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Antimicrobial susceptibility of enteric bacteria recovered from feedlot cattle administered chlortetracycline in feed

Tammy M. Platt MS1, Guy H. Loneragan BVSc, PhD2, H. Morgan Scott DVM, PhD3, Bo Norby CMV, MPVM, PhD4, Daniel U. Thomson PhD, DVM5, Michel S. Brown PhD6, Samuel E. Ives DVM, PhD7, and Mindy M. Brashears PhD8
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  • 1 Feedlot Research Group, Department of Agricultural Sciences, College of Agriculture, Science and Engineering, West Texas A&M University, Canyon, TX 79016.
  • | 2 Feedlot Research Group, Department of Agricultural Sciences, College of Agriculture, Science and Engineering, West Texas A&M University, Canyon, TX 79016.
  • | 3 Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843.
  • | 4 Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843.
  • | 5 Department of Clinical Sciences, College of Veterinary Medicine, Kansas State University, Manhattan, KS 66506.
  • | 6 Feedlot Research Group, Department of Agricultural Sciences, College of Agriculture, Science and Engineering, West Texas A&M University, Canyon, TX 79016.
  • | 7 Cactus Feeders, 2209 W 7th St, Amarillo, TX 79106.
  • | 8 International Center for Food Industry Excellence, Department of Animal and Food Sciences, College of Agricultural Sciences and Natural Resources, Texas Tech University, Lubbock, TX 79409.

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.

Animals—20 steers.

Procedures—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 log2 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 and Clinical Relevance—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.

Contributor Notes

Supported in part by The Beef Checkoff program through the Cattlemen's Beef Board and National Research Initiative of the USDA Cooperative State Research, Education and Extension Service (grant No. 2004-35212-14864).

Address correspondence to Dr. Loneragan.