Conjugative IncHI2/HI2A plasmids harbouring mcr-9 in colistin-susceptible Escherichia coli isolated from diseased pigs in Japan

Colistin is a last resort antimicrobial used for the treatment of gram-negative bacterial infections. Plasmid-mediated colistin resistance (mcr) genes are a cause of global concern, and, thus far, mcr-1–10 have been identified. In a previous study, we screened mcr-1–5 in Escherichia coli derived from diseased pigs in Japan and reported a high prevalence of mcr-1, -3 and -5. However, the previous report on the prevalence of mcr genes was inaccurate. In the present study, we aimed to clarify the prevalence of all reported variants of mcr in E. coli derived from the diseased pigs, which were previously screened for mcr-1–5. Additionally, we also characterized the mcr-9-positive E. coli , which was detected in this study. We screened mcr in 120 E. coli strains from diseased pigs and mcr-positive E. coli and an mcr-carrying plasmid were also characterized. One mcr-9-positive colistin-susceptible E. coli strain was detected (0.8 %). Plasmid-mediated mcr-9 was transferred to E. coli ML4909 as the recipient strain, and it was located on IncHI2/HI2A plasmid p387_L with other antimicrobial resistance genes (ARGs). The region harbouring ARGs including mcr-9, was similar to that on the Klebsiella pneumoniae chromosome harbouring mcr-9 isolated in Japan. mcr-3, -5 and -9 were detected (4.2 %) in colistin-susceptible strains. mcr-9 was found to be disseminated via the plasmid IncHI2/HI2A p387_L and transferred and inserted into chromosomes via a transposon. Our results suggest that mcr genes should be monitored regularly, regardless of their susceptibility to colistin.