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

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 Enterobacterales and 8294 non-fermenting Gram-negative bacilli. The percentage of colistin-resistant strains among those analyzed was 3.98%. Species-specific analysis focused on Klebsiella spp., Escherichia coli, Enterobacter spp., Citrobacter spp., Pseudomonas spp., and Acinetobacter spp. Klebsiella 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 E. coli was low (0.18-1.25%). Citrobacter spp. showed no resistance in the last three years of the study, and Enterobacter spp. maintained relatively stable resistance (3-5%). Resistance in Pseudomonas spp. remained below 1%, while Acinetobacter spp. showed a resistance rate of 5.43%. Several distinct resistance phenotypes were identified among Klebsiella spp., Pseudomonas spp., and Acinetobacter spp. strains, reflecting both endemic and sporadic circulation patterns. The study highlights a persistent presence of colistin resistance, especially in Klebsiella 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.