Ornidazole and anaerobic bacteria: in vitro sensitivity and effects on wound infections after appendectomy.

The sensitivities of 68 clinical isolates of Bacteroides fragilis, 18 of Clostridium perfringens, and 11 of other Clostridium species were tested against ornidazole alone and in combination with ampicillin and gentamicin. A concentration of 3.1 microgram of ornidazole/ml inhibited 98% of the strains of B. fragilis, with greater sensitivity when ampicillin and gentamicin were also present. A concentration of 6.2 microgram of ornidazole/ml inhibited 16 of 18 strains of C. perfringens and all 11 strains of other Clostridium species. Concentrations in serum and tissue were determined after intravenous infusion of 500 mg of ornidazole 15 min prior to appendectomy. During the operation the concentration in serum was 7.90 +/- 0.57 microgram/ml, and in appendix tissue, 5.26 +/- 0.60 microgram/g. In the series of 200 patients undergoing appendectomy, six patients treated with ornidazole and 12 patients treated with placebo developed a wound infection. In patients with perforated appendix, the rate of wound infection was 7.1% in those given ornidazole and 63.6% in those given placebo (P = 0.004). Not a single B. fragilis was isolated from appendix swabs or wound exudates after prophylaxis with ornidazole.