Characterization of quinolone resistance in Salmonella enterica serovar Typhimurium and its monophasic variants from food and patients in China

•High rates of resistance to nalidixic acid in the isolates were observed.•A high proportion of isolates with reduced susceptibility to ciprofloxacin were detected.•Single GyrA mutation with predominance of S83Y and D87N types frequently occurred in the isolates.•PMQR-encoding genes of aac(6′)-Ib-cr, oqxAB, qnrS, and qnrA were frequently identified. Objectives: The study aimed to characterize the quinolone resistance of Salmonella enterica serovar Typhimurium and its monophasic variant (Salmonella enterica serovar 1,4,[5],12:i:-) isolated from food and patients in China. Methods All of the isolates were assessed for quinolone susceptibility via the broth microdilution method. Then, the isolates were checked for mutations within quinolone resistance-determining regions of gyrA, gyrB, parC, and parE, and were examined for plasmid-mediated quinolone resistance genes. Results: High rates of resistance to nalidixic acid in the S. Typhimurium (70.7%) and S. 1,4,[5],12:i:- (41.9%) isolates were observed, while a considerable proportion of isolates with reduced susceptibility to ciprofloxacin and levofloxacin were also detected. The high frequency of mutations in GyrA (60.8%) and a variety of genes [aac(6′)-Ib-cr (23.2%), oqxAB (19.2%), qnrS (13.6%), and qnrA (3.2%)] conferring quinolone resistance in these Salmonella isolates were noteworthy. Lastly, the isolates carrying qnrS for transferability and transmission of the quinolone resistance were analyzed by conjugation. Multiple locus variable-number tandem repeat analysis profiles indicated that some qnrS-positive isolates were clonally related, whilst the other isolates were genetically divergent. This suggested that both clonal spread of resistant strains and horizontal transmission of the plasmid-mediated resistance genes contributed to the dissemination of qnrS-positive Salmonella isolates. Conclusion: This study highlights the prevalence of quinolone-resistant S. Typhimurium and S. 1,4,[5],12:i:- in China, posing a threat to public health.