title: Changes in antimicrobial susceptibility in a population of Escherichia coli isolated from feedlot cattle administered ceftiofur crystalline-free acid.
abstract: OBJECTIVE
  To determine effects of administration of ceftiofur crystalline-free acid (CCFA) on antimicrobial susceptibility of Escherichia coli in feedlot cattle.
  
  
  METHODS
  61 feedlot steers.
  
  
  METHODS
  A cohort study was conducted. Steers were housed in pens (5 pens with 10 steers and 1 pen with 11 steers). Five steers in each pen were administered CCFA, and 5 served as control steers (1 pen had 6 control steers). The CCFA administration included a single-dose regimen (6.6 mg/kg, SC, on day 0), two-thirds-dose regimen (4.4 mg/kg, SC, on day 0), and 3-dose regimen (6.6 mg/kg, SC, on days 0, 6, and 13). Fecal samples were collected on days 0, 2, 6, 9, 13, 16, 20, and 28. Fecal samples were collected immediately before CCFA administration. Minimum inhibitory concentrations of 15 antimicrobials were determined for 3 E coli isolates/fecal sample. Escherichia coli were enumerated by use of direct-plating techniques.
  
  
  RESULTS
  Resistance to 1 or more antimicrobials was detected in 986 of 1,441 (68.4%) isolates recovered. Administration of CCFA was associated with a transient increase in the population of ceftiofur-resistant isolates. Susceptibility returned to day 0 values (ie, samples collected immediately before CCFA administration) approximately 2 weeks after completion of CCFA administration. Agreement between ceftiofur resistance and co-resistance to ampicillin, chloramphenicol, streptomycin, sulfisoxazole, and tetracycline was almost perfect (kappa 0.97). We did not detect variation in susceptibility of E coli recovered from commingled control steers.
  
  
  CONCLUSIONS
  Administration of CCFA provided selection pressure that favored transient expansion of multiple-resistant variants.
authors.affiliation: Feedlot Research Group, Department of Agricultural Sciences, College of Agriculture, Science and Engineering, West Texas A&M University, Canyon, TX 79016, USA.
authors.forename: T Courtney
authors.initials: TC
authors.lastname: Lowrance
authors.forename: Guy H
authors.initials: GH
authors.lastname: Loneragan
authors.forename: David J
authors.initials: DJ
authors.lastname: Kunze
authors.forename: Tammy M
authors.initials: TM
authors.lastname: Platt
authors.forename: Samuel E
authors.initials: SE
authors.lastname: Ives
authors.forename: H Morgan
authors.initials: HM
authors.lastname: Scott
authors.forename: Bo
authors.initials: B
authors.lastname: Norby
authors.forename: Alejandro
authors.initials: A
authors.lastname: Echeverry
authors.forename: Mindy M
authors.initials: MM
authors.lastname: Brashears
mesh_terms: D000821:Animal Feed / Q000032:analysis; D000818:Animals; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000494:pharmacology; D002417:Cattle / Q000382:microbiology*; D002511:Cephalosporins / Q000008:administration & dosage* / Q000494:pharmacology*; D004032:Diet / Q000662:veterinary; D004305:Dose-Response Relationship, Drug; D024901:Drug Resistance, Multiple, Bacterial / Q000187:drug effects*; D004926:Escherichia coli / Q000187:drug effects*; D008297:Male; D008826:Microbial Sensitivity Tests; D013997:Time Factors
chemical_list: D000900:Anti-Bacterial Agents; D002511:Cephalosporins; C053503:ceftiofur