Objective: Today, plant-based active agents with antimicrobial activity are being investigated by many researchers to overcome the problem of resistant bacteria. In this study, the activity of carvacrol and colistin combination against colistin- resistant Acinetobacter baumannii strains were evaluated.

Method: In order to determine the antimicrobial activity of carvacrol and colistin against eight A. baumannii strains, the minimum inhibitory concentrations (MIC) were determined by broth microdilution method. Combined activities of colistin and carvacrol were studied with Checkerboard Synergy Test.

Results: In the study, the antimicrobial activity of carvacrol against eight A. baumannii strains tested was determined within the range of MIC values of 64-128 μg/ml. As a result of the combined activity of colistin and carvacrol, the synergistic effect was found in five of eight strains. The FICI values were 0.5 for two strains, 0.375 for the other two strains and 0.25 for one. Additive effects, and FICI values of 1, 0.625 and 0.53, were obtained in the other three A. baumannii strains.

Conclusion: According to our results, carvacrol showed antibacterial activity against A. baumannii strains. When combined with colistin, the synergistic effect was determined in five of the eight strains and the additive effect was found in the remaining three. In other words, carvacrol caused a decrease in colistin resistance by increasing antibacterial activity of colistin.