Chinese pig farms breed drug-resistant bacteria

Half of all pigs live in China ? and well over half of them eat feed laced with antibiotic "growth promoters". Now Chinese and US researchers have found that this practice is spawning a tide of antibiotic-resistant bacteria.

Animals in crowded feedlots are constantly exposed to bacterial infections, so producers put low doses of antibiotics into their feed. The measure may boost profits, but it also favours the survival of bacteria with genes that help them resist the antibiotics. Antibiotic resistance genes (ARGs) can migrate between bacteria, creating superbugs that are hard or impossible to treat and that can infect people.

The European Union banned antibiotic growth promoters for this reason in 2006. But they are still permitted in the US, where recently released figures show around half of all samples of the food-poisoning bacterium salmonella, taken from retail poultry meat, are resistant to at least three antibiotics.

The impact of antibiotic growth promoters in China, where industrial pig production is expanding, has been unclear. China is the biggest antibiotics producer and consumer in the world, and feeds nearly half its production to animals.

To investigate, Yong-Guan Zhu of the Chinese Academy of Science in Xiamen, James Tiedje of Michigan State University in East Lansing, and colleagues tested soil along with fresh and composted manure at three big Chinese pig farms in different provinces. They also tested compost and manure from Chinese pigs not fed antibiotics, and soil from a remote forest.

Using a quantitative PCR assay to pick up specific DNA sequences, the researchers measured the amounts of 244 different ARGs in their samples. They used the amount of a gene common to all bacterial in the sample as a measure of the total bacteria present, to establish how many ARGs there were for a given number of bacterial cells.

Clearer picture

"We were able to measure many more kinds of resistance than had been studied before," Tiedje says. The team's approach gave a clearer picture than traditional investigation methods, in which bacteria in a sample are grown in the lab and then tested for their antibiotic resistance. Many bacterial strains do not culture well and so this older approach risks underestimating antibiotic resistance.

In the farm samples, the team found 149 ARGs that, between them, confer resistance to all classes of antibiotics ? and the levels of some were "enriched" 28,000-fold compared to those in the soil samples not taken from farms. "Composting the manure and spreading that on soil reduced the load of some, but not all, types of ARGs," says Tiedje.

Worryingly, the samples also contained enzymes that help ARGs move between bacteria. They even contained the antibiotics themselves, so selection pressure for the resistant bacteria continues outside the pig.

Overall figures, he says, suggest the Chinese are using about the same amount of antibiotics per animal as US farmers ? but with six times as many pigs, the impact is much greater. "The concentration and number of types of resistance genes is a concern," says Tiedje. "This study shows animal farming is breeding for antibiotic resistance to a greater extent than shown by previous data. Some would be expected to migrate to human pathogens."

Antibiotic use is not monitored on Chinese farms, and Tiedje fears some sites where very large amounts are used could be resistance hotspots. We need more detailed information on antibiotic use to track its impact, says Lance Price of George Washington University in Washington DC. No such information is available, even for US farms.

Journal reference: PNAS, doi.org/kgg

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