Investigation of Synergistic, Additive and Antagonist Effect of Antimicrobial Combinations Used for Brucella spp., with E-test Combination Method

INTRODUCTION: The genus Brucella, which causes permanent infections in the reticuloendothelial system, is a significant cause of morbidity and mortality in humans. Clinically, a combination of two or three antibiotics is usually used for the treatment of brucellosis, but there is not enough information about the in vitro efficacy of these combinations.
METHODS: The in-vitro activity of the combination of doxycycline-rifampin, rifampin-cotrimoxazole, doxycycline-streptomycin, ciprofloxacin-cotrimoxazole and ciprofloxacin-streptomycin, which are most frequently used in clinical practice, was investigated by E-test combination method.
RESULTS: The results were evaluated using the Fractional Inhibitor Concentration Index and synergistic, additive and antagonist effects of these drugs were calculated. While the lowest MIK of the drug against to studied strains was detected as cotrimoxazole (as, 0.016 µg/ml), the highest MIK of the drug was detected as rifampin (as, 1.5 µg/ml). The combination of doxycycline-rifampin showed a synergistic activity to all tested strains (%100), and the combination of rifampin-cotrimoxazole exhibited antagonist activity to two strains (5%). The ratio of synergistic activities of ciprofloxacin-streptomycine and rifampin-cotrimoxazole combinations were found as 57.5% and 52.5%, orderly. The synergistic effect rates of combinations of doxycycline-streptomycin and ciprofloxacin-cotrimoxazole were 32.5% and 25%, respectively. The combination of rifampin with doxycycline showed a excellent synergistic effect against all strains.
DISCUSSION AND CONCLUSION: When clinical treatment is planned, testing the effectiveness of the antimicrobial combination will be beneficial in order for the treatment to be successful.In this context, we believe that in vitro detection of combination effects and single antibiotic susceptibility in the antimicrobial susceptibility tests against Brucella species would be beneficial.