Prevalence and molecular characteristics of mcr-1 gene in Salmonella typhimurium in a tertiary hospital of Zhejiang Province.

Objectives: mcr-1 gene has been widely reported in the world. This study aimed to analyze the prevalence and molecular characteristics of mcr-1 gene in Salmonella typhimurium from Quzhou People's Hospital.

Materials and methods: A total of 62 S. typhimurium isolates were isolated and preserved in our laboratory from 2007 to 2016. PCR was used to screen plasmid-mediated colistin resistance gene, mcr-1 . For mcr-1 -positive isolates, susceptibilities to colistin and other antibiotics were assessed using broth microdilution or agar dilution methods. The genetic location of mcr-1 was determined by analysis of pulsed-field gel electrophoresis profiles of S1-digested genomic DNA and subsequent Southern blot hybridization. The multi-locus sequence type and other drug resistance genes found in the mcr-1 -positive isolates were analyzed by performing whole genome sequencing. Genetic environment of mcr-1 gene was analyzed by RAST and Easyfig.

Results: A total of three S. typhimurium isolates were identified to be mcr-1 positive, with the prevalence rate of 4.8% (3/62). The minimum inhibitory concentration values of colistin for all these isolates were 8 µg/mL. The three mcr-1 -positive isolates carried mcr-1 gene on two different types of plasmids having the sizes of ~54.7-78.2 kb and 310.1 kb, respectively. All the three isolates belonged to ST34 and carried various resistant genes.

Conclusion: Colistin-resistant, mcr-1 -positive S. typhimurium isolates belonging to ST34 have been isolated from Quzhou People's Hospital. Surveillance needs to be strengthened to identify colistin resistance and prevent the spread of drug-resistant bacteria in the hospital.