Dynamic Changes in Antimicrobial Resistance in Fecal Escherichia coli from Neonatal Dairy Calves: An Individual Follow-Up Study

SIMPLE SUMMARY: Antimicrobial resistance in food animals is a global public health concern. In dairy farms, young calves typically carry high levels of antimicrobial-resistant (AMR) Escherichia coli, and may act as a potential reservoir. Fecal E. coli were isolated and tested for susceptibilities to eight common antimicrobials from 19 newborn dairy calves using individual follow-up at daily and weekly intervals. Shedding of AMR E. coli first appeared at 2–3 days after birth. The majority of fecal E. coli from neonatal calves (≤28 days old) were resistant to streptomycin, sulfisoxazole, and tetracycline. A tetracyclines-associated resistance gene (tetB) was predominant in the fecal E. coli from neonatal calves, and was also detected in maternal colostrum samples from the mothers of the tested calves. These results suggest the potential of antimicrobial resistance genes being shared between neonatal calves and their mothers’ colostrum. Neonatal calves with a history of treatment with ceftiofur also shed AMR E. coli resistant against beta-lactams. Moreover, these results provide new insights for controlling the spread of antimicrobial resistance on dairy farms. ABSTRACT: The prevalence of antimicrobial-resistant (AMR) Escherichia coli 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 E. coli from neonatal calves were characterized both at phenotypic and genotypic levels by individual follow-up sampling. Antimicrobial resistance profiles of E. coli isolates from the maternal colostrum were also determined. Most of the fecal AMR E. coli emerged in the calves at 2–3 days of age. The tetB was the most prevalent resistance gene detected among AMR fecal E. coli 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 E. coli as the calves aged. More than half of the fecal E. coli 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 E. coli from neonatal calves, and suggest that the feeding of colostrum and ceftiofur administration contribute to the higher prevalence of AMR E. coli in young dairy calves.