A Novel Deletion Mutation in pmrBContributes to Concurrent Colistin Resistance in Carbapenem-Resistant Escherichia coliSequence Type 405 of Clinical Origin

We report the first clinical Escherichia colistrain EC3000 with concomitant chromosomal colistin and carbapenem resistance. A novel in-frame deletion, Δ6-11 (RPISLR), in pmrBthat contributes to colistin resistance was verified using recombinant DNA techniques. Although being less fit than the wild-type (WT) strain or EC3000 revertant (chromosomal replacement of WT pmrBin EC3000), a portion of serially passaged EC3000 strains preserving colistin resistance without selective pressure raises the concern for further spread.