Isolation and characterization of small qnrS1-carrying plasmids from imported seafood isolates of Salmonella enterica that are highly similar to plasmids of clinical isolates

Dissemination of plasmid-mediated quinolone resistance among pathogenic bacteria is a concern for public health because of decreased sensitivity to fluoroquinolones and increased potentials to develop high fluoroquinolone resistance. Two qnrS1-positive isolates of Salmonella enterica Corvallis (468) and Typhimurium (484) from imported seafood (Thailand and Vietnam) were tested for quinolone sensitivity using disk agar diffusion and the Sensititre system. The presence of qnr genes, qnr-carrying plasmids, and mutations in the quinolone resistance determining regions were also determined. Minimal inhibitory concentrations of nalidixic acid for isolates 468 and 484 were 8 and 16 μg mL(-1) , respectively, and those of ciprofloxacin were 1 and 2 μg mL(-1), respectively. Disk agar diffusion indicated that isolate 468 was moderately resistant to moxifloxacin, and isolate 484 was resistant to moxifloxacin and moderately resistant to norfloxacin. Isolates 468 and 484 carried a mutation on parC, but not on gyrA, gyrB, or parE. Sequences of qnrS1-carrying plasmids from isolates 468 and 484, sized 10,039 and 10,047 bp, were nearly identical (> 99% similarity) to each other and to published sequences of plasmids from clinical isolates of Salmonella Typhimurium isolated in the United Kingdom and Taiwan, indicating a dissemination of qnrS1-carrying plasmids among different serovars of Salmonella from geographically separated sources. This is the first complete sequence of a qnrS1-carrying plasmid from imported seafood isolate of S. enterica.