Chromosomal Locations of mcr-1 and blaCTX-M-15 in Fluoroquinolone-Resistant Escherichia coli ST410

To the Editor: Recently, Yi-Yun Liu et al. reported on the discovery of mcr-1, a plasmidborne resistance gene mediating resistance to colistin, in isolates obtained from humans and animals (1). Since the original publication, mcr-1 with or without the insertion element ISApl1 has been detected on plasmids of different incompatibility groups, including IncI2, IncHI2, and IncX4, and in many different countries (1–3). Because colistin is a last-resort parenteral antimicrobial drug, the transfer of mcr-1 by conjugation or through mobilizable plasmids raises concern about the emergence of pan-resistant Enterobacteriaceae.