Background and purpose: The spread of the plasmid-mediated, colistin-resistance gene mcr-1  into New Delhi metallo-β-lactamase (NDM)-producing bacterial isolates can cause untreatable infections. In this study, we conducted a molecular characterization of a novel, conjugative, bla  _NDM-5-positive IncFII plasmid (pNDM-EC16-50) together with an mcr-1 -bearing IncI2 plasmid in a single Escherichia coli  ST167 clinical isolate EC16-50.
Methods and results: EC16-50, which belongs to the E. coli  strain ST167 and phylogroup A, was identified to co-produce NDM-5 and MCR-1. S1-PFGE and Southern blotting showed that bla  _NDM-5 and mcr-1  genes were located on ~95 kb and ~65 kb plasmids, respectively. A conjugation assay revealed that both bla  _NDM-5- and mcr-1 -bearing plasmids were self-transmissible. Comparative plasmid analysis suggested that bla  _NDM-5-harboring F2:A-:B-plasmid might have evolved from the well-reported NDM-carrying pMC-NDM-like plasmid via recombination with a locally emerged pSJ_94 plasmid, whereas the mcr-1 -carrying IncI2 plasmid was similar to previously reported mcr-1 -bearing plasmids in China.
Conclusion and impact: This study represents the first report of the NDM-5 carrying IncFII- but not IncX3-type plasmid in an MCR-1-producing E. coli  isolate. More striking was the dissemination of mcr-1  in a successful epidemic NDM-5-producing E. coli  clone ST167, which could facilitate the spread of colistin resistance in carbapenemase-producing E. coli  isolates.
Keywords: Escherichia coli , ST167, MCR-1, IncFII plasmid, NDM-5