Microbiological landscape in critical care: 14-year analysis of 42,722 ICU patients using MIMIC-IV database

OBJECTIVES: Healthcare-associated infections affect nearly one-third of intensive care unit (ICU) patients and contribute substantially to mortality. Most surveillance systems overlook the diversity of organisms and site-specific resistance patterns that guide empirical therapy. We aimed to describe the distribution of pathogens, resistance trends, and related outcomes across ICUs in the largest single-institution analysis to date. METHODS: We conducted a retrospective cohort study using the MIMIC-IV database covering 2008 to 2022. The study included adult ICU patients with at least one positive microbiological culture. Organisms were grouped by infection site and ICU type. Resistance phenotypes-MRSA, VRE, ESBL, CRE, and MDR-were defined according to CDC criteria. Temporal trends were analyzed across five periods. Multivariable logistic and negative binomial regression models assessed independent predictors of hospital mortality, resistance, and length of stay, adjusting for resistance phenotype, infection site, ICU type, demographics, and time period. RESULTS: We identified 42,722 ICU patients with 1,151,165 positive cultures, representing 497 unique organisms and 2,306,527 microbiological records. Gram-negative organisms predominated: Escherichia coli (18.9%), Klebsiella pneumoniae (10.7%), and Pseudomonas aeruginosa (10.3%). Among gram-positive organisms, Staphylococcus aureus accounted for 15.0% of isolates. Vancomycin resistance among all Enterococcus tested was 59.6% (9871/16,573), far exceeding the 8-12% reported in national surveillance data. Other resistance rates included ESBL 28.5%, MDR 18.8%, MRSA 6.2%, and CRE 2.3%. Antimicrobial resistance declined from 30.4% (2008-2010) to 25.8% (2020-2022) overall, though organism-specific trajectories diverged: K. pneumoniae resistance increased by 14.5 percentage points while P. aeruginosa decreased by 13.9 points. The 40-65 age group had the highest MDR rate (8.0%, P < 10⁻³⁶), and females had higher E. coli prevalence consistent with UTI epidemiology. CONCLUSION: Gram-negative bacteria remain dominant in ICU infections, but gram-positive resistance-particularly VRE-has reached alarming levels. Variation across sites and units highlights the need for tailored empirical therapy. Implementing site- and unit-specific antibiograms could help reduce resistance and improve outcomes in critical care.