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

Reports of transmissible colistin resistance show the importance of comprehensive colistin resistance surveillance. Recently, a new allele of the mobile colistin resistance ( ) gene family designated , which shows variation in genetic context and colistin susceptibility, was reported. We tested over 100 and isolates with 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 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.