Co-spread of oqxAB and blaCTX-M-9G in non-Typhi Salmonella enterica isolates mediated by ST2-IncHI2 plasmids

In this study, 130 non-Typhi Salmonella enterica isolates from chickens were analysed for the prevalence of plasmid-mediated quinolone resistance (PMQR) determinants as well as the co-existence of oqxAB and extended-spectrum β-lactamase (ESBL) genes. The genes oqxAB, aac(6′)-Ib-cr, blaCTX-M-9G and blaTEM were present alone or in combination in 40 (30.8%), 40 (30.8%), 55 (42.3%) and 2 (1.5%) isolates, respectively. Most of the oqxAB–blaCTX-M-9G-positive isolates (17/28) carried transferable ST2-IncHI2 plasmids containing an oqxAB cassette and blaCTX-M-14 flanked by insertion sequences IS10 or ISEcp1 upstream and IS903 downstream. The oqxAB–blaCTX-M-9G-positive isolates from a local area showed similar pulsed-field gel electrophoresis (PFGE) patterns, whilst the isolates from different areas were genetically divergent, suggesting that both clonal expansion in local areas and horizontal transmission contributed to the spread of ST2-IncHI2 plasmids containing oqxAB and blaCTX-M-14. This is the first report on the prevalence of ST2-IncHI2 plasmids concomitantly carrying oqxAB and blaCTX-M-14 in Salmonella and also the first description of the genetic environment of oqxAB–blaCTX-M. The genetic linkage of oqxAB–blaCTX-M-9G in non-Typhi Salmonella likely facilitates the spread of antibiotic-resistant Salmonella and poses a threat for clinical treatment of salmonellosis.