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, qacED1 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.