Emergence of Transferablemcr-9Gene-Carrying Colistin-ResistantSalmonella entericaDessau 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.Salmonellais one of the major pathogens responsible for foodborne illnesses; however, there is minimal information regarding the presence ofmcrgenes in foodborneSalmonellastrains. The aim of this study was to investigate the presence ofmcrgenes among 178Salmonellastrains 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 themcr-9gene were analyzed using next-generation sequencing. Transferability of themcr-9carrying colistin-resistantSalmonellastrain was tested using broth-mating conjugation. Thirteen of the 178Salmonellaisolates showed colistin resistance, but only one strain,SalmonellaDessau ST14 (KUFSE-SAL043) from a traditional chicken market in Korea, carried anmcrfamily gene,mcr-9. This strain also carried other acquired antimicrobial resistance genes such asbla(TEM-1B),qnrS1, andaac(6 ')-Iaa. Only the IncX1 plasmid replicon type was detected in this strain. In the strain KUFSE-SAL043, themcr-9gene was located between two insertion sequences, IS903Band IS26, followed by the downstream regulatory genesqseB-like andqseC-like, which were located between IS1Rand Delta IS1R. Conjugation tests revealed that themcr-9gene was successfully transferred toEscherichia coliJ53 at a mean frequency of 2.03 x 10(-7). This is the first report of a transferablemcr-9gene inSalmonellaisolated 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 ofEnterobacteriaceaestrains in humans, livestock, and the environment.