This study evaluated the association between administering beef steers with a therapeutic dose of tulathromycin in response to bovine respiratory disease with the antimicrobial and multi-drug resistance profiles of gastrointestinal tract commensals, Escherichia coli and Enterococcus spp., and foodborne pathogens, Salmonella enterica and Campylobacter spp., isolated from fecal samples. Individual fecal samples were collected on days 0, 14 and 28 from 70 beef steers, housed in a single pen, which were treated or not with tulathromycin. Samples were subjected to bacterial culture for isolation of bacterial species followed by antimicrobial susceptibility testing of isolates to determine the minimum inhibitory concentrations using the broth microdilution method. Generalized linear mixed effects models were fitted to estimate the prevalence of the bacterial species as well as the prevalence of resistant isolates, over time and between tulathromycin-treated and non-treated cattle, and multi-drug resistant isolates, to a range of clinically relevant antimicrobials in both human and veterinary medicine. Model-adjusted mean prevalence of E. coli, Enterococcus spp., S. enterica, and Campylobacter spp. were 99.5%, 85.9%, 1.5%, and 17.7%, respectively. The prevalence of erythromycin-resistant Enterococcus spp. was significantly higher on day 14 (59.7%) compared to day 28 (22.2%). There was also a higher prevalence of erythromycin-resistant Enterococcus spp. in samples from treated (59.3%) compared to non-treated animals (27.6%). Multi-drug resistance (≥ 3 antimicrobial classes) was observed in 8.4%, and 62.7% of E. coli and Enterococcus isolates, respectively. In conclusion, the administration of tulathromycin was significantly associated with an increased prevalence of erythromycin-resistant in Enterococcus spp.
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