Involvement of NADH Oxidase in Competition and Endocarditis Virulence in Streptococcus sanguinis

Here we report for the first time that the Streptococcus sanguinis nox gene encoding NADH oxidase is involved in both competition with Streptococcus mutans and virulence for infective endocarditis. A S. sanguinis nox mutant was found to fail to inhibit the growth of Streptococcus mutans under microaerobic conditions. In the presence of oxygen, the recombinant Nox protein of S. sanguinis could reduce oxygen to water and oxidize NADH to NAD + . The oxidation of NADH to NAD + was diminished in the nox mutant. The nox mutant exhibited decreased levels of extracellular H 2 O 2 ; however, the intracellular level of H 2 O 2 in the mutant was increased. Furthermore, the virulence of nox mutant was attenuated in a rabbit endocarditis model. The nox mutant was also shown to be more sensitive to blood killing and oxidative and acid stresses and reduced growth in serum. Thus, NADH oxidase contributes to multiple phenotypes related to competitiveness in the oral cavity and systemic virulence.