Spread of oqxAB in Salmonella enterica serotype Typhimurium predominantly by IncHI2 plasmids.

OBJECTIVES To investigate the prevalence and genetic environment of the multiresistance gene oqxAB in Salmonella enterica serotype Typhimurium isolated from food-producing animals. METHODS In this study, 63 Salmonella enterica serotype Typhimurium isolates were analysed for the presence of plasmid-mediated quinolone resistance determinants and mutations in the quinolone resistance-determining region by molecular methods (PCR/sequencing). The oqxAB-positive isolates were typed by pulsed-field gel electrophoresis (PFGE). Plasmids carrying oqxAB were studied by conjugation/transformation, replicon typing, Southern hybridization, long-range PCR and restriction fragment length polymorphism (RFLP). RESULTS The oqxAB, aac(6')-Ib-cr and qnrS1 genes were present alone or in combination in 20 (31.7%), 23 (36.5%) and 1 (1.6%) isolate, respectively. The oqxAB-positive isolates were clonally related, as determined by PFGE. All of the oqxAB-aac(6')-Ib-cr-positive isolates carried transferable IncHI2-type plasmids containing an oqxAB cassette and an incomplete class 1 integron harbouring aac(6')-Ib-cr, blaOXA-1, catB3, arr3, qacEΔ1 and sul1. Meanwhile, 6 of 15 plasmids carrying both oqxAB and aac(6')-Ib-cr showed identical RFLP patterns. CONCLUSIONS The results suggest that both clonal expansion and horizontal transmission of IncHI2-type plasmids containing oqxAB and aac(6')-Ib-cr may be involved in the spread of oqxAB in Salmonella Typhimurium isolates in food-producing animals in China. There is a great need to monitor the potential dissemination of this multiresistance gene.