Gram-negative bacilli resistant to carbapenems (GNB-CR) are of critical priority, according to the WHO, for research and development of new antibiotics. Most of the new β-lactams with β-lactamase inhibitors are not effective for the treatment of infections caused by NDM-producing GNB-CR, which have emerged in different countries, including Brazil. Polymyxin B (POLB), despite its undeniable toxicity, still remains a therapeutic option, but some bacteria are intrinsically resistant, imposing additional/insurmountable therapeutic difficulties. Our objective was to evaluate the distribution of carbapenemases and susceptibility to ceftazidime-avibactam (CZA) and POLB among GNB resistant (R) or intermediate (I – susceptible, increased exposure) to meropenem (MEM).

A retrospective study was conducted with bacteria recovered from blood cultures of patients from a tertiary hospital in southern Brazil (11/2024-05/2025). They were identified by MALDI-TOF; susceptibility to MEM and CZA was assessed by disk diffusion (BrCAST). The MIC of POLB was determined by broth microdilution (BrCAST). The presence of carbapenemase was evaluated by NG-Test® CARBA-5 (NDM, IMP, VIM, OXA-48, KPC) and/or by HRM-qPCR (blaKPC, blaGES, blaNDM-1, blaIMP, blaVIM and blaOXA-48-like).

During the study, 514 GNB R or I to MEM were recovered, with 13.2% (68/514) from blood cultures, mostly K. pneumoniae (45.6%, 31/68), P. aeruginosa (17.6%) and A. baumannii (10.3%). E. coli (8.8%), Enterobacter sp. (5.9%), S. marcescens (4.4%), K. oxytoca (2.9%), M. morganii (2.9%) and P. mosselii (1.5%) were also recovered, ensuring diversity. Among Enterobacterales (n = 48), 10 were MEM I and 38 (79.2%) R. All non-fermenters (n = 20) were MEM R. Susceptibility to CZA was assessed for all isolates, except A. baumannii and P. mosselii, with 55% (33/60) R. Of these, 56.5% were K. pneumoniae (n = 13) and P. aeruginosa (n = 10), all NDM producers, except 1 S. marcescens blaKPC. Among CZA R isolates, 24.2% (8/33) were also resistant to POLB, including 1 S. marcescens and 2 M. morganii, intrinsically resistant.

Treating severe infections caused by GNB-CR is challenging, especially considering the spread of NDM-producing isolates. Therefore, microbiological surveillance programs, infection control and rational use of antibiotics should be strengthened, in addition to continued investment in therapeutic and diagnostic strategies.