The <i>mcr-9</i> Gene of <i>Salmonella</i> and Escherichia coli Is Not Associated with Colistin Resistance in the United States

Abstract

Reports of transmissible colistin resistance show the importance of comprehensive colistin resistance surveillance. Recently, a new allele of the mobile colistin resistance (<i>mcr</i>) gene family designated <i>mcr-9</i>, which shows variation in genetic context and colistin susceptibility, was reported. We tested over 100 <i>Salmonella enterica</i> and <i>Escherichia coli</i> isolates with <i>mcr-9</i> from the National Antimicrobial Resistance Monitoring System (NARMS) in the United States for their susceptibility to colistin and found that every isolate was susceptible, with an MIC of ≤1 μg/ml. Long-read sequencing of 12 isolates revealed <i>mcr-9</i> on IncHI plasmids that were either independent or integrated into the chromosome. Overall, these results demonstrate that caution is necessary when determining the clinical relevance of new resistance genes.