In Vitro Activity of Eravacycline, Aztreonam–Avibactam, and Cefiderocol Against Multidrug-Resistant Gram-Negative Clinical Isolates from a Romanian Tertiary Hospital

Background/Objectives: Carbapenem-resistant Gram-negative bacteria—especially NDM- and OXA-48-producing Klebsiella pneumoniae and carbapenem-resistant Acinetobacter baumannii (CRAB)—have limited treatment options, and in vitro data from Eastern Europe on the reserve agents eravacycline, aztreonam–avibactam, and cefiderocol are scarce. We evaluated their in vitro activity against consecutive multidrug-resistant (MDR) Gram-negative isolates from a Romanian tertiary hospital, stratified by species and carbapenemase profile. Methods: Seventy-seven consecutive, non-duplicate MDR Gram-negative isolates were collected over a 12-week period (January–March 2026) at a single tertiary center and tested by gradient diffusion (Liofilchem MIC Test Strip). MTS was used as an exploratory screening method; reference broth microdilution (BMD) remains the standard for categorical susceptibility confirmation. MICs were interpreted using FDA and EUCAST v16.0 breakpoints. Carbapenemases were detected by immunochromatography (18 isolates), and carbapenemase-encoding genes were detected by Xpert Carba-R PCR (40 isolates of K. pneumoniae). Susceptibility is reported with Wilson 95% confidence intervals. Results: K. pneumoniae (n = 51) predominated; NDM+OXA-48 was the most common profile (43.1%), and NDM was present in 64.7%. Eravacycline inhibited 90.2% of K. pneumoniae, and aztreonam–avibactam inhibited 92.2%, both covering all NDM-only isolates (9/9). Cefiderocol inhibited 84.3% of K. pneumoniae overall, but only 6/9 (66.7%) of NDM-only producers versus 20/22 (90.9%) of NDM+OXA-48 co-producers, whereas eravacycline and aztreonam–avibactam remained active against all nine NDM-only isolates. Cefiderocol was the only agent active against P. aeruginosa (91.7%); aztreonam–avibactam showed uniformly high MICs against non-fermenters, and all 10 CRAB isolates had low eravacycline MICs (MIC50 0.064 µg/mL). Conclusions: The three agents were complementary: aztreonam–avibactam and eravacycline covered NDM-producing K. pneumoniae, whereas cefiderocol was less active in vitro; cefiderocol alone covered P. aeruginosa. Because MICs were generated by gradient diffusion, these exploratory benchmarks require confirmation by reference broth microdilution; gradient strips agree well with BMD for aztreonam–avibactam but show weaker, agent-dependent agreement for eravacycline and cefiderocol, so they are not yet a validated substitute across all three agents.