Evaluation of the synergistic effect of a combination of colistin and tigecycline against multidrug-resistant Acinetobacterbaumannii.
Güner, Müberra Devrim
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Objective: Acinetobacter baumannii species cause nosocomial infections and can subsequently develop multidrug resistance (MDR). The objective of this study was to evaluate the susceptibility of A. baumannii to a novel combination of colistin and tigecycline, which may provide a faster and more efficacious treatment via a synergistic effect. Methods: We included 50 MDR A. baumannii samples that were isolated in our clinics between 2009 and 2014. We used broth microdilution (BMD) and the E-test to evaluate the effects of colistin and tigecycline, and the E-test to assess the interaction of the colistin-tigecycline combination. The interaction between the two antibiotics was evaluated using the fractional inhibition concentration (FIC) index and was classified as follows: FIC?0.5, synergistic; 0.5<FIC<1, partially synergistic; FIC=1, additive; 1<FIC<4, indifferent; and FIC?4, antagonistic. Results: No tigecycline and colistin resistance was determined by BMD or E-test. The interaction between colistin and tigecycline, when used in combination, was 2% synergistic, 6% additive, 88% indifferent, and 4% antagonistic. Conclusion: Although combination therapy is suggested for MDR A. baumannii infections, our results suggest that the synergistic effect of the colistin-tigecycline combination is insufficient to make it an optimal treatment choice.