Changes in antibiotic susceptability of Escherichia coli isolated from steers exposed to antibiotics during the early feeding period.

The influence of therapeutic choices on antibiotic resistance of intestinal bacteria may have food safety consequences. Changes in antibiotic susceptibility of Escherichia coli to antibiotics currently approved for prevention and treatment of bovine respiratory disease were evaluated in 260 feedlot steers. Susceptibilities to antimicrobial compounds were compared among three treatment groups at three times between arrival at the feedlot and harvest to assess changes over the course of the feeding period. No significant change was found in the resistance of E. coli to tilmicosin, florfenicol, and enrofloxacin, which were used to prevent and treat respiratory disease in this study. Despite an absence of exposure to ampicillin and ceftiofur, a significant increase in resistance was observed for these two antimicrobial drugs that declined by the end of the feeding period. In this study, use of approved antimicrobials early in the feeding period for the prevention and treatment of bovine respiratory disease had little effect on antimicrobial resistance of E. coli isolated from cattle near the time of slaughter.