Abstract
BackgroundSalmonella Typhimurium ST213 was first detected in the Mexican Typhimurium population in 2001. It is associated with a multi-drug resistance phenotype and a plasmid-borne blaCMY-2 gene conferring resistance to extended-spectrum cephalosporins. The objective of the current study was to examine the association between the ST213 genotype and blaCMY-2 plasmids.ResultsThe blaCMY-2 gene was carried by an IncA/C plasmid. ST213 strains lacking the blaCMY-2 gene carried a different IncA/C plasmid. PCR analysis of seven DNA regions distributed throughout the plasmids showed that these IncA/C plasmids were related, but the presence and absence of DNA stretches produced two divergent types I and II. A class 1 integron (dfrA12, orfF and aadA2) was detected in most of the type I plasmids. Type I contained all the plasmids carrying the blaCMY-2 gene and a subset of plasmids lacking blaCMY-2. Type II included all of the remaining blaCMY-2-negative plasmids. A sequence comparison of the seven DNA regions showed that both types were closely related to IncA/C plasmids found in Escherichia, Salmonella, Yersinia, Photobacterium, Vibrio and Aeromonas. Analysis of our Typhimurium strains showed that the region containing the blaCMY-2 gene is inserted between traA and traC as a single copy, like in the E. coli plasmid pAR060302. The floR allele was identical to that of Newport pSN254, suggesting a mosaic pattern of ancestry with plasmids from other Salmonella serovars and E. coli. Only one of the tested strains was able to conjugate the IncA/C plasmid at very low frequencies (10-7 to 10-9). The lack of conjugation ability of our IncA/C plasmids agrees with the clonal dissemination trend suggested by the chromosomal backgrounds and plasmid pattern associations.ConclusionsThe ecological success of the newly emerging Typhimurium ST213 genotype in Mexico may be related to the carriage of IncA/C plasmids. We conclude that types I and II of IncA/C plasmids originated from a common ancestor and that the insertion and deletion of DNA stretches have shaped their evolutionary histories.
Highlights
Salmonella Typhimurium ST213 was first detected in the Mexican Typhimurium population in 2001
PCR replicon typing was performed for incompatibility groups that had been reported to be associated with either pSTV or blaCMY-2, such as IncFII, IncFIB, IncA/ C, IncHI2 and IncI1 [14,15,21,22]
Resistance to neither kanamycin nor nalidixic acid was transferred (Figure 2). These results indicate that the multi-drug resistant (MDR) phenotypes of ST213 strains can be explained largely by the presence of IncA/C plasmids
Summary
Salmonella Typhimurium ST213 was first detected in the Mexican Typhimurium population in 2001. It is associated with a multi-drug resistance phenotype and a plasmid-borne blaCMY-2 gene conferring resistance to extended-spectrum cephalosporins. Plasmids are part of the flexible genome, which is defined by the high plasticity and modularity of its genetic elements and high rates of gene acquisition and loss [2]. They are typically composed of conserved backbone modules coding for replication, maintenance and transfer functions as well as variable accessory modules. In Salmonella enterica, blaCMY-2 is frequently carried by IncA/C or IncI1 plasmids [11,12,14,15]
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