Investigation of synergistic, additive and antagonist effect of antimicrobial combinations used for Brucella spp., with etest combination method

The genus Brucella causing chronic infections via involving the reticuloendothelial system cells is a significant morbidity and sometimes mortality reason for human. In routine clinical practice, antimicrobial combinations consisting two or three antibiotics are used, however, there is less data about the in vitro activity of these combinations. In this study, in vitro activity of antimicrobial combiantions those are used against to Brucella spp. was aimed. A total 40 Brucella spp. strains were included in this study, which were isolated from blood cultures in Medical Microbiology Laboratory of Inonu University Tugut Ozal Medical Center between 2010 and 2012. In vitro activities of five antibiotics such as doxycycline, rifampin, cotrimoxazole, streptomycine and ciprofloxacin were studied with Etest strips. From these drugs, in vitro activities of five commonly-used combinations including doxycycline-rifampin, rifampin-cotrimoxazole, doxycycline-streptomycine, ciprofloxacin-cotrimoxazole and ciprofloxacin-streptomycine were investigated with Etest combination method. The obtained data was evaluated with FIC index, and synergistic, additive and antagonsit effects of these drugs were calculated. While the lowest MIC of the drug against to studied strains was detected as cotrimoxazole (as, 0.016 ?g/ml), the highest MIC of the drug was detected as rifampin (as, 1.5 ?g/ml). Regarding the MIC50 and MIC90 values, the most effective and less effective rank of the antibiotics was listed as fellows: cotrimoxazole 0.032 ?g/ml and 0.064 ?g/ml; doxycycline 0.064 ?g/ml and 0.094?g/ml; ciprofloxacin 0.094 ?g/ml and 0.75 ?g/ml; streptomycine 0.25 ?g/ml and 0.50 ?g/ml; and rifampin 0.50 ?g/ml and 0.75 ?g/ml. The combination of doxycycline-rifampin showed synergistic activity to all studied strains (%100), and the combination of rifampin-cotrimoxazole exhibited antagonist activity for two strains (5%). The ratio of synergistic activities of ciprofloxacin-streptomycine and rifampin-cotrimoxazole combinations were found as 57.5% and 52.5%, orderly. In this study, cotrimoxazole was found as the most active antibiotic, but rifampin was associated with the highest MIC levels. However, lower ratios of synergistic activity was observed with the combiantions of cotrimoxazole. In contrast, rifampin showed excellent synergistic activity to all studied strains, particularly in the combination of doxycycline. In this contex, in addition the antimicrobial susceptibility test performed for each antibiotic, it will be benefit to measure in vitro activity of the combinations against to brucella.