Diversity of Extended-Spectrum Cephalosporin-Resistant Escherichia coli in Feces from Calves and Cows on Pennsylvania Dairy Farms.

The global incidence of human infections associated with extended-spectrum β-lactamase (ESBL)-producing Escherichia coli is increasing. Dairy animals are reservoirs of ESBL-producing E. coli, especially, third-generation cephalosporin (3GC)-resistant strains. To further understand the diversity of 3GC-resistant E. coli across animals of different age groups (e.g., pre- and postweaned calves, lactating cows, and dry cows) and farms, we used pulsed-field gel electrophoresis (PFGE) to characterize 70 fecal isolates from 14 dairy farms located in nine Pennsylvania counties. Results of this analysis indicated that 3GC-resistant E. coli were highly diverse and grouped into 27 PFGE clades (80% similarity cutoff) and 24 unique antimicrobial resistance patterns were observed among the isolates. For eight farms, clonal E. coli with the same resistance patterns were isolated from two or more age groups, indicating that strains were carried in both the calves and adult cows within the same herd. However, there were also several isolates with the same resistance pattern that were distributed to different clades, including isolates from different animal age groups on the same farm, suggesting different strains of E. coli within a farm harbored the same resistance-conferring elements. Results of this analysis indicated that 3GC-resistant E. coli were highly diverse, associated with multidrug resistance, and circulated through different (noncommingled) animal groups on individual farms.