title: Antimicrobial susceptibility of enteric bacteria recovered from feedlot cattle administered chlortetracycline in feed.
abstract: OBJECTIVE
  To evaluate administration of chlortetracycline in feed of cattle as a method to select for tetracycline resistance among enteric bacteria in feedlot settings.
  
  
  METHODS
  20 steers.
  
  
  METHODS
  Steers were randomly assigned to an exposed cohort (n = 10) or an unexposed cohort (control cohort; 10). Chlortetracycline (22 mg/kg) in cottonseed meal was administered to the exposed cohort on days 0 through 4, 6 through 10, and 12 through 16. The control cohort was administered only cottonseed meal. Fecal samples were collected from 16 steers on days -7, 0, 2, 6, 8, 12, 14, 19, 22, 26, and 33, and Escherichia coli and Enterococcus spp were isolated. Minimum inhibitory concentration (MIC) of selected antimicrobials was estimated.
  
  
  RESULTS
  Overall, 56.0% and 31.4% of E coli and Enterococcus isolates, respectively, were resistant to tetracycline. Exposure to chlortetracycline was associated with a significant temporary increase in log(2) MIC for both genera but returned to preexposure values by day 33. Averaged across time, the proportion of tetracycline-resistant E coli and Enterococcus isolates was significantly greater in exposed than in unexposed steers. Although all ceftiofur-resistant E coli isolates were coresistant to tetracycline, exposure to chlortetracycline led to a significant decrease in the proportion of E coli resistant to ceftiofur during exposure.
  
  
  CONCLUSIONS
  Exposure to chlortetracycline was associated with a temporary increase in the likelihood of recovering resistant bacteria. Exposure to chlortetracycline decreased the likelihood of recovering ceftiofur-resistant E coli isolates, even though isolates were coresistant to tetracycline. These findings warrant further investigation.
authors.affiliation: Department of Agricultural Sciences, College of Agriculture, Science and Engineering, West Texas A&M University, Canyon, TX 79016, USA.
authors.forename: Tammy M
authors.initials: TM
authors.lastname: Platt
authors.forename: Guy H
authors.initials: GH
authors.lastname: Loneragan
authors.forename: H Morgan
authors.initials: HM
authors.lastname: Scott
authors.forename: Bo
authors.initials: B
authors.lastname: Norby
authors.forename: Daniel U
authors.initials: DU
authors.lastname: Thomson
authors.forename: Michael S
authors.initials: MS
authors.lastname: Brown
authors.forename: Michel S
authors.initials: MS
authors.lastname: Brown
authors.forename: Samuel E
authors.initials: SE
authors.lastname: Ives
authors.forename: Mindy M
authors.initials: MM
authors.lastname: Brashears
mesh_terms: D000821:Animal Feed; D000818:Animals; D002417:Cattle; D002418:Cattle Diseases / Q000382:microbiology / Q000517:prevention & control*; D002751:Chlortetracycline / Q000008:administration & dosage / Q000494:pharmacology*; D024881:Drug Resistance, Bacterial; D004755:Enterobacteriaceae / Q000187:drug effects*; D004756:Enterobacteriaceae Infections / Q000517:prevention & control / Q000635:transmission / Q000662:veterinary*; D004781:Environmental Exposure; D004926:Escherichia coli / Q000187:drug effects; D004927:Escherichia coli Infections / Q000188:drug therapy / Q000517:prevention & control / Q000662:veterinary; D008297:Male; D008826:Microbial Sensitivity Tests / Q000662:veterinary*
chemical_list: D002751:Chlortetracycline