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Emergence of Transferable mcr-9 Gene-Carrying Colistin-Resistant Salmonella enterica Dessau ST14 Isolated from Retail Chicken Meat in Korea.

Colistin is an important antibiotic currently used to manage infections caused by multidrug-resistant pathogens in both humans and livestock animals. A new mobile colistin-resistance (mcr-9) gene was recently discovered; this discovery highlighted the need for rigorous monitoring of bacterial resistance against colistin. Salmonella is one of the major pathogens responsible for foodborne illnesses; however, there is minimal information regarding the presence of mcr genes in foodborne Salmonella strains. The aim of this study was to investigate the presence of mcr genes among 178 Salmonella strains isolated from chicken meat in Korea. Antimicrobial susceptibility was measured using the broth microdilution method. Bioinformatics characterization of colistin-resistant strains and genetic environment of the mcr-9 gene were analyzed using next-generation sequencing. Transferability of the mcr-9 carrying colistin-resistant Salmonella strain was tested using broth-mating conjugation. Thirteen of the 178 Salmonella isolates showed colistin resistance, but only one strain, Salmonella Dessau ST14 (KUFSE-SAL043) from a traditional chicken market in Korea, carried an mcr family gene, mcr-9. This strain also carried other acquired antimicrobial resistance genes such as blaTEM-1B, qnrS1, and aac(6')-Iaa. Only the IncX1 plasmid replicon type was detected in this strain. In the strain KUFSE-SAL043, the mcr-9 gene was located between two insertion sequences, IS903B and IS26, followed by the downstream regulatory genes qseB-like and qseC-like, which were located between IS1R and ΔIS1R. Conjugation tests revealed that the mcr-9 gene was successfully transferred to Escherichia coli J53 at a mean frequency of 2.03 × 10-7. This is the first report of a transferable mcr-9 gene in Salmonella isolated from chicken meat in Korea, highlighting the possibility of transfer of colistin resistance. Therefore, the wide use of colistin should be reconsidered, and a One Health perspective should be adopted to monitor the antimicrobial resistance of Enterobacteriaceae strains in humans, livestock, and the environment.