Antimicrobial drug resistance of fecal Escherichia coli and Salmonella spp. isolates from United States dairy cows.

Antimicrobial resistance is a growing concern for public and animal health. Threats to public health could come from the transfer of pathogens from animals to people via indirect contact such as through food or by direct contact with animals. In addition, concern has been raised for the potential transfer of resistance determinants from animals to humans through commensal bacterial flora such as Escherichia coli. Isolates of E. coli and Salmonella spp. from dairy cows on farms in 21 states were evaluated for resistance to a panel of 16 antimicrobial drugs. Resistance patterns for E. coli were compared to those of Salmonella spp. when they were isolated concurrently on the same farm or from the same fecal sample. Overall, most of the E. coli isolates (85.3%) and Salmonella spp. isolates (87.2%) were susceptible to all antimicrobials in the panel. The resistance profiles for E. coli with and without concurrent isolation of Salmonella were comparable with the exception of tetracycline resistance, which was more common among the E. coli isolated with Salmonella spp. The resistance patterns for E. coli and Salmonella spp. isolated concurrently were not significantly different for any of the antimicrobials evaluated. The data from this study demonstrate that the majority of commensal E. coli and Salmonella spp. recovered from feces of dairy cows harbored no resistance to a broad range of antimicrobial drugs. Further studies are indicated to better understand the factors that influence the frequency of resistance in commensal E. coli and Salmonella spp. on dairy operations.