Background: Infections that are caused by multi drug resistant (MDR) Acinetobacter baumannii strains are very common. There are difficulties in treatment of these infections and colistin remains the most important option. It is advised to use colistin in combination with other antibiotics instead of single use. In this study, the synergistic effects of colistin and sulbactam combination in MDR Acinetobacter baumannii strains by diagonal gradient strip test method was investigated and a comparison with the checkerboard method was done.
Methods: In this study, 15 MDR Acinetobacter baumannii strains were evaluated. The synergistic effect of colistin and sulbactam combination on these strains was investigated by the diagonal strip test (strips were placed at a 90° angle) and checkerboard method. For the determination of synergy, the fractional inhibitory concentration index (FICI) of all strains were calculated.
Results: In the diagonal gradient strip test and checkerboard method, sulbactam’s combination MIC values decreased in all strains. Following the FIC index calculations, the outcomes were: in the gradient strip method there was a synergistic effect in 3 strains (20%), in 8 strains (53.3%) there was an additive effect, and in 4 strains (26.7%) an indifferent effect was determined. None of the strains showed an antagonist effect. In the checkerboard method, 14 strains (93.3%) showed a synergistic effect and an additive effect in 1 strain (6.7%).
Conclusions: In this study, the colistin/sulbactam combination is determined to be 20% synergistic and 60% partial synergistic effective of MDR Acinetobacter baumannii isolates. Diagonal gradient strip test method was used for the first time in this study to research the in vitro synergistic effect of colistin/sulbactam combination on MDR Acinetobacter baumannii isolates. There is a need for larger scale studies about this topic. Although there are some limitations in guidelines related to colistin susceptibility for gradient strip test, it is a method that can be used for other antibiotic combinations. For more accurate and precise results, it would be useful to plan in vitro and in vivo studies concurrently.