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Debate over Impact of Antibacterial Cleaning Agents Rejoined

Triclosan may not be a household word. But this antibacterial agent is a common household ingredient of products such as soaps and lotions, and it also is impregnated into many other household items, including cutting boards, clothes, and toys. The profuse deployment of triclosan is part of a wider, but perhaps ill-advised, campaign by consumers and manufacturers of consumer products against any and all bacteria. Little do they realize that this assault could backfire—converting this zealous quest for cleanliness into yet another means for inducing antibiotic resistance, according to Herbert Schweizer of Colorado State University (CSU) and his collaborators, who reported their findings in the February issue of Antimicrobial Agents and Chemotherapy (45:428-432).

The CSU scientists exposed Pseudomonas aeruginosa to triclosan and, like the legendary Hydra that sprouted two new heads every time Hercules decapitated it, the bacteria not only grew more resistant to this agent but also gained resistance to several antibiotics. For instance, exposing P. aeruginosa to triclosan raised its minimal inhibitory concentrations (MICs) for antibiotics such as tetracycline and erythromycin by as much as 500-fold, Schweizer says. Perhaps more alarmingly, the MIC for the antibiotic ciprofloxacin, a relatively recent addition to the pharmacopoeia, also went up, but only by 94-fold, he adds.

"The types of mutations selected by exposure to triclosan belong to the same family of mutations that you get through exposure to antibiotics," Schweizer says. The mutations disable regulatory proteins that prevent expression of genes encoding multidrug resistance efflux pumps. Once produced, these pumps bail antibiotics, antibacterial agents, and other kinds of prokaryote poisons from P. aeruginosa, whose genome encodes at least ten kinds of such pumps, he says. "I'm not saying triclosan causes multiple drug resistance, but it certainly can compound existing resistance. We don't know yet how this relates to the real world." For example, although P. aeruginosa strains sometimes have two such pumps operating at once, he and his CSU collaborators do not know whether the bacteria will activate these and other pumps when they encounter triclosan in the environment.

Some researchers are skeptical about the relevance of the CSU findings to other antibiotic resistance phenomena. They do not "relate to the real world at all," says Philip Tierno, director of microbiology and diagnostic immunology at New York University Medical Center. P. aeruginosa is "an extraordinarily resistant organism to a wide variety of chemicals, germicides, and antibiotics," he points out. For instance, it is much less permeable to such agents than are most gram-negative bacteria. Furthermore, he says, "it has numerous resistance mechanisms that are intrinsic to it, many of which have not been characterized." Moreover, because Triclosan does not sterilize and mixtures of microbial species generally are present, interspecies competition persists, "making selection of resistant bacteria unlikely" when such mixtures encounter triclosan or similar antibacterial agents, he says.

Others are not so quick to dismiss the impact of these agents. "Overuse of these antibacterials in homes may create the kind of environment where organisms like Pseudomonas will propagate," says Stuart Levy of Tufts University School of Medicine. "That has yet to be proven, but it makes sense to me. I just came back from England where they had some Staphylococcus aureus in hospital isolates that had higher-than-normal levels of triclosan resistance. Where did they come from? They are obviously being selected."

As for Pseudomonas, it is a serious problem in its own right, says Levy. "It is one of the worst things we have faced in treating leukemia patients since I started treating these patients 30 years ago. We can never keep up, and we never have the right antibiotics. It is an interesting point to say that triclosan is there to get rid of truly harmful bacteria like staph, but that other organisms like Pseudomonas are intrinsically resistant, and they are not normal pathogens," he says. "My point is that if a single antibacterial is used in all products in the home, you will select for resistant organisms—intrinsic or acquired—which will change the microbiology, with consequences that are unknown."

That could place those with weakened immune systems at risk. "If you're going to bring Aunt Martha home from the hospital after treatment for cancer, do you want her to confront Pseudomonas and Acinetobacter, and all these resistant infectious organisms on the counters?" Levy says. "This argument would be without substance if these antibacterials were serving any purpose. I don't think antibacterials have a place in a healthy household."

But Tierno says that failure to use antibacterials carries its own risk. Bacteria can accumulate in fissures in cutting boards, where even careful cleansing may not dislodge them. Similarly, they can congregate within microscopic cavities in toys. Americans are notoriously negligent about washing their hands. All this contributes to foodborne or other contact-dependent illnesses. We need antibacterials to "save us from our lazy ways," he says.

David Holzman
David Holzman is a freelance science writer in Lexington, Mass.