Sümeyye AKYÜZ, Mehmet PARLAK
New Trends in Medicine Sciences - 2026;7(1):9-15
Increasing resistance rates in multidrug -resistant Acinetobacter baumannii (MDR -AB) strains, a significant cause of hospital infections, necessitate alternative treatment strategies. In this study, we investigated the in vitro interactions of colistin (C) and the novel beta-lactam/beta -lactamase inhibitor combination ceftolozane/tazobactam (C/T) against MDR -AB isolates using E -test and checkerboard methods. Thirty MDR -AB isolates obtained from intensive care unit (ICU) patients were included in the study. Isolate identification was performed using the Becton Dickinson (BD) Phoenix(TM) 100 and VITEK(R) mass spectrometry (MS) systems. Routine antimicrobial susceptibility testing was performed with the BD Phoenix 100 system, and results were interpreted according to European Committee on Antimicrobial Susceptibility Testing (EUCAST) guidelines. In vitro interactions of the C/T -C combination were evaluated using the fractional inhibitory concentration (FIC) index. Agreement between the methods was analyzed using simple agreement ratios and Cohen's kappa coefficient. According to the FIC index results, while no synergistic interaction was observed with the E -test, the checkerboard method showed synergy in two isolates (6.7 %). Antagonistic interactions were detected in 20 % of isolates by the E -test, but not by the checkerboard method. Overall agreement between the two methods was 50 %, with a Cohen's kappa coefficient of 0.13, indicating poor agreement. Synergistic activity between C/T and C antibiotics was determined using the checkerboard method, and no antagonism was observed. However, since the detected low rates of synergy are limited to in vitro conditions, further in vitro and in vivo experimental studies are required to assess their clinical applicability.
