The mcr-9 Gene of Salmonella 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 (mcr) gene family designated mcr-9 was reported, which shows variation in genetic context and colistin susceptibility. We tested over one hundred Salmonella enterica and Escherichia coli with mcr-9 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 twelve isolates revealed mcr-9 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.