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Changes in microbial populations of Escherichia coli isolated from feedlot cattle administered chlortetracycline in the feed


1Department of Veterinary Integrative Biosciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, Texas 77843; 2Food and Feed Safety Research Unit, U.S. Department of Agriculture, Agricultural Research Service, Southern Plains Agricultural Research Center, College Station, Texas 77845; 3Feedlot Research Group, West Texas A&M University, Canyon, Texas 79016

Objective – To determine the effects of administration of chlortetracycline on the microbial population of Escherichia coli isolated from feedlot cattle as measured by biological fitness.

Animals – Twenty feedlot steers.

Procedure – Tetracycline-resistant E. coli were isolated from fecal samples of exposed and unexposed steers on days 0, 8, 14, 26 and 33. 210 E. coli isolates were tested for phenotypic ceftiofur resistance. They were also cultured in two types of MacConkey broth to measure growth over a 24-hour period.

Results – For 210 E. coli isolates (from exposed and unexposed steers on days 0 and 14), growth curves were plotted for culture in MacConkey broth and MacConkey broth with tetracycline. Growth curves for some isolates from Day 14 exposed steer samples cultured in MacConkey broth with tetracycline shift to the right. In addition, 28 of 210 isolates displayed phenotypic ceftiofur resistance.

Conclusions – A growth curve shift to the right indicates an increased lag phase for some isolates as a result of the CTC treatment. Further analysis is needed to determine the cause of this shift. More research is needed to confirm a decrease in phenotypic ceftiofur resistance and  PCR detection of blaCMY-2 for each isolate is also necessary to determine an appropriate association between decreased ceftiofur resistance and increased fitness costs under selection pressure.