Distribution of Carbapenemases and Resistance to Last-Line Drugs in Gram-Negative Bacilli Recovered From Patients Hospitalized in Tertiary Hospitals in Southern Brazil

Resistance to carbapenems among non-fermenting gram-negative bacilli and enterobacteria represents a growing challenge in hospital clinical practice, especially in high-complexity institutions. This study aimed to describe the local epidemiology of carbapenem-resistant Enterobacterales, Pseudomonas aeruginosa, and Acinetobacter baumannii in a hospital group in Porto Alegre, focusing on the diversity of carbapenemases, resistance to polymyxin B and tigecycline, and analysis of synergy between ceftazidime-avibactam (CZA) and aztreonam (ATM). Retrospective study based on laboratory data collected between December 2024 and March 2025 from the central laboratory of a group composed of four geographically distinct institutions: Hospital A (875 beds), a general hospital with outpatient care; Hospital B (237 beds), specialized in trauma; Hospital C (166 beds), focused on women’s health; and an Emergency Care Unit (17 beds). Bacterial identification and susceptibility testing were performed according to standardized protocols. Interpretation followed BrCAST guidelines. Carbapenemase detection was performed using enzymatic inhibition tests and/or immunochromatographic assays for KPC, NDM, OXA-48, VIM, and IMP. A total of 357 carbapenem-resistant gram-negative bacilli strains were analyzed: 189 Enterobacterales (52.9%), 67 P. aeruginosa (18.8%), and 101 A. baumannii (28.3%). Among Enterobacterales, KPC was the most frequent carbapenemase (55.5%), followed by metallo-β-lactamases (33.3%) and coproduction of KPC and NDM (10%). In P. aeruginosa, NDM expression predominated (43.3%). Resistance to polymyxin B was observed in 27% of Enterobacterales, 7.5% of P. aeruginosa, and 18.8% of A. baumannii. Among 95 A. baumannii isolates, 49.5% showed minimum inhibitory concentrations >0.5 µg/mL for tigecycline. For 41 metallo-β-lactamase-producing Enterobacterales, synergy between CZA and ATM was observed in 100% of cases. The data reiterate the concerning occurrence of multidrug resistance among gram-negative bacilli in the evaluated hospitals. The significant presence of resistance to polymyxin B, an antibiotic often used as a last resort, reinforces the need for continuous surveillance and strict infection control measures. The CZA/ATM synergy represents a promising alternative, especially for Enterobacterales.