To investigate further the involvement of the oxygen-dependent antimicrobial defense mechanism against systemic infection by anaerobic bacteria, we evaluated the susceptibility of Cybb−/− mice to intraperitoneal chamber inoculation with a lethal dose (100-μl volume, 1 × 109 CFU/ml) of P. gingivalis. Interestingly, we found that Cybb−/− mice were resistant to infection with either the wild-type or P. gingivalis Rbr− strains, as evidenced by the survival of all the mice following systemic challenge up to the end of the experiment (Figure 1). This evidence concerns the gene CYBB and infection.