Investigation of the Synergic Effect of the Colistin/Sulbactam Combination in Carbapenem-Resistant Acinetobacter baumannii complex Strains with Time-Kill and Checkerboard Methods

Introduction: Infections caused by carbapenem-resistant Acinetobacter strains have become very common in recent years, and the most frequently used medicinal treatment is colistin. Combination treatments should also be applied to prevent development of resistance to colistin. This study examines the in vitro synergic effect of the colistin/sulbactam combination in carbapenem-resistant Acinetobacter strains with the time-kill and checkerboard methods. Materials and Methods: Twenty carbapenem-resistant Acinetobacter baumannii-calcoaceticus complex strains, which were isolated from various clinical samples, were included in this study. Strains were identified with mass spectrometry, and antibiotic sensitivity results were determined with the VITEK 2 (R) system. The in vitro effect and synergic activity of the colistin, sulbactam, and colistin/sulbactam combination on the carbapenem-resistant strains were determined using the time-kill and checkerboard methods. Seventeen strains were examined with the time-kill method, and twenty strains were examined using the checkerboard method. The fractional inhibitory concentration index of strains was calculated for detection of synergic effect. Results: Using the time-kill method applied on the colistin/sulbactam combination showed that the combination had a synergic effect on all 17 strains, while sulbactam alone did not have a bactericidal effect in the studied concentrations. When applying the checkerboard method, it was determined that the colistin/sulbactam combination had a synergic effect on 17 of the strains (85%) and an additive effect on 3 strains (15%), sulbactam had a low effect alone (15%), and colistin was effective on all strains. Conclusion: Study results indicated that the colistin/sulbactam combination had a high level of synergic effect on all studied strains using both methods.