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Studies identify food sources of disease and drug resistance

October 04, 2006

As the recent U.S. outbreak of E. coli infections caused by contaminated spinach demonstrates, the safety of the food we eat cannot be taken for granted. Two studies in the Nov. 1 issue of The Journal of Infectious Diseases, now available online, further illustrate the point, one adding a new bacterial culprit to the mix and the other showing that use of antibiotics as growth promoters in livestock increases the risk of antibiotic resistance in humans.

In one study, investigators led by Katri Jalava, DVM, of the Finnish National Public Health Institute, and J. Pekka Nuorti, MD, DSc, of the National Centers for Disease Control and Prevention, traced an outbreak of Yersinia pseudotuberculosis infection among children in a Finnish town to carrots grown on a single farm. An epidemiologic investigation linked illness to eating raw carrots. Laboratory tests confirmed that the bacteria in infected children's stool samples were indistinguishable from the bacteria isolated from the farm.

The authors noted that this marked the first time that the bacterium had been recovered from an epidemiologically implicated source of food-borne illness. They pointed out that it is well known as a pathogen in wild mammals, and that the farm stored the carrots in a barn in open containers for months. "A combination of direct contact with wildlife feces during storage and cross-contamination during washing and peeling," they concluded, "are the most likely contributing factors." To prevent such outbreaks, "regulations addressing the production, storage and shipping conditions for fresh produce are needed."

In the other study, Edward A. Belongia, MD, and colleagues at the Marshfield Clinic Research Foundation, Marshfield, Wis., and elsewhere examined poultry exposure as a risk factor for antibiotic resistance by Enterococcus faecium, a gut bacterium that is an increasing cause of hospital infections. A drug combination called quinupristin-dalfopristin, also known as Synercid, is used to treat serious E. faecium infections that are resistant to the first-choice antibiotic. Synercid is related to virginiamycin, an antibiotic that has long been used as a growth promoter in U.S. livestock but is now banned in Europe. The question Dr. Belongia and colleagues asked was, does exposing poultry to virginiamycin lead to Synercid-resistant E. faecium in humans?

The group isolated E. faecium in stool samples from 105 newly hospitalized patients and 65 healthy vegetarians, and in 77 samples of conventional retail poultry and 23 antibiotic-free poultry meat samples.

Laboratory tests showed that the bacteria isolated from patients and vegetarians had no pre-existing resistance to Synercid. Resistance was rare among antibiotic-free poultry, but a majority of bacterial isolates from conventional poultry samples were resistant. After exposure to virginiamycin, E. faecium from conventional poultry and from patients who consumed poultry became resistant to Synercid more often than E. faecium from vegetarians or from antibiotic-free poultry. Some of the resistance was attributed to a specific gene, and both the gene and resistance were associated with touching raw poultry meat and frequent poultry consumption.

In an editorial commenting on the studies, Niels Frimodt-Møller, MD, DMSc, and Annette M. Hammerum, PhD, MSc, of the Danish National Center for Antimicrobials and Infection Control, observed that the findings are "examples of how industrialization of food production carries and even amplifies risk for unaware consumers." To reduce or remove the risk of food contamination as documented by the Finnish team, they noted, requires multiple measures at multiple levels, such as growing and storing carrots away from animals and encouraging hygienic practices for harvesters and harvesting machinery. To reduce or remove the risk of resistant gut bacteria, however, the editorial authors say the answer is easy: "Ban antibiotic growth promoters!"

Infectious Diseases Society of America