Photo by onex10/flickr

Along with their usual rations of grain and prepared feed, factory-farmed hogs and chickens in the United States dine on a steady diet of antibiotics. The animals are given the drugs not to prevent or cure illness but simply because low-level doses stimulate them to grow faster than untreated animals. This may be good for agribusiness's bottom lines, but an increasing body of research shows that it might be very bad for public health.

Several scientific examinations of pork and poultry operations in this country have shown that anti-microbial-resistant "superbugs," such as flesh-eating methicillin-resistant Staphylococcus Aureus (MRSA) and certain tough-to-kill strains of E. coli, are showing up not only in farm animals but also in the humans who tend them—and even in members of their families who don't work on the farms.

Before the Boston study, scientists generally viewed the mutations that led to resistance as chance events.

Now, a group of researchers at Boston University has discovered a mechanism that causes these superbugs to develop. It could mean that the problem is even worse than previously imagined. Their results are reported in the current issue of the journal Molecular Cell.

In earlier studies, the scientists had found that drugs that kill bacteria do so in part by stimulating the production of free radicals in those bacteria—not unlike the ones in humans that contribute to heart disease, cancer, and other maladies. However, when antibiotics are administered to the bacteria at low levels, as they are on factory farms, instead of killing the bugs the free radicals cause genetic mutations—far more than would normally occur. Some of those mutations lead to new strains of bacteria that can survive what were once lethal doses of drugs.

"Our work indicates that it is much more dangerous than previously thought," said Jim Collins, a professor of biomedical engineering and one of the paper's authors. "The low-level antibiotics are boosting up the mutation rate and not killing off the bacteria. As result, you have created a zoo of mutants."

Before the Boston study, scientists generally viewed the mutations that led to resistance as chance events. "A mutation would emerge randomly in a bacterial population, and the antibiotic would kill off everybody but that guy, and that guy would breed, creating a resistant strain," Collins said. "We now know that the antibiotics are acting as mutagens themselves."

But it even gets scarier. The brave new bugs that result from these mutations can carry resistance not only to the antibiotic that had been administered, but to others as well. "You could have antibiotic A being delivered to the bacteria and have a mutant bug arise that could still be killed by high levels of antibiotic A, but now has mutations that give it resistance to antibiotic B, antibiotic C, and antibiotic D," Collins said.

There is a simple solution to agriculture's contribution to this problem: outlaw the use of subtherapeutic antibiotics on farm animals. The European Union did it in 2006. Somehow it still manages to produce hogs and chickens. So a ban here is hardly what I'd call a radical idea.