Retail meats host drug-resistant bacteria

There’s a good chance your grocer’s meats are tainted with bacterial germs that are relatively invulnerable to antibiotics, a trio of new studies finds. As a result, anyone sickened by these microbes could find the arsenal of drugs available to fight their infection small to nonexistent.

Thorough cooking makes meats safe, but contamination may occur in the kitchen. Artville

Three years ago, the Washington, D.C.­based National Research Council and Institute of Medicine jointly published an analysis of the human-health impacts of low doses of antibiotics added to livestock feed to promote the animals’ growth. The report concluded that at least some antibiotic-resistant disease in people is due to meat from animals treated with these growth promoters (SN: 7/18/98, p. 39: http://www.sciencenews.org/pages/sn_arc98/7_18_98/fob7.htm).

However, the analysis failed to establish whether such ties to human disease were common or just isolated anomalies.

Taken together, three papers in the Oct. 17 New England Journal of Medicine now offer a “proverbial ‘smoking gun,'” tying livestock growth promoters to risk of serious human disease, according to an editorial in the same journal by Sherwood L. Gorbach of the Tufts University School of Medicine in Boston.

In the first study, a team of microbiologists, including David G. White of the Food and Drug Administration (FDA) in Laurel, Md., screened for Salmonella enterica in supermarket ground meats. Of 200 packages of chicken, beef, turkey, and pork, more than 20 percent contained this microbe. The scientists uncovered 13 strains. “In four samples, we actually found more than one [strain],” which is quite unusual, notes White.

More importantly, of the 45 salmonella samples isolated, 35 were resistant to at least one antibiotic and 24 showed resistance to at least three. Of the eight samples tainted with a strain known as Salmonella typhimurium, five of the infections were resistant to 6 antibiotics, and two chicken samples bore a form of this strain resistant to 12 such drugs. The latter form, White says, appears to be “the most resistant salmonella [ever] discovered.”

In a second study, researchers looked for the bacterium Enterococcus faecium in chickens purchased at 26 groceries in four states. L. Clifford McDonald of the Centers for Disease Control and Prevention in Atlanta and his colleagues found that 86 percent of the 407 birds they tested carried this germ. In 68 percent of the tainted chicken, the E. faecium was resistant to a potent antibiotic cocktail that goes by the name of Synercid.

“We all carry enterococci bacteria in our gut,” explains McDonald, now at the University of Louisville, Ky. If people get sick from other causes, however, these bacteria can cause disease. Doctors used to prescribe vancomycin for such infections. However, heavy use of vancomycin by hospitals has led to many E. faecium becoming resistant to it (SN: 4/24/99, p. 268). That’s why, starting 2 years ago, doctors turned to Synercid.

The widespread resistance to this drug now seen in meatborne bacteria appears to stem from farm use since 1974 of a related antibiotic—virginiamycin—as a growth promoter, McDonald says.

There is some good news. McDonald’s team found that less than 1 percent of human stool samples tested exhibited Synercid-resistant enterococci. However, McDonald adds, sometimes a resistant germ will only colonize the gut after people have taken a course of antibiotics. As more people take Synercid, he worries, their vulnerability to resistant E. faecium may climb.

In the third study, scientists in Denmark demonstrate that a single tainted meal can seed the gut with antibiotic-resistant E. faecium for up to 2 weeks. Researchers at the Statens Serum Institut in Copenhagen worked with healthy volunteers who showed no evidence of hosting antibiotic-resistant E. faecium. Twelve men and women each drank a glass of milk laced with 10 million bacteria from pigs and chickens resistant to either vancomycin or virginiamycin. Then, the scientists tested the volunteers’ stool daily for a week and again at the end of a second week. A day or two after drinking the tainted milk, all the participants began shedding antibiotic-resistant enterococci. Eight continued to shed them through day 6. One continued to do so through day 14.

The finding lends support to a 1997 move taken by the European Commission to ban use of vancomycin-related growth promoters in livestock, asserts epidemiologist Dominique L. Monnet, one of the Danish study’s authors. That ban has led to dramatically lower rates of resistant enterococci in Danish meat (SN: 8/5/00, p. 95).

At some point, enough data accumulate so that people can say, “It’s time to take action,” Gorbach maintains. “And I think we’re at that point. It’s time to put a ban on [growth-promoting] use of antibiotics in animal feed.”

U.S. regulators are investigating the issue. Last year, FDA solicited “a request for comments and for scientific data” on human health impacts of livestock growth promoters.

Janet Raloff

Janet Raloff is the Editor, Digital of Science News Explores, a daily online magazine for middle school students. She started at Science News in 1977 as the environment and policy writer, specializing in toxicology. To her never-ending surprise, her daughter became a toxicologist.

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