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Applied and Environmental Microbiology, September 2005, p. 4986-4991, Vol. 71, No. 9
0099-2240/05/$08.00+0     doi:10.1128/AEM.71.9.4986-4991.2005

Changes in Antimicrobial Susceptibility of Native Enterococcus faecium in Chickens Fed Virginiamycin

Patrick F. McDermott,^1* Patti Cullen,¹ Susannah K. Hubert,¹ Shawn D. McDermott,¹ Mary Bartholomew,² Shabbir Simjee,^1, and David D. Wagner¹

Division of Animal and Food Microbiology, Center for Veterinary Medicine, U.S. Food and Drug Administration, Laurel, Maryland,¹ Scientific Support Staff, Center for Veterinary Medicine, U.S. Food and Drug Administration, Rockville, Maryland²

Received 28 January 2005/ Accepted 23 March 2005

The extent of transfer of antimicrobial resistance from agricultural environments to humans is controversial. To assess the potential hazard posed by streptogramin use in food animals, this study evaluated the effect of virginiamycin exposure on antimicrobial resistance in Enterococcus faecium recovered from treated broilers. Four consecutive broiler feeding trials were conducted using animals raised on common litter. In the first three trials, one group of birds was fed virginiamycin continuously in feed at 20 g/ton, and a second group served as the nontreated control. In the fourth trial, antimicrobial-free feed was given to both groups. Fecal samples were cultured 1 day after chickens hatched and then at 1, 3, 5, and 7 weeks of age. Isolates from each time point were tested for susceptibility to a panel of different antimicrobials. Quinupristin/dalfopristin-resistant E. faecium appeared after 5 weeks of treatment in trial 1 and within 7 days of trials 2 to 4. Following removal of virginiamycin in trial 4, no resistant isolates were detected after 5 weeks. PCR failed to detect vat, vgb, or erm(B) in any of the streptogramin-resistant E. faecium isolates, whereas the msr(C) gene was detected in 97% of resistant isolates. In an experimental setting using broiler chickens, continuous virginiamycin exposure was required to maintain a stable streptogramin-resistant population of E. faecium in the animals. The bases of resistance could not be explained by known genetic determinants.

Present address: Eli Lilly-Elanco, Kingsclere Road, Basingstoke RG21 6XA, United Kingdom.