Current drugs are not always effective in severe anaerobic infections, especially those involving Bacteroides fragilis, even when the organism appears susceptible by in vitro testing. Five persons with severe and/or refractory anaerobic infections were treated with metronidazole administered either orally or intravenously. Of the four with Bacteroides fragilis group infections, three were bacteremic and two had not responded to prior treatment with clindamycin. In one patient aortic valvulitis was documented by a new murmur and echocardiographic findings. In all patients clinical and/or bacteriologic cures of their infections were attained without concomitant surgical procedures. In one patient removal of his penile prosthesis was required for complete clinical resolution. In all persons, metronidazole was tolerated well. Serum drug levels were measured in the bacteremic patients. New pharmacokinetic information is presented from patients treated intravenously by continuous and by intermittent infusion. In addition, a method of measuring serum bactericidal concentrations is described, and these measurements are correlated with both drug levels and responses to treatment. Serum bactericidal concentrations of ≥1:8 were associated with response whereas lower concentrations were associated with persistent infection. Metronidazole, which has rapid bactericidal activity in vitro against B. fragilis and other anaerobes, appears to be safe and effective intravenously as well as orally in the treatment of severe anaerobic infections.
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