From infection to clinical cure: the role of colonization, molecular mechanisms and choice of antibiotic therapy in infections caused by multidrug-resistant Gram-negative bacilli

Antimicrobial resistance (AMR) represents one of the most urgent global public health threats, with multidrug-resistant Gram-negative bacilli (MDR-GNB) such as carbapenemase-producing Enterobacterales (CRE), carbapenem-resistant Acinetobacter baumannii (CRAB), and carbapenem-resistant Pseudomonas aeruginosa (CRPA) playing a leading role. This PhD project provides an integrated analysis of molecular resistance mechanisms, the clinical role of colonization, and the impact of novel therapeutic options through both single-center and multicenter observational studies. In the first study, conducted in intensive care units, 36.4% of patients colonized with CRAB developed bloodstream infections (BSI) caused by the same strain. Multivariable analysis identified burns, respiratory tract colonization, multisite carriage, and cardiovascular disease as independent risk factors for progression to infection. Based on these variables, a predictive score (AUC 0.82) was derived and validated, offering a pragmatic tool to guide early empirical therapy and minimize unnecessary use of anti-CRAB agents. A second prospective cohort including 343 patients with infections due to metallo-β-lactamase (MBL)-producing Enterobacterales, mostly NDM-producing Klebsiella pneumoniae, showed a 30-day mortality rate of 29.7%. The combination of ceftazidime/avibactam plus aztreonam was independently associated with reduced mortality compared with colistin-based regimens (aHR 0.39; p=0.02), defining its role as the treatment of choice for MBL-producing strains. In parallel, meropenem/vaborbactam demonstrated excellent activity against KPC-producing K. pneumoniae (clinical success 77%, 30-day mortality 15.4%), with high tolerability and favorable outcome even when used as monotherapy. The compassionate use of novel antimicrobials provided further insights into real-world management of highly resistant infections. Sulbactam/durlobactam achieved complete microbiological eradication in a patient with colistin- and cefiderocol-resistant CRAB, while eravacycline proved effective in a surgical-site infection caused by NDM-producing K. pneumoniae unresponsive to ceftazidime/avibactam plus aztreonam. Preliminary data from the national multicenter RESISTIMIT registry (1525 bloodstream infections) confirmed the high mortality associated with MBL-producing Enterobacterales (attributable mortality 14%) and CRAB (15%), highlighting early administration of active therapy and effective source control as the strongest modifiable predictors of survival. Overall, this work provides a comprehensive translational perspective on infections caused by MDR-GNB, demonstrating that the integration of rapid molecular diagnostics, understanding of resistance mechanisms, and prompt, mechanism-based antibiotic selection can substantially improve clinical outcomes.