Antimicrobial Resistance of Salmonella and Characterization of Three mcr-1-harboring Isolates from Retail Snail Aquatic Products in Mainland China

To understand the antimicrobial resistance and the mobile colistin resistance gene (mcr) carrying status of Salmonella isolates recovered from retail snail aquatic products in mainland China. Methods: A total of 346 Salmonella isolates collected from mainland China in 2018 were tested for their susceptibility to a panel of antimicrobial compounds by broth microdilution method; mcr genes were detected by PCR, and positive isolates were subjected to whole genome sequencing and downstream bioinformatics analysis. Results: The resistant isolates accounted for 84.10% (291/346) of the collection and the resistance rate to SMX was the highest at 80.64% (279/346). One hundred and thirty-five isolates (39.02%) expressed a multi-drug resistance (MDR) phenotype and ninty-seven antimicrobial resistance profiles were recorded. Three Salmonella isolates representing different serotypes recovered from Shaanxi, Sichuan, and Chongqing provinces were positive for the mcr-1 gene positive with a rate of 0.87% (3/346), and all were multi-drug resistant. These three mcr-1-positive isolates contained IncHI2 plasmids that were not transferable. Genomic analysis of 106 mcr-1-bearing-IncHI2 plasmids showed that these plasmids were mainly from clinical settings (45.2%, 48/106) in China (75.40%, 80/106), and were identified from E. coli (62.26%, 66/106), Salmonella (33.02%, 35/106) and Klebsiella (4.72%, 5/106). Conclusion: Salmonella isolated from retail snail aquatic products in China in 2018 demonstrated high level antimicrobial resistance. In addition, mcr-1-bearing IncHI2-pST3 plasmids play an important role as carriers of colistin resistance with the potential for broader dissemination with the assistance of helper plasmids, a feature that warrants active surveillance for antimicrobial resistance.