The aim of this study was to characterize the plasmids carrying antimicrobial resistance (AMR) determinants in multiple Salmonella serotypes recovered from the commercial swine farms environment after manure application on land. Manure and soil samples were collected on day0 before and after manure application on six farms in North Carolina and sequential soil samples were recollected on day7, 14, and 21 from the same plots. All environmental samples were processed for Salmonella and their plasmid contents were further characterized. A total of fourteen isolates including Johannesburg (n=2), Ohio (n=2), Rissen (n=1), Typhimurium var5- (n=5), Worthington (n=3), and 4,12:i:- (n=1) representing different farms were selected for plasmid analysis. Antimicrobial susceptibility test was done by broth microdilution against a panel of 14 antimicrobials on the 14 confirmed transconjugants after conjugation assays. The plasmids were isolated by the modified alkaline lysis and PCRs were performed on purified plasmid DNA to identify the AMR determinants and the plasmid replicon types. The plasmids were sequenced to further analyze, compare profiles and create phylogenetic trees. A class 1 integron with ANT(2")Ia-aadA2 cassette was detected in the 50-kb IncN plasmids identified in S. Worthington. We identified 100-kb and 90-kb IncI1 plasmids in S. Johannesburg and S. Rissen carrying the blaCMY-2 and tet(A) genes, respectively. An identical 95-kb IncF plasmid was widely disseminated among the different serotypes and across different farms. Our study provides evidence on the importance of horizontal dissemination of resistance determinants through plasmids in multiple Salmonella serotypes distributed across commercial swine farms after manure application.IMPORTANCE. The horizontal gene transfer of antimicrobial resistance (AMR) determinants located on plasmids is considered to be the main reason for the rapid proliferation and spread drug resistance. The deposition of manure generated in swine production systems into the environment is identified as a potential source of AMR dissemination. In this study, AMR gene carrying plasmids were detected in multiple Salmonella serotypes across different commercial swine farms in North Carolina. The plasmid profiles were characterized based on Salmonella serotype donors and incompatibility (Inc) groups. We found different Inc plasmids showed the evidence of AMR genes transfer in multiple Salmonella serotypes. We detected an identical 95-kb plasmid that was widely distributed across swine farms in NC. These conjugable resistance plasmids were able to persist on land after swine manure application. Our study provides strong evidence of AMR determinants dissemination present in plasmids of multiple Salmonella serotypes in the environment after manure application.
Source: AASV