Over the last decade, increasing attention has been focused on plasmids that harbour the antimicrobial resistance gene bla CMY-2, which encodes an AmpC-type beta-lactamase that hydrolyzes third-generation cephalosporins [11–13]. In Salmonella enterica, bla CMY-2 is frequently carried by IncA/C or IncI1 plasmids [11, 12, 14, 15]. In a previous study, we examined the genetic variation of a Salmonella enterica serovar Typhimurium population isolated from human and food-animal sources from four geographic regions

in Mexico [16]. Multilocus sequence typing (MLST) and Xba I macro-restriction showed two predominant genotypes, ST19 and ST213. ST19 has been THZ1 chemical structure reported worldwide and is the most abundant Typhimurium genotype in the MLST database [17], while ST213 has only been reported in Mexico. Clonal complex analysis supported ST19 as the founder genotype, while ST213 was determined to be a derived genotype replacing ST19. We found a non-random distribution of virulence and antimicrobial resistance accessory genes across chromosomal backgrounds, and several associations among core and accessory genetic markers were detected. First, the Salmonella virulence plasmid (pSTV) was found in ST19 strains, but not in ST213 strains. Second, the plasmid-borne bla CMY-2 gene was found

only MGCD0103 nmr in ST213 strains. Third, the most abundant integron, the integron profile one (IP-1; dfrA12, orfF and aadA2), was found only in ST213 strains. Fourth, the Salmonella learn more genomic island (SGI1) was found in a subgroup of ST19 strains carrying pSTV [16]. The general picture obtained from that study was a population composed of two main genotypes marked by the presence of different accessory genes. The emergence of the multi-drug resistant (MDR) ST213 genotype associated with resistance to expanded spectrum cephalosporins is Branched chain aminotransferase a public health threat in

Mexico where this clone has rapidly disseminated throughout certain regions, causing severe and fatal infections in infants [18]. The objective of the current study was to examine the association between the recently emerged genotype MDR ST213 and bla CMY-2 plasmids. ST213 isolates were analyzed by plasmid profiling, PCR replicon typing [19], plasmid Pst I restriction profiles [12, 20], Southern hybridization, plasmid PCR screening and sequencing of regions scattered throughout the IncA/C plasmids [8], and by their conjugation abilities. We found two divergent types of IncA/C plasmids: one composed of plasmids possessing or lacking the bla CMY-2 region and the other lacking bla CMY-2. We discuss our results in the context of epidemiological findings in Mexico, and we present evolutionary hypotheses regarding the origin of the two genetic types of IncA/C plasmids.