Antibiotic resistance is coming to dinner

Bacteria that are immune to several antibiotics are showing up in a broad range of foods on grocery store shelves, new studies show.

It’s a recipe for rising illness and deaths from food poisoning, according to data reported in Orlando, Fla., this week at the American Society for Microbiology meeting.

When virulent, the microbes can induce gut-wrenching poisoning. There also is a risk that, once bacteria mingle on the cutting board or in a diner’s gastrointestinal tract, even benign bacteria that happen to be resistant to drugs could share their resistance genes with more dangerous microbes. Later, infections caused by the modified microbes might prove intractable.

Bacteria can develop drug resistance in the environment, in hospitals, and even within a person treated with antibiotics. However, animal agriculture is playing a disproportionately large role, says Burke A. Cunha, who heads the division of infectious diseases at Winthrop-University Hospital in Mineola, N.Y.

“The volume of antibiotics used in animal feeds equals or exceeds that used to treat infections in humans,” he notes in the April 28 Lancet. What’s more, he told Science News, “many of the antibiotics that have been used to supplement animal feeds are the very ones most likely to induce resistance.”

If livestock develop resistant bacteria (SN: 7/18/98, p. 39:, the bugs can taint meat or foods exposed to the animals’ wastes, Cunha says.

Several reports in Orlando included accounts of such tainted foods. For example, Michael Teuber of the Swiss Federal Research Institute in Zurich isolated Enterococcus faecalis from salamis, which were fermented and made from raw meat. The normally harmless bacteria, which serve as an indicator of fecal contamination, were resistant to five common antibiotics: chloramphenicol, erythromycin, streptomycin, streptothricin, and kanamycin.

In lab tests, Teuber incubated E. faecalis with other “not very closely related bugs,” such as lactococcus and Listeria. The salamis’ bacteria readily donated their resistance genes to the others, he reports. The enterococcus carried these genes on one of its two plasmids, which are circular strips of DNA that many bacteria freely swap with each other.

When Teuber’s group examined single-serving-size salamis in the United States, it found no bacteria. The difference, Teuber says, is that these little salamis had been sterilized during production.

However, David D. Wagner and his colleagues at the Food and Drug Administration in Laurel, Md., identified enterococcus species, including E. faecalis, in brand-name retail cuts of raw meat from local supermarkets. To rule out in-store tainting, they included only factory-packaged meat. Overall, the bacteria turned up in 67 percent of the chicken, 34 percent of the turkey, and 66 percent of the beef. At least some of the microbes in each contaminated sample proved resistant to multiple antibiotics, though not necessarily to the same drug combos.

The birds were resistant to more drugs than the beef. For example, 70 percent were immune to penicillin and 39 percent to streptomycin. Beef microbes were resistant to neither. However, bacteria from both poultry and beef showed high rates of resistance to feed-additive antibiotics such as tetracycline.

In response to this study, FDA has just launched a year-long trial to test meats weekly in Iowa. Wagner says it’s a prelude to a national monitoring program for drug resistance in meats.

Shaohua Zhao of FDA last year tested for antibiotic-resistant bacteria in U.S. imports of foods other than meat. Of the 187 samples contaminated with Salmonella, 8 percent showed bacterial resistance to antibiotics, and the rate was highest in imports from developing nations. Some resisted as many as six different drugs. The antibiotic-resistant bacteria showed up primarily in fresh and frozen seafood, but there was one case in parsley and another in cheese.

The new reports “follow a theme that we saw through the ’90s—that [bacterial] resistance is not to one or two agents, but to four, five, and six,” notes Stuart B. Levy, director of the Center for Adaptation Genetics and Drug Resistance at Tufts University in Boston.

“We like to think that our food supply is perfectly safe, and I think it can be if we address [the resistance problem] more aggressively,” he says. “But that is going to take time and additional funds from the state and federal governments.”

Until then, Levy recommends that consumers assume a defensive approach by cooking foods to germ-killing temperatures, thoroughly washing all foods to be eaten raw, and meticulously cleaning all food-preparation surfaces after any contact with raw foods.

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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