[In vitro activity of an ofloxacin-metronidazole combination against anaerobic bacteria. Kinetics of the action of metronidazole against Bacteroides fragilis].

The activity of metronidazole combined with ofloxacin was investigated by the checkerboard method in liquid medium against 60 obligate anaerobes. The bacteriostatic effect of the combination was assessed by calculating the FIC index. Two metronidazole resistant strains of Propionibacterium acnes (MIC greater than 32 mg/l) were inhibited by 0.125 mg/l of this former antibiotic in presence of an ofloxacin concentration equal to half the value of the MIC. On the 58 other anaerobic strains, the combination of metronidazole plus ofloxacin had an additive bacteriostatic effect (30 strains) or a synergistic effect (26 strains). No antagonism was noted with any strain. For selected anaerobic or mixed infections the combination of metronidazole and ofloxacin may be useful. Killing curves demonstrated that, under good anaerobic conditions, metronidazole acted rapidly against Bacteroides fragilis. At a concentration of 8 mg/l, a decrease of log10 of the population was observed after 2 hours. After a contact ranging from 1 to 8 hours, depending of the investigated strains, a bactericidal effect was observed with metronidazole.