Phenotypic profile and molecular mechanism of resistance in carbapenemase-producing Enterobacterales and Pseudomonas aeruginosa isolates from Brazilian hospitals: implications for the introduction of imipenem-relebactam

Background/Objectives Carbapenemase-producing Enterobacterales and P. aeruginosa are critical threats to global public health, especially in high-burden regions such as Brazil. Imipenem-relebactam (IMR), a combination of a carbapenem with a β-lactamase inhibitor, is a promising treatment option against resistant Gram-negative bacteria. This study aimed to characterize phenotypic resistance and molecular mechanisms in clinical isolates from Brazilian hospitals and assess IMR activity. Methods A prospective multicenter study was conducted across 12 hospitals in Rio de Janeiro. A total of 150 Enterobacterale s and 100 P. aeruginosa isolates resistant to carbapenems were collected. Isolates were identified by MALDI-TOF and screened for carbapenemase genes (KPC, NDM, VIM, IMP, OXA-48) using PCR. Susceptibility to IMR was determined by broth microdilution following EUCAST guidelines. Next-generation sequencing (NGS) was performed on a subset of multidrug-resistant isolates. Results IMR resistance was identified in 34.5% of K. pneumoniae and 74% of P. aeruginosa isolates. Among Enterobacterales, 21.1% of KPC-producers and 88.9% of OXA-48-producers were resistant to IMR. The bla_KPC gene was predominant, but NDM was increasingly detected. In P. aeruginosa , resistance was largely unrelated to carbapenemase production, implicating porin loss and efflux pumps. NGS revealed extensive co-resistance and multiple virulence genes in K. pneumoniae isolates. Conclusion This study highlights the emergence of significant resistance to imipenem-relebactam in Brazil, driven by both enzymatic and non-enzymatic mechanisms. Ongoing molecular surveillance and tailored treatment strategies are essential to address the evolving threat of multidrug-resistant Gram-negative infections in endemic regions.