Emergence Of A Salmonella Enterica Serovar Typhimurium St34 Isolate, Cfsa629, Carrying A Novel Mcr-1.19 Variant Cultured From Egg In China

Objectives: This study aimed to characterize the genomic features of a Salmonella enterica serovar Typhimurium ST34 isolate, CFSA629, which carried a novel mcr-1 variant, designated as mcr-1.19, mapped to an ESBL-encoding IncHI2 plasmid.Methods: Antimicrobial susceptibility assays as well as WGS were carried out on isolate CFSA629. The complete closed genome was obtained and then explored to obtain genomic features. Plasmid sequence comparison was performed for pCFSA629 with similar plasmids and the mcr-1 genetic environment was analysed.Results: S. Typhimurium ST34 CFSA629 expressed an MDR phenotype to six classes of compound and consisted of a single circular chromosome and one plasmid. It possessed 11 resistance genes including 2 ESBL genes that mapped to the chromosome and the plasmid; an IS26-flanked composite-Like transposon was identified. A novel mcr-1 variant (mcr-1.19) was identified, which had a unique SNP (G1534A) that gave rise to a novel MCR-1 protein containing a Val512Ile amino acid substitution. Plasmid pCFSA629 possessed a conjugative plasmid transfer gene cluster as well as an antimicrobial resistance-encoding gene cluster-containing region that contained two IS26 composite-Like transposonal modules, but was devoid of any plasmid-mediated quinolone resistance genes. The background of mcr-1.19 consisted of an ISApl1-mcr-1-PAP2-ter module.Conclusions: We report on an MDR S. Typhimurium ST34 CFSA629 isolate cultured from egg in China, harbouring an mcr-1.19 variant mapped to an IncHI2 plasmid. This highlights the importance of surveillance to mitigate dissemination of mcr-encoding genes among foodborne Salmonella. Improved surveillance is important for tackling the dissemination of mcr genes among foodborne Salmonella around the world.