| Summary: | The prevalence of antimicrobial-resistant (AMR) <i>Escherichia coli</i> is typically higher in the feces of young dairy calves than in the feces of older cattle; however, the underlying factors contributing to this difference are poorly understood. In this study, AMR fecal <i>E. coli</i> from neonatal calves were characterized both at phenotypic and genotypic levels by individual follow-up sampling. Antimicrobial resistance profiles of <i>E. coli</i> isolates from the maternal colostrum were also determined. Most of the fecal AMR <i>E. coli</i> emerged in the calves at 2–3 days of age. The <i>tetB</i> was the most prevalent resistance gene detected among AMR fecal <i>E. coli</i> from <7-day-old calves, and was also detected in two isolates from the maternal colostrum. Weekly sampling revealed changes in the phenotype of AMR fecal <i>E. coli</i> as the calves aged. More than half of the fecal <i>E. coli</i> isolates acquired additional resistance to beta-lactams by 21–28 days of age, and minimum inhibitory concentrations were higher in ceftiofur-exposed calves than in unexposed calves. Our findings reveal the dynamic changes in AMR fecal <i>E. coli</i> from neonatal calves, and suggest that the feeding of colostrum and ceftiofur administration contribute to the higher prevalence of AMR <i>E. coli</i> in young dairy calves.
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