Susceptibility of Brucella melitensis to fluoroquinolones

In vitro activity of four fluoroquinolones and four other antibacterial agents was tested against clinical isolates of Brucella melitensis. Initially all the 146 isolates studied were inhibited by 0.06-0.5 mg/l of ciprofloxacin and fleroxacin and 0.12-0.5 mg/l of pefloxacin or norfloxacin. One of these isolates developed resistance during therapy with ciprofloxacin, with a rise in MIC from 0.06 mg/l to more than 5.0 mg/l. This strain also showed cross-resistance to all other quinolones. All the isolates remained susceptible to tetracycline, gentamicin, rifampicin and trimethoprim-sulfamethoxazole. None of the quinolones showed in vitro synergy with other antibiotics.