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Bugs. An E. coli toxin makes Salmonella enterica (picture) resistant to antibiotics.

Resistance Without Antibiotics

Antibiotic resistance, an increasing medical problem that renders drugs useless, is mainly caused by overexposing bacteria to antibiotics. But that's not the only way. Researchers have now found that bacteria can become resistant to a host of antibiotics through exposure to a toxin from another bug.

In many environments, bacteria wage a continuous war against each other. Some species fight with an antimicrobial peptide called a microcin to which they are resistant themselves. A team led by microbiologist Steve Carlson at the U.S. Department of Agriculture's Agricultural Research Service lab in Ames, Iowa, was interested in harnessing microcins as antibiotics but wondered whether bacteria would eventually learn how to evade these too. To test this, the researchers used two bacteria that both live in the gut: a strain of Escherichia coli that secretes microcin 24 and a Salmonella enterica strain that is susceptible to it.

The researchers grew Salmonella in a petri dish, then added E. coli. As reported in the August issue of Applied and Environmental Microbiology, the Salmonella quickly developed defenses to the E. coli microcin. But it could also withstand a wide variety of antibiotics like ampicillin, chloramphenicol, tetracycline, rifampin, and even ciprofloxacin, a heavy-hitter used as a last resort against resistant Salmonella. The explanation, the team figured, could be that the microcin turned on the so-called multiple antibiotic resistance system, a pump that some bacteria use to get rid of almost anything harmful. Their hunch proved right: By disabling the pump, they could prevent Salmonella from becoming resistant.

A worrying implication is that bacteria may acquire resistance even when antibiotics are used prudently or not at all, says Carlson--although it's not clear how often this happens.

The study also casts doubt on researchers' plans to employ special killer bacteria to combat infections, says microbiologist Herbert Schweizer at Colorado State University in Fort Collins, such as the use of microcin-secreting E. coli to treat Salmonella infections in chickens. If they induce general resistance, Schweizer says, such bugs may well do more harm than good.

Related sites

The multiple antibiotic resistance system
Alliance for the Prudent Use of Antibiotics