Comparative phenotypic and genotypic characterization of temporally related nontyphoidal Salmonella isolated from human clinical cases, pigs, and the environment in North Carolina.
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Nontyphoidal Salmonella infections caused by antimicrobial-resistant (AMR) strains are of great public health concern. We compared the phenotypic and genotypic relationships among temporally and spatially related AMR Salmonella isolates (n=1058) representing several predominant serovars, including Salmonella Typhimurium, Salmonella Typhimurium var. 5-, Salmonella Derby, Salmonella Heidelberg, Salmonella Muenchen, Salmonella Schwarzengrund, and Salmonella Rissen of human clinical cases (n=572), pig (n=212), and farm environment (n=274) origin in North Carolina. Antimicrobial susceptibility testing was performed using the broth microdilution method, and genotypic resistance determinants, including class I and II integrons, were identified. Overall, Salmonella isolates exhibited the highest frequency of resistance to tetracycline (50%), followed by sulfisoxazole (36%) and streptomycin (27%). We identified 16 different antimicrobial resistance genes, including extended spectrum and AmpC -lactamases-producing genes (bla(TEM), bla(PSE), and bla(CMY-2)), in all the -lactam- and cephalosporin-resistant Salmonella isolates from humans, pigs, and the environment. Class I integrons of 1-kb and 1.2-kb size were identified from all the three sources (humans, 66%; pigs, 85%; environment, 58%), while Class II integrons of 2-kb size were identified only in pig (10%) and environmental (19%) isolates. We detected genotypic similarity between Salmonella Typhimurium isolated from humans, pigs, and the environment while serovars Derby, Heidelberg, and Muenchen exhibited genotypic diversity. Detection of AMR Salmonella isolates from humans, pigs, and the environment is a concern for clinicians and veterinarians to mitigate the dissemination of AMR Salmonella strains.