title: Lower prevalence of antibiotic-resistant Enterococci on U.S. conventional poultry farms that transitioned to organic practices.
abstract: BACKGROUND
  In U.S. conventional poultry production, antimicrobials are used for therapeutic, prophylactic, and nontherapeutic purposes. Researchers have shown that this can select for antibiotic-resistant commensal and pathogenic bacteria on poultry farms and in 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
  We investigated 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® 2 Compact System and tested for susceptibility to 17 antimicrobials using the Sensititre™ microbroth dilution system. Data were analyzed using SAS software (version 9.2), and statistical associations were derived based on generalized linear mixed models.
  
  
  RESULTS
  Litter, feed, and water samples were Enterococcus positive. The percentages of resistant Enterococcus faecalis and resistant Enterococcus faecium were significantly lower (p < 0.05) among isolates from newly organic versus conventional poultry 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 multidrug resistant (MDR; resistant to three or more antimicrobial classes), compared with 10% of isolates from newly organic poultry houses (p = 0.02); 84% of E. faecium isolates from conventional poultry houses were MDR, compared with 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.
authors.affiliation: Maryland Institute for Applied Environmental Health, University of Maryland, School of Public Health, College Park, Maryland, USA. ars@umd.edu
authors.forename: Amy R
authors.initials: AR
authors.lastname: Sapkota
authors.forename: R Michael
authors.initials: RM
authors.lastname: Hulet
authors.forename: Guangyu
authors.initials: G
authors.lastname: Zhang
authors.forename: Patrick
authors.initials: P
authors.lastname: McDermott
authors.forename: Erinna L
authors.initials: EL
authors.lastname: Kinney
authors.forename: Kellogg J
authors.initials: KJ
authors.lastname: Schwab
authors.forename: Sam W
authors.initials: SW
authors.lastname: Joseph
mesh_terms: D000822:Animal Husbandry; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D016983:Enterococcus / Q000145:classification / Q000187:drug effects / Q000302:isolation & purification*; D013293:Enterococcus faecalis / Q000145:classification / Q000187:drug effects / Q000302:isolation & purification; D016984:Enterococcus faecium / Q000145:classification / Q000187:drug effects / Q000302:isolation & purification; D016908:Gram-Positive Bacterial Infections / Q000453:epidemiology / Q000382:microbiology / Q000662:veterinary*; D008826:Microbial Sensitivity Tests; D008873:Mid-Atlantic Region / Q000453:epidemiology; D011200:Poultry / Q000254:growth & development / Q000382:microbiology*; D011201:Poultry Diseases / Q000453:epidemiology* / Q000382:microbiology; D015995:Prevalence; D011446:Prospective Studies
chemical_list: D000900:Anti-Bacterial Agents