Abstract
The objective of this study was to evaluate the association between antimicrobial resistance (AMR) in 106 fecal generic Escherichia coli isolates from calves in cow-calf herds, based on either phenotype or the presence of resistance genes, and the occurrence of virulence factors stx1, stx2, and eae. Three virulence genes and 23 AMR genes for six antimicrobial families were examined using DNA hybridization and PCR. Antimicrobial susceptibility of the isolates was tested using microbroth dilution (Sensititre®, TREK Diagnostic Systems, Cleveland, OH) and the 2002 National Antimicrobial Monitoring System (NARMS) panel. The 106 isolates examined in this study were a stratified random subset from a larger study of AMR in cow-calf herds; 88.7% of the selected isolates were resistant to at least one antimicrobial, and 89.6% of the selected isolates were positive for at least one resistance gene. At least one virulence factor was identified in 48.1% (95%CI, 37.7–58.7) of the isolates. The most common virulence gene detected was stx2 followed by eae. Neither AMR measured phenotypically nor the presence of AMR genes were associated with the presence of above Shiga-toxigenic E. coli virulence factors in this population of healthy beef calves.