Plasmid-mediated quinolone resistance determinant qnrS1 found in Salmonella enterica strains isolated in the UK

Objectives: To determine the prevalence of qnr genes in selected Salmonella enterica and Escherichia coli isolated in the UK. Methods: One hundred and eighteen S. enterica and 103 E coli were screened for qnrA, qnrB and qnrS by PCR. Transferability of qnr plasmids was assessed and isolated plasmids compared with previously identified qnr plasmids by restriction fragment length polymorphism analysis and hybridization experiments. PCRs and sequencing identified co-transferred beta-lactamase genes and mutations in the quinolone resistance-determining region of gyrA. Results: Only six S. enterica strains belonging to four serotypes (Stanley, Typhimurium, Virchow and Virginia) were positive for qnrS1. qnrSl was present on plasmids of 13.5 kb (TPqnrS-1 a and -1 b) in Typhimurium and Virginia isolates, 44 kb (TPqnrS-2) in two Virchow isolates and >148 kb (TPqnrS-3a and -3b) in two Stanley isolates. bla'rEM-1 and a group 9 bla(CTX-M) were co-transferred on TPqnrS-2 and TPqnrS-3b. Hybridization of a qnrS1 probe to digested qnrS1 plasmids suggested qnrS1 on TPqnrS-2 may be located in a similar genetic environment to Shigella qnrS plasmid pAH0376, but in a different environment in the other plasmids. Conclusions: This is the first report of plasm id-mediated quinolone resistance in a Salmonella isolate from the UK; five isolates were associated with foreign travel to, or food imported from, the Far East. The presence of qnrSl on different plasmid backbones in several Salmonella serotypes suggests successful dissemination of plasmids or qnrS1. It is of concern that qnrS1 is being identified in Salmonella serotypes that are commonly implicated in human infection in the UK. Coupled with beta-lactam resistance, it may compromise treatment of vulnerable patient groups.