Antimicrobial Resistance Genes in Multidrug-Resistant Salmonella enterica

Salmonella enterica is a prevalent food-borne pathogen which can carry multi-drug resistance (MDR) and could pose a threat to human health. Identifying the genetic elements associated with MDR in Salmonella isolated from animals, food, and humans can help determine the sources of MDR in food animals and their impact on human health. Representatives of MDR S. enterica serovars most frequently isolated from healthy animals, retail meat, and human infections in the U.S. and Canada were subjected to detailed genetic analysis (n=56). These included U.S. slaughter (n=12), retail (n=9), and human (9) isolates, and Canadian slaughter (n=9), retail (n=9), and human (n=8) isolates. These isolates were assayed by microarray for antimicrobial resistance and MDR plasmid genes. Genes detected encoded resistance to aminoglycosides (alleles of aac, aad, aph, strA/B); beta-lactams (bla(TEM), bla(CMY), bla(PSE-1)); chloramphenicol (cat, flo, cmlA); sulfamethoxazole (sulI); tetracycline (tet(A, B, C, D) and tetR); and trimethoprim (dfrA). Similar resistance genes were detected regardless of serovar, source, or location. Hybridization with IncA/C plasmid gene probes indicated that 27/56 isolates carried a member of this plasmid family; however these plasmids differed in several highly variable regions. Cluster analysis based on genes detected separated most of the isolates into two groups, one with IncA/C plasmids and one without IncA/C plasmids. Other plasmid replicons were detected in all but one isolate, and included I1 (25/56), N (23/56) and FIB (10/56). The presence of different mobile elements along with similar resistance genes suggest that these genetic elements may acquire similar resistance cassettes, and serve as multiple sources for MDR in Salmonella from food animals, retail meat, and human infections