The First Report of mcr-1-Carrying Escherichia coli, Isolated from a Clinical Sample in the North-East of Romania

Colistin resistance poses a significant clinical challenge, particularly in Gram-negative bacteria. This study investigates the occurrence of plasmid-mediated colistin resistance among <i>Enterobacterales</i> isolates (<i>Escherichia coli</i>, <i>Klebsiella pneumoniae</i>, and <i>Enterobacter</i> spp.) and non-fermentative rods (<i>Acinetobacter baumannii</i> and <i>Pseudomonas aeruginosa</i>). We analyzed 114 colistin-resistant isolates that were selected, based on resistance phenotypes, and isolated between 2019 and 2023. To achieve this, we used the rapid immunochromatographic test, NG-Test^® MCR-1; multiplex PCR for <i>mcr</i>-1 to <i>mcr</i>-8, and real-time PCR for <i>mcr</i>-1 and <i>mcr</i>-2. One <i>E. coli</i> isolate was identified as carrying the <i>mcr</i>-1 gene, confirmed by NG-Test^® MCR-1, multiplex PCR and whole-genome sequencing. This strain, belonging to ST69, harbored four plasmids, harboring different antimicrobial resistance genes, with <i>mcr</i>-1 being located on a 33,304 bp circular IncX4 plasmid. No <i>mcr</i>-2 to <i>mcr</i>-8-positive isolates were detected, prompting further investigation into alternative colistin resistance mechanisms. This is the first report of a <i>mcr</i>-1-positive, colistin-resistant <i>E. coli</i> isolated from a human clinical sample in the North-East of Romania.