Phenotypic co-occurrence of resistance to aminoglycosides and fluoroquinolones in clinical isolates of Acinetobacter baumannii

Background and Aim: Acinetobacter baumannii is a major nosocomial pathogen associated with multidrug resistance, limiting therapeutic options in hospital settings. This study aimed to investigate antimicrobial susceptibility patterns and the co-occurrence of resistance to aminoglycosides and fluoroquinolones among clinical isolates of A. baumannii recovered from a tertiary hospital in Shahroud, Iran. Methods: In this descriptive cross-sectional study, six non-duplicate clinical isolates of A. baumannii were collected from hospitalized patients. Species identification was confirmed by phenotypic methods and PCR detection of the intrinsic blaOXA-51-like gene. Antimicrobial susceptibility testing was performed using the Kirby–Bauer disk diffusion method and interpreted according to CLSI M100 (2024) guidelines. Data were analyzed descriptively, and hierarchical clustering was used for exploratory visualization of resistance patterns. Results: All isolates (6/6, 100%) were classified as multidrug-resistant (MDR), and two isolates (2/6, 33.3%) met the criteria for extensively drug-resistant (XDR). All isolates (6/6, 100%) were resistant to ciprofloxacin. High resistance rates were observed among aminoglycosides, including amikacin (5/6, 83.3%), gentamicin (5/6, 83.3%), and tobramycin (5/6, 83.3%). Resistance to ceftazidime was observed in 5/6 isolates (83.3%), while resistance rates for cefepime, piperacillin–tazobactam, and ampicillin–sulbactam were 3/6 (50.0%) each. Resistance to carbapenems was detected in 4/6 isolates (66.7%) for meropenem and 3/6 (50.0%) for imipenem. Concurrent resistance to fluoroquinolones and aminoglycosides was observed across all isolates, indicating co-occurrence of resistance phenotypes. Conclusion: The present study demonstrates a high burden of multidrug resistance and frequent co-occurrence of resistance to aminoglycosides and fluoroquinolones among clinical isolates of A. baumannii. These findings highlight the need for continued regional antimicrobial resistance surveillance and strengthened infection control strategies.