Molecular Epidemiology and Colistin Resistant Mechanism of mcr -Positive and mcr -Negative Clinical Isolated Escherichia coli .

Transmissible colistin resistance mediated by the mcr gene has been reported worldwide, but clinical isolates of mcr -negative colistin-resistant Escherichia coli are rarely reported. The aim of this study was to evaluate the mechanism of colistin resistance among mcr -positive and mcr -negative E. coli clinical isolates by performing a molecular epidemiological surveillance. For the first time ever, we show nearly the same isolation ratio for mcr -negative and mcr -positive colistin-resistant clinical isolates (47.5 and 52.5%, respectively), with no demonstrable nosocomial transmission. We provide evidence for the prevalence of the mcr -positive IncX4 plasmid and its high potential for horizontal transfer, with no obvious sequence type (ST) preference. In addition, the minimal inhibitory concentrations (MICs) of colistin of the mcr -negative E. coli isolates were obviously higher than those of mcr -positive isolates. Apart from the usually detected genes, i.e., pmrAB, phoPQ , and mgrB , other genes may be associated with the colistin resistance in mcr -negative E. coli . To the best of our knowledge, this is the first paper to report the molecular epidemiological surveillance and the proper mechanism of colistin resistance in mcr -negative E. coli clinical isolates. Together, the results show that colistin resistance was prevalent not only in the mcr -positive clinical E. coli isolates but also in the mcr -negative isolates.