Co-occurrence of mcr-2 and mcr-3 genes on chromosome of multidrug-resistant Escherichia coli isolated from healthy individuals in Thailand

The aim of this study was to characterize three strains of colistin-resistant E. coli isolated from feces samples of healthy individuals in Thailand. The three strains, namely, SY_EC03, SY_EC07, and SY_EC10 were identified as ST165, ST1602, and ST34. All isolates exhibited multidrug-resistant phenotype, which is mediated by accumulation of various antimicrobial resistance genes. SY_EC03 contained mcr-1.1 while SY_EC07 co-harbored mcr-2.3 and mcr-3.4, and SY_EC10 co-harbored mcr-1.1 and mcr-3.5. Genomic analysis revealed that mcr-1.1 of the two strains were located on IncI2 plasmid with genetic environment of ISApl1-mcr-1.1-PAP2, which is a composite transposon Tn6330 with single-ended. Regarding mcr-2.3, the gene was identified within the composite transposon of ISKpn71-mcr-2.3-ISSpu2-ISKpn71, which was located on a novel mobile genetic element (MGE) that was integrated into the chromosome by phage integrase. For mcr-3.4 and mcr-3.5, the genes were confirmed to locate on the chromosome by S1-PFGE/DNA hybridization. Hence, to the best of our knowledge, this is the first report on co-occurrence of mcr-2 and mcr-3 on chromosome of E. coli. More interestingly, mcr-2 was found to locate on a novel MGE, which had never been described. In addition, we also report the co-occurrence of plasmidic mcr-1.1 and chromosomal mcr-3.5 which is extremely rare. Since all these bacteria were isolated from healthy individuals and the identified STs have been found in a variety of origins, all these clones may serve as reservoir for horizontal and vertical transmission of mcr genes. Strategic action plans to control and prevent the spread of mcr genes are urgently needed.