Trends in Colistin Resistance and Multidrug-Resistant Phenotypes Among Gram-Negative Bacilli: A Retrospective Analysis

Colistin has re-emerged as a last-resort antibiotic for treating infections caused by multidrug-resistant (MDR) Gram-negative bacilli (GNB). However, increasing resistance threatens its efficacy. This study aimed to evaluate colistin resistance trends among clinical isolates of Gram-negative bacilli isolated over a five-year period at a large Emergency Hospital in North-Eastern Romania. A total of 23,143 GNB strains were isolated during the study period, including 14,531 <i>Enterobacterales</i> and 8294 non-fermenting Gram-negative bacilli. The percentage of colistin-resistant strains among those analyzed was 3.98%. Species-specific analysis focused on <i>Klebsiella</i> spp., <i>Escherichia coli</i>, <i>Enterobacter</i> spp., <i>Citrobacter</i> spp., <i>Pseudomonas</i> spp., and <i>Acinetobacter</i> spp. <i>Klebsiella</i> spp. exhibited the highest prevalence of colistin resistance, accounting for over 80% of all colistin-resistant strains, with annual resistance rates fluctuating between 12.97% and 21.64%. Colistin resistance among <i>E. coli</i> was low (0.18-1.25%). <i>Citrobacter</i> spp. showed no resistance in the last three years of the study, and <i>Enterobacter</i> spp. maintained relatively stable resistance (3-5%). Resistance in <i>Pseudomonas</i> spp. remained below 1%, while <i>Acinetobacter</i> spp. showed a resistance rate of 5.43%. Several distinct resistance phenotypes were identified among <i>Klebsiella</i> spp., <i>Pseudomonas</i> spp., and <i>Acinetobacter</i> spp. strains, reflecting both endemic and sporadic circulation patterns. The study highlights a persistent presence of colistin resistance, especially in <i>Klebsiella</i> spp., underlining the importance of ongoing surveillance. Despite low resistance in other species, the emergence of resistant strains underscores the need for robust antimicrobial stewardship and infection control policies.