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Title: Analysis of antimicrobial resistance genes detected in multidrug-resistant salmonella enterica serovar typhimurium isolated from food animals

Author
item Glenn, Lashanda
item Lindsey, Rebecca
item FRANK, JOSEPH - University Of Georgia
item Meinersmann, Richard - Rick
item Englen, Mark
item Cray, Paula
item Frye, Jonathan

Submitted to: Microbial Drug Resistance
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/17/2011
Publication Date: 9/1/2011
Citation: Glenn, L.M., Lindsey, R.L., Frank, J.F., Meinersmann, R.J., Englen, M.D., Cray, P.J., Frye, J.G. 2011. Analysis of antimicrobial resistance genes detected in multidrug-resistant salmonella enterica serovar typhimurium isolated from food animals. Microbial Drug Resistance. 17(3):407-418.

Interpretive Summary: Antimicrobial resistance (AR) in foodborne bacteria is a concern for both animal and human health when treatment is warranted. Particularly concerning is when bacteria develops resistance to multiple antimicrobials which can further limit therapy. This is especially true when multi drug resistance (MDR) occurs in foodborne pathogens such as Salmonella enterica. MDR has been found in Salmonella enterica serovar Typhimurium isolated from animals as part of the National Antimicrobial Resistance Monitoring System (NARMS). Some of these bacteria are resistant to five or more antimicrobials including: ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (aka ACSSuT resistance phenotype). To investigate the genes that may be responsible for this MDR, animal isolates resistant to least ACSSuT were selected between the years 1997 to 2007. One isolate from healthy cattle, poultry, and swine were selected for a total of 33 isolates. These isolates were analyzed by a microarray test which can detect specific genes in a bacterial isolate responsible for AR as well as plasmids, which are DNA elements that can move AR genes between bacteria. The distribution of the genes detected separated most of the isolates into two groups, A (15 isolates) and B (16 isolates). Those in group A were often a specific type called definitive phage type DT104, and were isolated mostly from swine. Isolates in group B had many Inc A/C plasmid genes detected, and were isolated mostly from cattle. Further analysis demonstrated significant linkage for the associations between plasmids, phage type, and animal source for these groups.

Technical Abstract: The development of multi drug resistance (MDR) in foodborne pathogens such as Salmonella enterica is a concern for both animal and human health. MDR Salmonella enterica serovar Typhimurium is the most prevalent penta-resistant serovar isolated from animals as part of the National Antimicrobial Resistance Monitoring System (NARMS). Penta-resistant isolates are most often resistant to ampicillin, chloramphenicol, streptomycin, sulfamethoxazole, and tetracycline (aka ACSSuT) and most often occur among serovar Typhimurium. To investigate MDR in S. Typhimurium (including variant 5-), one isolate each from cattle, poultry, and swine with at least the ACSSuT phenotype were selected for each year from 1997-2007 (n=33). Total DNA was analyzed with a microarray designed to detect antimicrobial resistance, Inc A/C, and HI1 plasmid genes. Hybridizations detected resistance and plasmid genes in all isolates. Cluster analysis based on this data separated the isolates into two major groups A and B (15 and 16 isolates, respectively). Isolates in group A were further characterized as definitive phage type DT104 or U302 and they were isolated most frequently from swine (7/15). The genes detected included intI1, blaPSE-1, floR, aadA, sulI, tet(G), and tetR, which were similar to those usually present in the class I integron found in Salmonella Genomic Island I (SGI1). Isolates in group B had many Inc A/C plasmid genes detected and they were isolated most often from cattle (9/16). The genes detected included blaCMY-2, floR, aac(3), aadA, aphA1, strA, strB, sulI, sulII, dfrA, dhf, tet(A)(B)(C)(D) and tetR, which are associated with Inc A/C plasmids encoding the MDR-AmpC phenotype. The remaining two isolates, C and D, did not cluster with any other isolates. Some of the genes detected in these isolates were different from those in groups A and B including 100 Inc HI1 plasmid genes in isolate C and 202 Inc HI1 genes in isolate D. PCR analyses confirmed the microarray findings, and Linkage Disequilibrium detected significant associations between plasmid Inc type, phage type, and animal source. These data suggest that MDR in S. Typhimurium (including var. 5-) is associated with DT104/SGI1 or Inc A/C MDR-AmpC encoding plasmids. Furthermore, these genetic elements have a high degree of similarity throughout the first 11 years of NARMS animal surveillance suggesting a high degree of stability among the AR genes found in this serotype.