Beyond Colonization: Dynamic Risk Stratification and Clinical Decision-Making for MDR Gram-Negative Infections in Critically Ill Patients.

Background: Colonization with multidrug-resistant (MDR) Gram-negative bacteria is common in critically ill patients, yet the meaning of a positive surveillance culture is not always clear at the bedside. For some patients, colonization remains asymptomatic; for others, it may represent the first step toward an invasive infection that can develop rapidly. Objectives: This review examines the transition from colonization to infection in intensive care unit (ICU) patients carrying MDR Pseudomonas aeruginosa, carbapenem-resistant or extended-spectrum β-lactamase-producing Enterobacterales, and carbapenem-resistant Acinetobacter baumannii (CRAB). Discussion: Across these pathogens, colonization should not be viewed as an isolated microbiological finding. Its clinical relevance depends on the organism involved, site and persistence of carriage, extent of colonization, prior antibiotic exposure, severity of illness, invasive devices, impaired host defences, and the epidemiological pressure within the ICU. Intestinal carriage of resistant Enterobacterales is particularly relevant because it may act as an endogenous reservoir for subsequent invasive infection. In patients colonized with CRAB, respiratory and multisite carriage appear to identify those at greatest risk of pneumonia and poor outcomes. For Pseudomonas aeruginosa, the relationship is less uniform but becomes more relevant in the presence of respiratory vulnerability, ICU exposure, and previous broad-spectrum antibiotic use. Conclusions: Colonization results should therefore support risk stratification, infection-prevention measures, and informed empirical treatment when infection is suspected, rather than automatically leading to antibiotic therapy.