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

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.

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.

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.

Combination of a Quinolone with doxycycline demonstrated good in vitro activity against B. melitensis. Further in vivo studies are necessary to support this suggestion.