Waste from large-scale Chinese pig farms has a heavy concentration of a range of genes that could confer resistance to nearly all major classes of antibiotics, according to a study published today in the Proceedings of the National Academy of Sciences.
These alarming findings are the latest in a growing body of evidence suggesting that agricultural use of antibiotics contributes to increasing rates of antibiotic resistance among bacteria that cause disease in humans.
One practice in particular, feeding animals subtherapeutic levels of antibiotics to help them grow faster, has received considerable scrutiny. The emergence of antibiotic-resistant pathogens promoted by chronic exposure to such low levels of antibiotics might occur in the animals themselves or—because the bulk of the antibiotics are excreted and wind up in manure or run-off—in the environment. Last year, the US Food and Drug Administration issued guidelines urging animal growers to phase out the use of antibiotics for growth promotion (http://jama.jamanetwork.com/article.aspx?articleid=1172040); some European countries have already phased out such nontherapeutic use.
Concern about the potential environmental effect of antibiotic resistance in agricultural waste in China prompted scientists from the Chinese Academy of Sciences partnering with James M. Tiedje, PhD, and his colleagues from Michigan State University, to assess antibiotic-resistance genes in pig waste from large-scale Chinese farms, which use nearly all major classes of antibiotics. The scientists used high-capacity polymerase chain reaction analysis to identify resistance genes in manure, composted manure, and soil on which manure had been spread.
They found 149 genes with the potential to confer antibiotic resistance in the agricultural waste and determined that such genes were highly concentrated in the samples compared with samples from nonagricultural forested areas. Additionally, they found a heavy concentration of metals such as zinc, copper, and arsenic, which are also added to animal feeds to promote growth.
Although the levels of antibiotics and heavy metals were comparable with those found on farms in other parts of the world, the diversity of such growth additives in the Chinese waste was far greater, the authors said, noting that the presence of so many different antibiotics and metals may promote the emergence of resistance genes. In addition, samples with higher concentrations of antibiotics and metals had a greater abundance of antibiotic-resistance genes. Tiedje explained in an interview that when environmental pressure selects for genes that help a microbe survive exposure to metals, some genes for antibiotic resistance may be passed along at the same time.
The diversity and abundance of antibiotic-resistance genes found in the Chinese agricultural waste is bad news for human health, not just in China but around the world, explained Tiedje. The resistance genes identified in the waste included genes that could confer resistance to antibiotics with critical medical applications, including macrolides, cephalosporins, aminoglycosides, and tetracycline.
“The problem of maintaining the effectiveness of antibiotics is a probability problem,” he said. “The more widely antibiotics are used, the more selection there is for resistance. There is also a greater likelihood of those resistance genes being transferred to human pathogens and transported globally.”
Tiedje noted that better management of agricultural antibiotic use and improved methods for handling animal waste to reduce the concentration of resistance genes may help curb the emergence of antibiotic resistance.