Carriage of Distinct mcr-1-Harboring Plasmids by Unusual Serotypes of Salmonella.

Colistin acts as a last-resort antibiotic against lethal infections by carbapenem-resistant Enterobacterial pathogens. Enterobacteriaceae carrying mobile colistin resistance (MCR) genes are rapidly emerging and threatening human health and food safety. Despite mcr-1 being prevalent in Escherichia coli, its dissemination in Salmonella is not well characterized. Herein, two unusual serotypes of colistin-resistant Salmonella isolates are reported first, namely serotype Ngor (ST5399) and Goldcoast (ST2529). Using whole genome sequencing, it is shown that mcr-1 is located on the IncHI2-like plasmid pTB501 (188,527 bp) of strain SSDFZ54 and the IncX4-type plasmid pTB602 (33,303 bp) in strain SSDFZ69, respectively. Furthermore, the backbone, tra- and antimicrobial resistance genes relative variable regions in the mcr-1-bearing IncHI2 plasmids are systematically characterized. Phylogenetic analysis shows that all IncHI2-type plasmids from non-Chinese regions are clustered together, suggesting a possible Chinese origin. Taken together, these findings extend the understanding of Salmonella as a vehicle of mcr-1 carriage and distribution.