In vitro Activity of Colistin in Combination with Tigecycline against Carbapenem-Resistant Acinetobacter baumannii Strains Isolated from Patients with Ventilator-Associated Pneumonia

Objective: This study investigated the minimum inhibitory concentration (MIC) values and in vitro activity of colistin in combination with tigecycline against carbapenem-resistant Acinetobacter baumannii strains isolated from patients with ventilator-associated pneumonia (VAP) using the E-test method.Methods: A total of 40 A. baumannii strains, identified using the Phoenix Automated Microbiology System (Becton, Dickinson and Co., Franklin Lakes, NJ, USA) by conventional methods, were included in this study.Pulsed-field gel electrophoresis was performed to examine the clonal relationships between isolates.The carbapenem resistance of the strains to colistin and tigecycline was assessed using the E-test method (Liofilchem, Roseto Degli Abruzzi, Italy).The in vitro activity of colistin in combination with tigecycline was evaluated using the fractional inhibitor concentration (FIC) index.Results: While only 1 of 40 A. baumannii strains was determined to be colistin resistant, 6 were tigecycline resistant.The MIC 50 , MIC 90 , and MIC intervals of the A. baumannii strains were 0.19, 1.5, and 0.064-4 μg/ml for colistin and 1, 8, and 0.094-256 μg/ml for tigecycline, respectively.No synergistic effect was observed using the FIC index; 8 strains exhibited an indifferent effect and 32 exhibited an antagonist effect.Three of the six strains that were resistant to tigecycline were indifferent; the remaining three were antagonistic.The colistin-resistant strain also exhibited an antagonist effect. Conclusion:In contrast to their synergistic effect against carbapenem-resistant A. baumannii isolates, colistin and tigecycline were highly antagonistic to carbapenem-resistant A. baumannii strains isolated from patients with VAP when the drugs were administered together.Therefore, alternative treatment options should be used during the treatment of VAP attributed to A. baumannii.