Soaps, household cleaners, sponges, and even mattresses and lip glosses now pack bacteria-killing ingredients. Does adding those ingredients make sense?

Traditionally people wash bacteria from their bodies and homes using soap and hot water, alcohol, chlorine bleach or hydrogen peroxide. Soap works by loosening and lifting dirt, oil and microbes from surfaces so that they can be easily rinsed away with water. General cleaners such as alcohol inflict sweeping damage to cells by demolishing key structures, after which the cleaners evaporate. Products containing antibacterial agents, in contrast, leave surface residues, notes Stuart Levy of the Tufts University School of Medicine

Spurring Evolution

This persistence is problematic. When a bacterial population that survives the first hit of an antibacterial agent vies with the lingering chemical, a small subpopulation armed with special defense mechanisms can evolve. This group then multiplies as its weaker relatives perish, and it will withstand attack the next time the chemical is applied. “What doesn't kill you makes you stronger” is the governing maxim here; antibacterial chemicals select for bacteria that can endure their presence.

Resistance to topical chemicals is not the only risk. When bacteria become tolerant to these compounds, they sometimes also become less sensitive to certain antibiotic medicines. This phenomenon, called cross-resistance, has already been demonstrated with triclosan, one of the most common chemicals in antibacterial products.

Genetic mutations can arise in bacteria exposed to triclosan for long periods and endow certain types of bugs with resistance to isoniazid, an antibiotic used for treating tuberculosis, explains Allison Aiello of the University of Michigan School of Public Health. Other mutations can allow microbes to supercharge their efflux pumps—protein machines in the cell membrane that can spit out several types of antibiotics, including ciprofloxacin, used to treat anthrax. These effects have been demonstrated only in the laboratory, not in households and other real-world environments, but Aiello suspects that the few household studies conducted so far might not have been long enough. “The potential is there,” she says.

Scientists have further concerns about antibacterial compounds. Both triclosan and its close chemical relative triclocarban are present in 60 percent of U.S. streams and rivers, points out Rolf Halden of the Johns Hopkins Bloomberg School of Public Health.

Both chemicals also end up in the sludge produced by wastewater treatment plants, which is used as fertilizer for crops, thereby opening a potential pathway for contamination of food, Halden says. “The concentrations in agricultural soil are very high,” and this abundance, “along with the presence of pathogens from sewage, could be a recipe for breeding antimicrobial resistance” in the environment, he adds.

Triclosan has also been found in human breast milk, though not in concentrations considered dangerous to babies, as well as in human blood plasma. There is no evidence that current concentrations of triclosan in the human body are harmful, but recent studies suggest that it compromises the function of the hormone systems in bullfrogs and rats.

No Benefit

Ultimately, the value of such antibacterial additives is questionable. An expert panel convened by the U.S. Food and Drug Administration determined that there is insufficient evidence for any benefit from consumer products containing them. “What is this stuff doing in households when we have soaps?” asks John Gustafson of New Mexico State University.

Some scientists argue that consumer use of antibacterial products is appropriate in the homes of people with weakened immune systems. In general, however, good hygiene means using regular soaps rather than antibacterial ones, experts say. “The main way to keep from getting sick,” Gustafson says, “is to wash your hands three times a day, and don't touch mucous membranes.”