title: Prevalence and pattern of antimicrobial susceptibility in Escherichia coli isolated from pigs reared under antimicrobial-free and conventional production methods.
abstract: OBJECTIVE
  To determine and compare levels and patterns of antimicrobial resistance among Escherichia coli isolated from pigs on farms that did not use antimicrobial agents versus pigs produced under conventional methods.
  
  
  METHODS
  Cross-sectional study. Sample Population-35 antimicrobial-free and 60 conventional swine farms.
  
  
  METHODS
  Farms were visited once, and fecal samples were collected from 15 finisher pigs if available. One E coli isolate from each sample was tested for susceptibility pattern to 14 antimicrobial agents by use of microbroth dilution.
  
  
  RESULTS
  E coli isolates were recovered from 1,381 (97.1%) of 1,422 fecal samples. Herd size was significantly larger for conventional swine farms. Resistance to ceftriaxone, ciprofloxacin, or nalidixic acid was not observed on any of the 95 farms. Three isolates from 2 conventional farms were resistant to ceftiofur. Conventional farms had significantly higher levels of resistance to ampicillin, sulfamethoxazole, tetracycline, and chloramphenicol, compared with antimicrobial-free farms. Fourteen percent of E coli isolates were susceptible or had intermediate resistance to all the tested antimicrobial agents. The 3 most frequent patterns of multiple resistance were streptomycin-tetracycline, sulfamethoxazole-tetracycline, and kanamycin-streptomycin-sulfamethoxazole-tetracycline.
  
  
  CONCLUSIONS
  Cessation of antimicrobial use did not appear to result in an immediate reduction in antimicrobial resistance in swine farms. Prospective studies of long-term antimicrobial usage and cessation are needed to estimate the extent to which food animal production may be contributing to antimicrobial drug resistance and might provide a direct measure of the rates of reversibility of antimicrobial drug resistance that might be achieved by curtailing antimicrobial usage.
authors.affiliation: Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824-1324, USA.
authors.forename: Christine A
authors.initials: CA
authors.lastname: Bunner
authors.forename: Bo
authors.initials: B
authors.lastname: Norby
authors.forename: Paul C
authors.initials: PC
authors.lastname: Bartlett
authors.forename: Ronald J
authors.initials: RJ
authors.lastname: Erskine
authors.forename: Frances P
authors.initials: FP
authors.lastname: Downes
authors.forename: John B
authors.initials: JB
authors.lastname: Kaneene
mesh_terms: D000822:Animal Husbandry / Q000379:methods*; D000818:Animals; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D015169:Colony Count, Microbial / Q000662:veterinary; D003430:Cross-Sectional Studies; D024881:Drug Resistance, Bacterial; D004926:Escherichia coli / Q000187:drug effects*; D004927:Escherichia coli Infections / Q000188:drug therapy / Q000662:veterinary*; D005243:Feces / Q000382:microbiology; D008826:Microbial Sensitivity Tests / Q000662:veterinary; D013552:Swine; D013553:Swine Diseases / Q000188:drug therapy*
chemical_list: D000900:Anti-Bacterial Agents