Antimicrobial Resistance of Salmonella and E. coli from Pennsylvania Dairy Herds

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The emergence and dissemination of bacterial antimicrobial resistance has become a major public health concern. A total of 444 manure composite samples were collected from 80 dairy farms in Pennsylvania, representing pre-weaned calves, post-weaned calves, dry cows, and lactating cows. E. coli and Salmonella were isolated, and tested for antimicrobial susceptibility. Salmonella was isolated from at least one sample from 51 (64%) farms and was more prevalent in adult animals than young animals. The predominant serotypes were Cerro, Montevideo and Kentucky. Salmonella isolates were mostly susceptible to all antimicrobials. E. coli were commonly resistant to tetracycline, streptomycin, sulfisoxazole and ampicillin. Resistance of up to 8 classes of antibiotics was observed in E. coli isolated from young animals. The blaCMY- and blaCTX-M-carrying E. coli were detected in 35% and 5% of the farms, respectively. The presence of multi-drug resistant E. coli suggested potential risks to human health associated with dairy farming.