Effect of Medium pH on Antibiotic Activity against Syrian Brucella spp. Isolates

Background: Brucellosis is an endemic zoonosis in Syria, affecting large numbers of animals. There are an increasing number of cases in humans. Brucella is a facultative intracellular pathogen, a small, non-motile, Gram-negative coccobacillus, which causes abortion in domestic animals and a febrile illness in humans. Methods: One hundred isolates collected from different Syrian regions were confirmed to be Brucella melitensis by biochemical tests. The minimum inhibitory concentration (MIC) of 6 antibiotics, alone and in combination, was determined at pH 7.0 and pH 5.0. Results: Ciprofloxacin and sparfloxacin were the most effective antibiotics tested at either pH value. In contrast, rifampicin had low activity and streptomycin was ineffective at either pH value. A combination of rifampicin-doxycycline revealed the highest synergistic activity at both test pH values (against 19/24 and 17/24 isolates, respectively) in vitro. Antagonistic activities were observed using a ciprofloxacin-streptomycin combination (against 9/24 and 13/24 isolates, respectively) as well as a ciprofloxacin-tetracycline combination (against 6/24 and 9/24 isolates, respectively). No differences were observed at both test pH values, when combining a Quinolone with rifampicin or doxycycline. Conclusion: Combination of a Quinolone with doxycycline demonstrated good in vitro activity against B. melitensis. Further in vivo studies are necessary to support this suggestion.