Efficacy of Metronidazole in experimental Bacteroides fragilis otitis media.

The efficacy of metronidazole in otitis media due to Bacteroides fragilis was evaluated in a guinea pig model. Fifty-nine animals received an injection of 10(8) live B. fragilis bacteria through the tympanic membrane into the right middle ear cavity and metronidazole therapy was started 7 days later. On day 14 after challenge the animals were sacrificed and their middle ears analysed. Intraperitoneal injection of 6 mg metronidazole (about 20 mg per kg) once daily, 6 mg twice daily and 15 mg (50 mg per kg) once daily reduced the incidence of culture positive ears from 13/17 among untreated controls to 7/17 (p = 0.08), 3/10 (p = 0.05) and 2/15 (p = 0.001), respectively. The therapeutic efficacy of metronidazole in experimental otitis media was less than that expected from the concentrations of drug recorded in serum and the drug levels presumed to be achieved in the middle ear effusion.