Molecular characterisation of antimicrobial resistance determinants and class 1 integrons of Salmonella enterica subsp. enterica serotype Enteritidis strains from retail food in China

Abstract

Abstract Salmonella is a genus responsible for a majority of foodborne disease outbreaks affecting public health worldwide, and Salmonella enterica subsp. enterica serotype Enteritidis (S. Enteritidis) is the most common pathogen that presents alarming rates of antimicrobial resistance. The aim of this study was to analyse the antimicrobial resistance of 124\xa0S. Enteritidis strains collected from retail foods in 39 cities across China and to conduct a comprehensive molecular characterisation of their antimicrobial resistance genes (ARGs) and class 1 integrons through PCR amplification, antibiotic susceptibility testing, and conjugation assays. The strains exhibited a multidrug resistance rate as high as 68.5% and were frequently resistant to nalidixic acid (97.6%) and ampicillin (74.2%). Correspondingly, the analysis of quinolone genes revealed that 97.5% of the strains harboured single mutations in gyrA; some possessed the plasmid-mediated quinolone resistance genes qnrS (0.8%), oqxAB (2.4%), as well as the blaTEM-1 (67.7%); and the extended spectrum beta-lactamase (ESBL) genes blaCTX-M-55 were detected in 2.4% of the strains. Class 1 integrons were observed in 11.3% (14/124) of the strains and integrated three different gene cassettes, namely, aadA22 (1/14), dfrA1-aadA1 (5/14), and dfrA17-aadA5 (8/14). Conjugation experiments and antimicrobial susceptibility testing showed that the integrons including antimicrobial resistance genes could be transferred to the recipient strain via mobile plasmids, rendering the recipient strain resistant. Both multilocus variable-number tandem-repeat analysis (MLVA) and core genome multilocus sequence typing (cgMLST) show a better discrimination in this study, hence a combination of MLVA and cgMLST analyses would improve the accuracy and cost-effectiveness. This study provides a systematic and comprehensive analysis of the antimicrobial resistance profiles of S. Enteritidis strains isolated from retail food in China. The present data will help to gain insight into the public health impact of S. Enteritidis and guide the development of control strategies.