Molecular genetic characteristics of mcr-9-harbouring Salmonella enterica serotype Typhimurium isolated from raw milk

•To the authors’ knowledge, this is the first report of an mcr-9-carrying strain from a food sample in China (raw milk of healthy cows); prevalence was low (0.83%).•The mcr-9-carrying Salmonella enterica serotype Typhimurium belonged to ST26, a novel sequence type for mcr-9-carrying S. enterica.•This mcr-9-plasmid was stable in its original host S. enterica, and was transferrable to Escherichia coli with a significant biological fitness cost in the recipient strain.•Whole-genome sequencing revealed that mcr-9 was located on the IncHI2A-type plasmid, surrounded by IS903B and IS26 in its flanking regions.•A functional cloning experiment found that mcr-9-pUC19 did not mediate colistin resistance in the absence of the qseBC system.