Submitted to: Microbial Drug Resistance
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/20/2012
Publication Date: 6/1/2013
Citation: Glenn, L.M., Lindsey, R.L., Folster, J.P., Pecic, G., Boerlin, P., Gilmour, M.W., Harbottle, H., Zhoa, S., Mcdermott, P., Cray, P.J., Frye, J.G. 2013. Antimicrobial resistance genes in multidrug-resistant Salmonella enterica isolated from animals, retail meats, and humans in the United States and Canada. Microbial Drug Resistance. 19(3):175-184. Interpretive Summary: Salmonella enterica is a cause of food-borne disease in humans. Although treatment is not often required following infection, if Salmonella is resistant to multiple antimicrobials (multi-drug resistant, MDR) the infection may be difficult to treat. Determining the genetics of MDR in Salmonella isolated from animals, food, and humans can help us understand the risk infection with food-borne MDR Salmonella may pose to human health. To study this, MDR S. enterica strains (also called serovars) most frequently found in animals, retail meat, and human infections in the U.S. and Canada were studied (n=56). These isolates were analyzed by microarray, a genetic test which allows us to study hundreds of MDR and plasmid genes. Plasmids are tiny pieces of DNA which can move between bacteria and often harbor MDR genes. Our results detected genes encoding resistance to multiple classes of antimicrobials including common ones such as streptomycin, penicillins, chloramphenicol, sulfamethoxazole, tetracycline, and trimethoprim. Similar resistance genes were detected regardless of serovar, source, or location. Plasmid genes from the IncA/C plasmid were detected in 27/56 isolates; however, the plasmids differed in several highly variable regions of their structure. Other plasmid families were detected in all but one isolate, and included IncI1 (25/56), IncN (23/56) and IncFIB (10/56). The presence of different plasmid mobile elements along with similar resistance genes suggest that these genetic elements may have collected similar resistance gene structures which may serve as multiple origins of MDR in Salmonella from food animals, retail meat, and human infections.
Technical Abstract: Salmonella enterica is a prevalent foodborne pathogen which can carry multi-drug resistance (MDR) and pose a threat to human health. Identifying the genetic elements associated with MDR in Salmonella isolated from animals, foods, and humans can help determine the sources of MDR in food animals and their impact on human health. S. enterica serovars carrying MDR and most frequently isolated from healthy animals, retail meat, and human infections in the U.S. and Canada were identified and isolates resistant to the largest number of antimicrobial compounds were chosen. Isolates were from U.S. slaughter (n=12), retail (n=9), and humans (9), and Canadian slaughter (n=9), retail (n=9), and humans (n=8; total n=56). 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 (blaTEM, blaCMY, blaPSE-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.