Livestock farms that stop using antibiotics may still be breeding grounds for drug-resistant germs, according to a new study. Scientists have found that bacteria in a group of Canadian pigs remained mostly impervious to two antibiotics years after farmers stopped dosing the animals. This antibiotic resistance could eventually make its way into hospitals and the human food supply, although experts caution that no link has yet been proved.
Farmers regularly treat cattle, pigs and chickens with antibiotics to damp low-level infections that slow the growth of these animals. But wily bacteria quickly evolve resistance. Livestock farms often brim with resistant bugs that can pass to humans and potentially spread resistance to other microbes. Scientists suspected that if farmers stopped using the drugs, the bacteria would lower their defenses to save energy, eventually kicking out the DNA that codes for antibiotic resistance.
To figure out if this happens, ecologist Martin Chenier of McGill University and his colleagues examined bacteria on a university farm that in January 2007 banned all antibiotics, including two commonly used varieties: tylosin and chlortetracycline. They monitored gut bacterial populations in 10 pigs by searching for bacteria resistant to common drugs in their waste.
To the team’s surprise, the entire bug community kept most of its armor against the antibiotics, even after 2 1/2 years. When the researchers grew the bacteria in the lab, for example, 70 to 100 percent of them were still resistant to chlortetracycline when the pigs were slaughtered.
“I didn’t expect such high levels of resistance would remain,” said Chenier, whose team will publish the results in the January issue of Microbial Ecology.
Many resistance genes are easy to shed, Chenier noted. Bacteria often store them on circular pieces of DNA known as plasmids, which are not part of the microbes’ primary genome. Keeping these plasmids around costs energy, and if they’re not needed, the bacteria should kick them out within days. But many of the microbes Chenier’s team studied harbored a chlortetracycline resistance gene on plasmids, even after years without the antibiotic.
The study highlights the indirect consequences of antibiotic use that scientists have long worried about, said microbiologist Julie Zilles of the University of Illinois, who was not involved in the work. Still, she said, it’s difficult to trace antibiotic resistance from farms to medical clinics.
