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

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 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 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.