The First Report of -1-Carrying , 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 isolates (, , and spp.) and non-fermentative rods ( and ). 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 -1 to -8, and real-time PCR for -1 and -2. One isolate was identified as carrying the -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 -1 being located on a 33,304 bp circular IncX4 plasmid. No -2 to -8-positive isolates were detected, prompting further investigation into alternative colistin resistance mechanisms. This is the first report of a -1-positive, colistin-resistant isolated from a human clinical sample in the North-East of Romania.