Epidemiological characteristics and risk factors associated with bacteremia caused by intrinsically colistin-resistant gram-negative microorganisms: a case–control study

OBJECTIVE: To identify risk factors and clinical characteristics of bacteremia caused by intrinsically colistin-resistant Gram-negative microorganisms (ICRMs) in intensive care unit (ICU) patients and to compare them with Klebsiella pneumoniae bacteremia. METHODS: This retrospective case-control study was conducted in the ICUs of a tertiary-care hospital in Türkiye between January 2023 and February 2024. Adult patients with ICRM-positive blood cultures were compared with those with colistin-susceptible or colistin-resistant Klebsiella pneumoniae bacteremia, and demographic, clinical, and outcome data were analyzed. RESULTS: In total, 439 patients were included: 148 with ICRM bacteremia, 191 with colistin-susceptible K. pneumoniae bacteremia, and 100 with colistin-resistant K. pneumoniae bacteremia. Exposure to colistin or polymyxin B within the previous three months was independently associated with ICRM bacteremia (OR = 2.87, 95% CI = 1.29-6.39, p = 0.010). Polymicrobial bacteremia was identified in 20% of ICRM cases (30/148), most commonly involving Enterococcus spp. (7/30). New-onset BSIs within fourteen days of the initial bacteremia were more frequent in the ICRM group (16.9%, 25/148) than in both K. pneumoniae groups (p < 0.05), with K. pneumoniae as the most common pathogen (40%, 10/25). ICRM bacteremia cases were more often community-acquired or healthcare-associated and were associated with lower Sequential Organ Failure Assessment (SOFA) scores and mortality rates than both K. pneumoniae groups (each p < 0.05). CONCLUSION: ICRM bacteremia poses a growing threat in critically ill patients due to intrinsic and increasing acquired resistance. Prior colistin or polymyxin-B exposure was identified as an independent risk factor, and these infections were more often polymicrobial and community- or healthcare-associated than K. pneumoniae bacteremia. Given the limited ICU-focused evidence on ICRM bacteremia, these findings highlight prior polymyxin exposure as a potentially modifiable risk factor and support antimicrobial stewardship efforts. Therefore, empirical therapy in high-risk patients should consider potential ICRM involvement.