Metronidazole susceptibility factors in Actinobacillus actinomycetemcomitans

Actinobacillus actinomycetemcomitans is a facultative anaerobic bacterium that displays moderate susceptibility to metronidazole and this study was undertaken to identify the factors involved. A. actinomycetemcomitans appeared two to four times less susceptible to metronidazole when grown in air supplemented with 5% CO2 than under anaerobic conditions. Ferredoxin-linked pyruvate:oxidoreductase activity was absent but each strain exhibited nitroreductase activity which corresponded directly with uptake of metronidazole and susceptibility to the drug under anaerobic conditions but not in air supplemented with 5% CO2. Nitroreductase activity therefore appears responsible for the susceptibility of A. actinomycetemcomitans to metronidazole.