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WASHINGTON (Reuters) -

It sounds like a good idea -- put a germ-killing disinfectant in toothpaste and soap to keep kids and adults safe from infection -- right? Wrong, Boston-based microbiologist Laura McMurry and colleagues at the Tufts University School of Medicine say.

McMurry said triclosan, a disinfectant widely used in products as diverse as kitchen sponges, soap, fabrics and plastics, is capable of forcing the emergence of ``superbugs'' that it cannot kill. And experiments have shown that it may not be the all-out germ-killer scientists once thought it was. Changing just one gene in the E. coli bacterium allowed it to resist triclosan's effects, McMurry said in a telephone interview. ``We were able to get resistance by simply changing an amino acid in the target.''

Triclosan is used so widely because it is what is known as a nonspecific biocide -- it kills all microbes. Like bleach and alcohol it was believed to interrupt so many cell processes there was no way any organism could develop resistance to it. ``It was just kind of thought it dissolved the membranes. If it does, then you are probably not going to get resistance. You would have to have a totally different membrane that would be resistant,'' McMurry said.

Most drugs used as antibiotics work on just a single process. For instance, penicillin stops many bacteria from building a strong cell wall by acting against one component, known as a mucopeptide. But this specific action means many bacteria, including the very common staphylococcus, can resist penicillin. That is why new generations of antibiotics have had to be developed.

MORE USE MEANS MORE CHANCE OF RESISTANCE The more a drug is used, the more chances bacteria have to evolve resistance. Unless all the bacteria in an infection are killed, the ones that survive exposure to a drug will be those that resist it in some way, while the weaker ones die first. Thus, a species of bacteria can evolve resistance, especially if this happens over and over again. Antibiotic-resistant bacteria are becoming a bigger and bigger problem. They range from penicillin-resistant gonorrhea to super-strains of staphylococcus that cannot be killed by vancomycin, the strongest antibiotic available.

For this reason, doctors are now being warned to cut back on frequent prescriptions of antibiotics except for people who really need them, and patients are being reminded to take their full course of drugs to make sure no resistant bacteria survive to breed more resistant bacteria. But no one had thought this evolutionary process was a problem with triclosan because it was thought to kill all bacteria. Then McMurry and her colleagues put this to the test, breeding bacteria that had various genetic mutations to see if they would resist triclosan. Writing in the most recent edition of the journal Nature, they said they had found one. It was a gene called fab1, which is involved in the creation of fatty acids in cells. McMurry said this could mean that bacteria could evolve resistance to triclosan, but she stressed that there is no evidence so far that this has happened in nature.

DAILY USE OF TRICLOSAN MAY BE UNWISE ‘We did find those triclosan-resistant mutants in the lab; we have not looked for them out in the real world. But the point is not that we've proved that it's really happened out there in the real world but that there is the potential.''

Considering this, she said, using triclosan daily in the home -- in products ranging from children's soaps to toothpaste to ``germ-free'' cutting boards -- may be unwise. "As I understand it, washing hands with soap, the goal of it is to wash off the bacteria. I think that unless it's in a setting where you are in a hospital or you are in a home with a really sick person, I think it is overkill,'' she said. "That's my suspicion. It's putting a chemical in there that I'm not sure is necessary.''

McMurry has not tested her mutant bacteria to see if they would resist triclosan in a real-life setting. "The amounts of triclosan employed in many of the hand soaps are quite high,'' she said. "I can't say with those high amounts that even my mutant would survive.'' But there is more than one way to fight off a drug. Sometimes bacteria evolve their own resistance, but they also have a habit of meeting and exchanging genes with one another. This means resistance to triclosan could be acquired, and not simply evolved.