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

Abstract

Transmissible colistin resistance mediated by the <i>mcr</i> gene has been reported worldwide, but clinical isolates of <i>mcr</i>-negative colistin-resistant <i>Escherichia coli</i> are rarely reported. The aim of this study was to evaluate the mechanism of colistin resistance among <i>mcr</i>-positive and <i>mcr</i>-negative <i>E. coli</i> clinical isolates by performing a molecular epidemiological surveillance. For the first time ever, we show nearly the same isolation ratio for <i>mcr</i>-negative and <i>mcr</i>-positive colistin-resistant clinical isolates (47.5 and 52.5%, respectively), with no demonstrable nosocomial transmission. We provide evidence for the prevalence of the <i>mcr</i>-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 <i>mcr</i>-negative <i>E. coli</i> isolates were obviously higher than those of <i>mcr</i>-positive isolates. Apart from the usually detected genes, i.e., <i>pmrAB, phoPQ</i>, and <i>mgrB</i>, other genes may be associated with the colistin resistance in <i>mcr</i>-negative <i>E. coli</i>. 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 <i>mcr</i>-negative <i>E. coli</i> clinical isolates. Together, the results show that colistin resistance was prevalent not only in the <i>mcr</i>-positive clinical <i>E. coli</i> isolates but also in the <i>mcr</i>-negative isolates.