In Vitro Activity of Cefiderocol, Eravacycline, and Imipenem–Relebactam Against Multidrug-Resistant Acinetobacter baumannii Clinical Isolates

<b>Background/Objectives</b>: <i>Acinetobacter baumannii</i> is a leading cause of healthcare-associated infections and is frequently associated with multidrug resistance, severely limiting therapeutic options. The increasing prevalence of carbapenem-resistant <i>A. baumannii</i> (CRAB) has intensified interest in novel antimicrobial agents such as cefiderocol, eravacycline, and imipenem-relebactam. <b>Methods</b>: A total of 80 multidrug-resistant (MDR) <i>A. baumannii</i> isolates recovered from various clinical specimens between April 2019 and October 2023 were included. Antimicrobial susceptibility testing was performed using disk diffusion, gradient test, and broth microdilution methods in accordance with EUCAST and CLSI recommendations. The minimum inhibitory concentrations (MIC's) for cefiderocol were evaluated with broth microdilution using iron-depleted cation-adjusted Mueller-Hinton broth as the reference method. The presence of carbapenem resistance-associated genes (<i>bla</i>OXA-23, <i>bla</i>OXA-24, <i>bla</i>OXA-51, <i>bla</i>OXA-58, <i>bla</i>IMP, and <i>tetA</i>) was investigated by polymerase chain reaction. <b>Results</b>: All isolates were resistant to imipenem and meropenem. Colistin resistance was detected in 7.5% of isolates. According to EUCAST criteria, cefiderocol susceptibility was observed in 77.5% of isolates by microdilution and in 81.25% by disk diffusion. Eravacycline demonstrated low MIC values, with MIC₅₀ and MIC₉₀ of 0.25 mg/L and 0.75 mg/L, respectively. In contrast, all isolates were resistant to imipenem-relebactam. The <i>bla</i>OXA-23 gene was detected in 82.5% and <i>bla</i>OXA-24 in 17.5% of isolates, while no <i>bla</i>OXA-58, <i>bla</i>IMP, or <i>tetA</i> genes were identified. No statistically significant association was found between cefiderocol resistance and OXA-type carbapenemase genes. <b>Conclusions</b>: Cefiderocol and eravacycline demonstrated promising in vitro activity against MDR <i>A. baumannii</i>, including colistin-resistant isolates, whereas imipenem-relebactam showed no activity. These findings support the potential role of cefiderocol and eravacycline as alternative treatment options for CRAB infections and highlight the multifactorial nature of cefiderocol resistance beyond OXA-type carbapenemase production.