Age and Diet Effects on Fecal Populations and Antibiotic Resistance of a Multi-drug Resistant Escherichia coli in Dairy Calves†

Dairy calves are colonized at a very young age by a multi-drug resistant Escherichia coli (MDR EC) and research studies indicate that the prevalence is not related to recent use of antimicrobials, but that diet and other environmental factors are likely involved. To further investigate the occurrence of this bacterium, we sampled dairy calves on southwestern United States farms at one week of age through 6 months, and determined not only prevalence, but fecal concentrations of the MDR EC. The influence of feeding pasteurized (PWM) versus non-pasteurized (NPWM) waste milk was examined, and the effect of weaning was investigated. The number of fecal samples positive for MDR EC as well as their populations decreased (P < 0.01) with increasing calf age. Slight differences were observed when comparing PWM and NPWM feeding, with MDR EC concentration and prevalence in the latter group generally decreasing at younger ages. No significant differences were observed in the fecal concentrations of MDR EC due to weaning. No clear differences were observed in resistance when comparing calves fed PWM or NPWM. Approximately 41% of the MDR EC isolates collected throughout the study were resistant to 10 or more antibiotics, with two primary phenotypes: ACSSuT and MDR-AmpC. Based on the results herein, it appears that neither pasteurization of the waste milk or weaning, has a significant effect on the prevalence or concentration of MDR EC, and based on the age-associated decline in prevalence, they survive in an immature digestive system with limited bacterial diversity and competition for resources.