Characterization of bla CMY Plasmids and Their Possible Role in Source Attribution of Salmonella enterica Serotype Typhimurium Infections

Abstract

Salmonella is an important cause of foodborne illness; however, identifying the source of these infections can be difficult. This is especially true for Salmonella serotype Typhimurium, which is found in diverse agricultural niches. Extended-spectrum cephalosporins (ESC) are one of the primary treatment choices for complicated Salmonella infections. In Salmonella, ESC resistance in the United States is mainly mediated by bla _CMY genes carried on various plasmids. In this study, we examined whether the characterization of bla _CMY plasmids, along with additional information, can help us identify potential sources of infection by Salmonella, and used serotype Typhimurium as a model. In the United States, monitoring of retail meat, food animals, and ill persons for antimicrobial-resistant Salmonella is conducted by the National Antimicrobial Resistance Monitoring System. In 2008, 70 isolates (70/581; 12.0%) (34 isolates from retail meat, 23 food animal, and 13 human) were resistant to ceftriaxone and amoxicillin/clavulanic acid. All were polymerase chain reaction (PCR)–positive for bla _CMY and 59/70 (84.3%) of these genes were plasmid encoded. PCR-based replicon typing identified 42/59 (71.2%) IncI1-bla _CMY plasmids and 17/59 (28.8%) IncA/C-bla _CMY plasmids. Isolates from chickens or chicken products with bla _CMY plasmids primarily had IncI1-bla _CMY plasmids (37/40; 92.5%), while all isolates from cattle had IncA/C-bla _CMY plasmids. Isolates from humans had either IncA/C- bla _CMY (n=8/12; [66.7%]) or IncI1- bla _CMY (n=4/12 [33.3%]) plasmids. All of the IncI1-bla _CMY plasmids were ST12 or were closely related to ST12. Antimicrobial susceptibility patterns (AST) and pulsed-field gel electrophoresis (PFGE) patterns of the isolates were also compared and differences were identified between isolate sources. When the source of a Typhimurium outbreak or sporadic illness is unknown, characterizing the outbreak isolate's bla _CMY plasmids, AST, and PFGE patterns may help identify it.