Genetic characterization of antimicrobial resistance in Salmonella enterica serovars isolated from dairy cattle in Wisconsin

Salmonella enterica is a pathogen of humans and animals, and is one of the most frequent causes of bacterial foodborne illness worldwide. People consuming contaminated foods or working with infected livestock have the potential to become infected with Salmonella and may require antimicrobial therapy. Antimicrobial resistance in Salmonella has become a problem worldwide due in part to the inappropriate use of antimicrobial agents in human and veterinary medicine. In this study, forty-five Salmonella isolates from diagnostic fecal samples of Wisconsin dairy cattle were serotyped and characterized by antimicrobial susceptibility testing using agar disk diffusion, antimicrobial resistance gene detection by PCR, plasmid analysis and conjugation studies. The predominant serovars detected were Kentucky, Newport, Typhimurium, Cerro, Dublin and Montevideo. Over half (51%) of all isolates were resistant to at least one antimicrobial agent, and 29% were resistant to 8–10 drugs. The most commonly observed resistance phenotypes were to streptomycin (44%), tetracycline (42%), sulfisoxazole (40%), chloramphenicol (35%), ampicillin (33%), and cefoxitin (33%). When resistance was detected phenotypically, a corresponding resistance gene was detected 86.2% of the time. Plasmids ranging in size from < 8 to 165 kb were detected in 45% of the isolates. A greater understanding of the factors associated with antimicrobial resistance in Salmonella should provide insights into the factors that contribute to the development of resistant pathogens during dairy production, which in turn can lead to strategies to minimize the spread of antimicrobial resistant Salmonella in the food supply.