Characterization of multidrug-resistant Salmonella enterica serovar Heidelberg isolated from humans and animals.

Salmonella enterica serovar Heidelberg has been recognized as one of the most common serovar associated with foodborne infections in the United States. It is also frequently isolated from nonhuman sources and has increasingly shown resistance to various antimicrobial agents. The present study was undertaken to identify the predominant antimicrobial resistance phenotypes and genotypes of Salmonella Heidelberg (n = 95) isolates of human, swine, and turkey origin. Antimicrobial susceptibility was done using Kirby-Bauer disk diffusion method with a panel of 12 antimicrobials. Pulsed-field gel electrophoresis genotyping was used to determine the diversity of the isolates. The antimicrobial resistance genes and carriage of Class 1 and 2 integrons were determined by polymerase chain reaction. All Salmonella Heidelberg isolates from swine were resistant to one or more of the antimicrobials tested and the majority (73.3%) showed multidrug resistance to streptomycin, tetracycline, and kanamycin (R-type: StTeKm). About 80% of the Salmonella Heidelberg isolates of human origin were pan-susceptible, however, one isolate showed multidrug resistance to 10 of 12 antimicrobials tested. Among the multidrug-resistant (MDR) Salmonella Heidelberg isolates, Class 1 integrons with variable sizes of 1.2 to 1.5 kb were detected in six isolates (three each) from humans and swine. DNA sequencing revealed that Class 1 integrons of both human and swine origin carried a gene encoding aminoglycoside adenyltransferase (aadA). Resistance genes identified in other loci include aphA1-Iab, strA, bla(TEM), and tetA (B). Both human and swine MDR strains of Salmonella Heidelberg carried the resistance phenotypes on self-transferable plasmids. Dendrogram analysis of pulsotypes indicated possible clonality of Salmonella Heidelberg between isolates of human and swine origin. The findings in this study indicate the increasing significance of swine as reservoirs of emerging MDR serovars, such as MDR Salmonella Heidelberg, is of public health significance.