First identification and Limited Dissemination of mcr-1 Colistin Resistance in Salmonella Isolates in Jiaxing.

Salmonella , a major foodborne pathogen, causes severe gastrointestinal disease in people and animals worldwide. Plasmid-borne mcr-1 , which confers colistin resistance in Salmonella, has significant epidemiological interest for public health safety. Here, we report the first evidence of mcr-1 -mediated colistin resistance in one multidrug-resistant strain，namely 16062 in this study, from 355 Salmonella isolates collected for Jiaxing foodborne pathogen monitoring in Zhejiang Province in 2015-2019. In addition to colistin, 16062 displayed multidrug resistance to various antimicrobials (β-lactams, quinolone, sulfonamide, florfenicol, ampicillin, streptomycin, nalidixic acid, aminoglycoside, and trimethoprim-sulfamethox). The mcr-1 -carrying IncX4 plasmid (p16062-MCR) in this study shares a conserved structure with other mcr -IncX4 plasmids. We found that other antimicrobial-resistance genes ( aac(6')-Ib-cr , aadA1 , aadA2 , aph(3')-Ia , oqxA , oqxB , sul1 , and cmlA1 ) are located on p16062-cmlA, an atypical IncHI2 plasmid, in isolate 16062. This is the first identification of transferable colistin resistance in foodborne Salmonella isolate collected in Jiaxing city, the 5-year monitoring of which revealed limited dissemination. By determining the genetic features of the plasmid vehicle, the characteristics of transferable mcr genes circulating in isolates from Jiaxing are now clearer.