Are Antimicrobial Soaps Breeding Tougher Bugs?

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Levy lays out this theory in his book "The Antibiotic Paradox": Antibacterial products leave residues on the surfaces where they are used. The active ingredients linger and continue to kill the bacteria, but not effectively or randomly. The naturally stronger bacteria that survived the initial assault develop new defense mechanisms against the chemicals. This selection process gives rise to a new generation that is resistant to the offending compounds.

Certain bacteria also develop "cross-resistance" -- transferring their new and improved defenses to bacteria fighting other types of antibiotics.

This is essentially the same scenario as the emergence of drug resistance from the overuse of antibiotic medications.

But Sansoni says that transferring the drug resistance phenomenon to hand cleaners is one of the "greatest suburban myths," for which, he says, there is no scientific evidence. Indeed, scientists looking for the emerging resistance have found it only in their own labs, in Petri dishes. Triclosan, for example, has been shown to make bacteria undergo mutations and create resistance -- but only in the laboratory setting. Not on your kitchen counter.

Levy is cautious: "Clearly, this is not a leading cause or a major cause of the resistance problem," he says. "But it is a potential." Given that it can happen in the laboratory, he is concerned that with increasing use, "these additives will contribute to the problem of resistance."

According to Allison Aiello, an epidemiologist at the University of Michigan School of Public Health, the specific way that triclosan works could cause problems. Triclosan targets a biochemical pathway in the bacteria, so any mutations at that site could mean that the chemical no longer gets to the target to kill the bacteria.

Beyond the drug-resistance worries, some scientists are concerned that antimicrobial soap is an indiscriminate killer.

Some bacteria are bad for us, but some are good. The antimicrobials kill both. And when the good bacteria are gone, there's more room for the bad bacteria to grow, raising our risk of becoming sick.

Besides, a germ-free environment may actually weaken our immune systems, some critics say. They are referring to the Hygiene Hypothesis -- the theory that children build their immune systems from infancy by putting in their mouths all those dirty objects they find lying around.

A number of studies have linked the development of allergies, asthma and skin problems in children to their having been raised in environments that are too sterile. "You need a little dirt," Levy says, "to train your immune system correctly."

Altering the Environment

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Scientists have discovered high levels of triclosan and triclocarban -- two of the common compounds found in soaps and detergents -- in our ecosystem. According to a U.S. Geological Survey report from 2002, the substances can be found in nearly 60 percent of our streams and rivers.

Kristopher McNeill, a University of Minnesota chemist studying the effect of triclosan on the ecosystem, says it might be a source of certain types of dioxins.

The chlorine that is routinely added to wastewater, McNeill says, reacts with triclosan. The chlorinated triclosan, in the presence of sunlight, is transformed yet again in a reaction that forms dioxin -- not the most toxic type of dioxin, but one that may be harmful to aquatic life and, as a result, the food chain.

"We, as a society, did everything wrong with triclosan," McNeill says. "We took an antimicrobial compound and put it into all kinds of products and thus into just about every household, workplace and public restroom. . . . Amazingly, we haven't had outbreaks of triclosan-resistant bacteria, but it's not for lack of trying on our part."

The takeaway message: If you are worried about MRSA, E. coli, SARS, influenza or simply the common cold, you already know you should wash your hands.

Thoroughly.

Regular soap and water will do.

Ranit Mishori is a family medicine resident at Georgetown University/Providence Hospital. Comments:health@washpost.com.