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 dampen 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 hypothesized 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 actually happens, ecologist Martin Chénier of McGill University in Montreal, Canada, 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 ½ years. When the researchers grew the bacteria in the lab, for example, 70% to 100% of them were still resistant to chlortetracycline when the pigs were slaughtered. "I didn't expect such high levels of resistance would remain," says Chénier, whose team will publish the results in the January issue of Microbial Ecology.
Many resistance genes are easy to shed, Chénier notes. 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 Chénier's team studied harbored a chlortetracycline resistance gene on plasmids, even after years without the antibiotic.
Chénier suspects the resistance genes may have stuck around because they're linked to other genes bacteria need to survive. One hypothesis is that the other genes on the plasmids protect the animals from high levels of copper and zinc, metals found in pig feed. And as long as bugs hold on to resistance, they can share it with other bacteria they encounter.
Most farm screens have traced resistance only in pathogenic bugs—those that cause disease. But such organisms make up just a small percentage of gut microbes in pigs, Chénier notes. The new data, he says, suggest that the common practice of using swine waste as a fertilizer is like spreading truckloads of antibiotic resistance on farmland. Those bacteria can share their resistance with other bacteria that happen to be on crops and in downstream aquatic ecosystems—bacteria that could cause illness, Chénier says. "This is a time bomb."
The study highlights the indirect consequences of antibiotic use that scientists have long worried about, says microbiologist Julie Zilles of the University of Illinois, Urbana-Champaign, who was not involved in the work. Still, she notes, it's difficult to trace antibiotic resistance from animal farms to medical clinics.
The impact on the human food supply is also unclear, says H. Scott Hurd, an animal food safety expert with the World Health Organization and the University of Iowa, Ames. When scientists trace bacteria through meat production, he says, they find the meat to be safe. Employees at slaughterhouses and meat-processing facilities follow guidelines to keep the pigs' gut bacteria from contaminating the rest of the meat and the facility, he notes. "Risk assessment shows us by the time food gets to the consumer, there's very little resistant bacteria left."
Still, Hurd says, although farmers have used chlortetracycline for 50 years, food-safety experts have monitored resistant strains only in the past decade. He agrees with Chénier about the need for more research on the spread of resistance genes from farms to the environment. "Clearly, we are creating resistant bacteria on the farm," Hurd says. The new study "raises concern," he says, "but doesn't answer questions."