An Intra-Hospital Spread of Colistin-Resistant Isolates-Epidemiological, Clinical, and Genetic Analysis.

appears to be a significant problem due to its ability to accumulate antibiotic-resistance genes. After 2013, alarming colistin resistance rates among carbapenem-resistant have been reported in the Balkans. The study aims to perform an epidemiological, clinical, and genetic analysis of a local outbreak of COLr CR-Kp. All carbapenem-resistant and colistin-resistant isolates observed among patients in the ICU unit of Military Medical Academy, Sofia, from 1 January to 31 October 2023, were included. The results were analyzed according to the EUCAST criteria. All isolates were screened for VIM, IMP, KPC, NDM, and OXA-48. Genetic similarity was determined using the Dice coefficient as a similarity measure and the unweighted pair group method with arithmetic mean (UPGMA). genes and plasmid-mediated colistin resistance determinants (, , , , and ) were investigated. There was a total of 379 multidrug-resistant isolates, 88% of which were carbapenem-resistant. Of these, there were nine (2.7%) colistin-resistant isolates in six patients. A time and space cluster for five patients was found. Epidemiology typing showed that two isolates belonged to clone A (pts. 1, 5) and the rest to clone B (pts. 2-4) with 69% similarity. Clone A isolates were coproducers of NDM-like and OXA-48-like and had -mediated colistin resistance (40%). Clone B isolates had only OXA-48-like and intact genes. All isolates were negative for , , , , and genes. The study describes a within-hospital spread of two clones of COLr CR-Kp with a 60% mortality rate. Clone A isolates were coproducers of NDM-like and OXA-48-like enzymes and had -mediated colistin resistance. Clone B isolates had only OXA-48-like enzymes and intact genes. No plasmid-mediated resistance was found. The extremely high mortality rate and limited treatment options warrant strict measures to prevent outbreaks.