Activity of metronidazole and its hydroxy metabolite against clinical isolates of Actinobacillus actinomycetemcomitans

The in vitro susceptibility of Actinobacillus actinomycetemcomitans to metronidazole and its hydroxy metabolite was determined by the agar dilution method with 73 A. actinomycetemcomitans strains, recent clinical isolates from 13 patients with localized juvenile periodontitis. The minimal inhibitory concentrations (MICs) for 50% and for 90% of the isolates were, respectively, 16 μg/ml and 32 μg/ml of metronidazole, and 4 μg/ml and 8 μg/ml of the hydroxy metabolite. Although the MICs of the hydroxy metabolite were lower than those of the parent compound, the presently used dosage of metronidazole in treatment of periodontal diseases may not result in sufficiently high drug levels in gingival crevice fluid or in plasma to inhibit all A. actinomycetemcomitans strains. However, the known synergistic and additive effects of these compounds may increase the clinical efficacy of metronidazole in the treatment of periodontal infections associated with A. actinomycetemcomitans.