In vitro susceptibility of Brucella melitensis to antibiotics.

The in vitro susceptibilities of 86 recent clinical isolates of Brucella melitensis to minocycline, streptomycin, co-trimoxazole, rifampin, and six fluoroquinolones were determined. Minocycline exhibited the lowest MIC and was followed by rifampin and streptomycin. Among the quinolones, WIN 57273 and ciprofloxacin were the most active agents. No antibiotic combination of these agents exhibited synergy against 15 selected isolates. In killing rate experiments, streptomycin exhibited the most rapid kill (less than 12 h), while a complete kill with minocycline, rifampin, and ciprofloxacin was delayed up to 48 h. The combinations of streptomycin with each of minocycline, rifampin, or ciprofloxacin exhibited the fastest kills (within 2 h), while with the other combinations, a complete kill was delayed up to 96 h. These results demonstrate the discrepancy between the results of various in vitro methods in evaluating the antibiotic susceptibility of B. melitensis.