Inc A/C Plasmids Are Prevalent in Multidrug-Resistant Salmonella enterica Isolates

Bacterial plasmids are fragments of extrachromosomal double-stranded DNA that can contain a variety of genes that are beneficial to the host organism, like those responsible for antimicrobial resistance. The objective of this study was to characterize a collection of 437 Salmonella enterica isolates from different animal sources for their antimicrobial resistance phenotypes and plasmid replicon types and, in some cases, by pulsed-field gel electrophoresis (PFGE) in an effort to learn more about the distribution of multidrug resistance in relation to replicon types. A PCR-based replicon typing assay consisting of three multiplex PCRs was used to detect 18 of the 26 known plasmid types in the Enterobacteriaceae based on their incompatibility (Inc) replicon types. Linkage analysis was completed with antibiograms, replicon types, serovars, and Inc A/C. Inc A/C plasmids were prevalent in multidrug-resistant isolates with the notable exception of Salmonella enterica serovar Typhimurium. Cluster analysis based on PFGE of a subset of 216 isolates showed 155 unique types, suggesting a variable population, but distinct clusters of isolates with Inc A/C plasmids were apparent. Significant linkage of serovar was also seen with Inc replicon types B/O, I1, Frep, and HI1. The present study showed that the combination of Salmonella, the Inc A/C plasmids, and multiple-drug-resistant genes is very old. Our results suggest that some strains, notably serovar Typhimurium and closely related types, may have once carried the plasmid but that the resistance genes were transferred to the chromosome with the subsequent loss of the plasmid.