jury is still out on the effectiveness of antibacterial soap compared to regular soap.