Surveillance of colistin-resistance and mcr genes in multi-drug resistant Enterobacteriaceae

Background. The plasmid-mediated bacterial colistin-resistant (mcr) gene is a global concern in clinical health care. This study aimed to clarify the prevalence of colistin resistance through nine mcr genes in ESBL-producing and CRE isolated Enterobacteriaceae in Japan. Methods. We collected strains from August 2016 to March 2017 from five tertiary hospitals. MICs were measured using the microdilution method. PCR was performed to detect mcr-1 to mcr-9 genes in all strains. Additionally, we performed whole-genome sequencing of the mcr gene-positive strain. Results. The rate of colistin resistance was 7.7%. The mcr-5 and mcr-9 gene were detected in one ESBL-producing E. coli strain (0.37%) and three CRE strains (1.1%), respectively. Since the ESBL-producing E. coli strain was the first clinical strain with mcr-5 in Japan, whole-genome sequencing analysis was performed for the strain. The sequenece type of the mcr-5 positive strain was ST1642 and it carried two distinct plasmids, ESBL gene-carrying pN-ES-6-1 and mcr-5.1-carrying pN-ES-6-2. Conclusions. We showed that the frequency of colistin resistance and mcr-positive strains is not high in Japan. Since the MIC for colistin was low in the mcr-5.1 and mcr-9 gene-positive strain, continuous monitoring of mcr genes is necessary.