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.