Characterization of plasmids harbouring qnrS1, qnrB2 and qnrB19 genes in Salmonella

The aim of this study was to identify and characterize plasmids carrying qnrS1, qnrB2 and qnrB19 genes identified in Salmonella strains from The Netherlands. The identification of plasmids may help to follow the dissemination of these resistance genes in different countries and environments. Plasmids from 33 qnr-positive Salmonella strains were transferred to Escherichia coli and analysed by restriction, Southern blot hybridization, PCR and sequencing of resistance determinants. They were also assigned to incompatibility groups by PCR-based replicon typing, including three additional PCR assays for the IncU, IncR and ColE groups. The collection included isolates from humans and one from chicken meat. Five IncN plasmids carrying qnrS1, qnrB2 and qnrB19 genes were identified in Salmonella enterica Bredeney, Typhimurium PT507, Kentucky and Saintpaul. qnrS1 genes were also located on three further plasmid types, belonging to the ColE (in Salmonella Corvallis and Anatum), IncR (in Salmonella Montevideo) and IncHI2 (in Salmonella Stanley) groups. Multiple events of mobilization, transposition and replicon fusion generate the complexity observed in qnr-positive isolates that are emerging worldwide. Despite the fact that the occurrence of qnr genes in bacteria from animals is scarcely reported, these genes are associated with genetic elements and located on plasmids that are recurrent in animal isolates.