Emerging coexistence of three PMQR genes on a multiple resistance plasmid with a new surrounding genetic structure of qnrS2 in E. coli in China

BACKGROUND: Quinolones are commonly used for treatment of infections by bacteria of the Enterobacteriaceae family. However, the rising resistance to quinolones worldwide poses a major clinical and public health risk. This study aimed to characterise a novel multiple resistance plasmid carrying three plasmid-mediated quinolone resistance genes in Escherichia coli clinical stain RJ749. METHODS: MICs of ceftriaxone, cefepime, ceftazidime, ciprofloxacin, and levofloxacin for RJ749 and transconjugant c749 were determined by the Etest method. Conjugation was performed using sodium azide-resistant E. coli J53 strain as a recipient. The quinolone resistance-determining regions of gyrA, gyrB, parC, and parE were PCR-amplified. RESULTS: RJ749 was highly resistant to quinolones, while c749 showed low-level resistance. S1-nuclease pulsed-field gel electrophoresis revealed that RJ749 and c749 both harboured a plasmid. PCR presented chromosomal mutation sites of the quinolone resistance-determining region, which mediated quinolone resistance. The c749 genome comprised a single plasmid, pRJ749, with a multiple resistance region, including three plasmid-mediated quinolone resistance (PMQR) genes (aac (6′)-Ib-cr, qnrS2, and oqxAB) and ten acquired resistance genes. One of the genes, qnrS2, was shown for the first time to be flanked by two IS26s. Three IS26-mediated circular molecules carrying the PMQR genes were detected. CONCLUSIONS: We revealed the coexistence of three PMQR genes on a multiple resistance plasmid and a new surrounding genetic structure of qnrS2 flanked by IS26 elements. IS26 plays an important role in horizontal spread of quinolone resistance.