Lower Prevalence of Antibiotic-resistant Enterococci On U.S. Conventional Poultry Farms That Transitioned to Organic Practices.
Environ Health Perspect. 2011 Aug 10. Epub 2011 Aug 10. PMID: 21827979
University of Maryland College Park, School of Public Health.
Background: In U.S. conventional poultry production, antimicrobials are used for therapeutic, prophylactic and non-therapeutic purposes. Researchers have shown that this can select for antibiotic-resistant commensal and pathogenic bacteria on poultry farms and poultry-derived products. However, no U.S. studies have investigated on-farm changes in resistance as conventional poultry farms transition to organic practices and cease using antibiotics. Objective: To evaluate the prevalence of antibiotic-resistant Enterococcus on U.S. conventional poultry farms that transitioned to organic practices. Methods: Poultry litter, feed, and water samples were collected from 10 conventional and 10 newly organic poultry houses in 2008 and tested for Enterococcus. Enterococcus (n=259) was identified using the Vitek®Compact 2 System, and tested for susceptibility to 17 antimicrobials using the Sensititre™ microbroth dilution system. Data were analyzed using SAS v9.2 and statistical associations were derived based on generalized linear mixed models. Results: Litter, feed and water samples were Enterococcus-positive. The percentages of resistant E. faecalis and resistant E. faecium were significantly lower (p<0.05) among isolates from newly organic versus conventional houses for two (erythromycin and tylosin) and five (ciprofloxacin, gentamicin, nitrofurantoin, penicillin and tetracycline) antimicrobials, respectively. Forty-two percent of E. faecalis isolates from conventional poultry houses were multi-drug resistant (MDR) (to≥3 antimicrobial classes) compared to 10% of isolates from newly organic houses (p=0.02), and 84% of E. faecium isolates from conventional houses were MDR compared to 17% of isolates from newly organic poultry houses (p<0.001). Conclusions: Our findings suggest that the voluntary removal of antibiotics from large-scale U.S. poultry farms that transition to organic practices is associated with a lower prevalence of antibiotic-resistant and MDR Enterococcus.