Antimicrobial susceptibility patterns of Escherichia coli phylogenetic groups isolated from bovine clinical mastitis

Determination of antimicrobial susceptibility (AMS) of Escherichia coli causing clinical mastitis (CM) according to the phylogenetic groups and its association with descriptors at the cow and herd level may help improve specific strategies for treatment and control of this pathogen in dairy herds. The aims of the present study were to (a) determine the frequency of phylogenetic groups of E. coli isolated from CM in dairy cows, and its association with cow-level descriptors (parity, lactation stage, CM severity, and affected quarter position), housing system, and season; and (b) determine and compare AMS among E. coli phylogenetic groups. A quadruplex PCR method was used to classify E. coli isolates into 1 of the 7 phylogenetic groups. Minimal inhibitory concentrations were determined for 10 antimicrobials, and survival analysis was performed to evaluate the AMS differences among E. coli phylogroups. Most E. coli isolates belonged to phylogroups A (52%) and B1 (38%). None of the cow- and herd-level descriptors were associated with the E. coli phylogenetic groups. Overall, E. coli isolates were mostly susceptible to ceftiofur (96.8%), sulfadimethoxine (75.5%), and cephalothin (74.5%). Based on the survival analysis, differences in AMS between phylogenetic groups of E. coli was observed only for cephalothin, in which strains of phylogroup A were inhibited at lower minimum inhibitory concentration than strains of phylogroup B1. Results of this study indicated low susceptibility of E. coli isolates identified from CM to most antimicrobials. In addition, differences in AMS can occur among E. coli phylogenetic groups, although they may be uncommon as they were limited to only one antimicrobial (i.e., cephalothin).