Oral mitis group streptococci reduce infectivity of influenza A virus via acidification and H2O2 production

<jats:p>Members of the mitis group streptococci are the most abundant inhabitants of the oral cavity and dental plaque. Influenza A virus (IAV), the causative agent of influenza, infects the upper respiratory tract, and co-infection with <jats:italic>Streptococcus pneumoniae</jats:italic> is a major cause of morbidity during influenza epidemics. <jats:italic>S</jats:italic>. <jats:italic>pneumoniae</jats:italic> is a member of mitis group streptococci and shares many features with oral mitis group streptococci. In this study, we investigated the effect of viable <jats:italic>Streptococcus oralis</jats:italic>, a representative member of oral mitis group, on the infectivity of H1N1 IAV. The infectivity of IAV was measured by a plaque assay using Madin-Darby canine kidney cells. When IAV was incubated in growing culture of <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic>, the IAV titer decreased in a time- and dose-dependent manner and became less than 100-fold, whereas heat-inactivated <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic> had no effect. Other oral streptococci such as <jats:italic>Streptococcus mutans</jats:italic> and <jats:italic>Streptococcus salivarius</jats:italic> also reduced the viral infectivity to a lesser extent compared to <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic> and <jats:italic>Streptococcus gordonii</jats:italic>, another member of the oral mitis group. <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic> produces hydrogen peroxide (H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>) at a concentration of 1–2 mM, and its mutant deficient in H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> production showed a weaker effect on the inactivation of IAV, suggesting that H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> contributes to viral inactivation. The contribution of H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> was confirmed by an inhibition assay using catalase, an H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>-decomposing enzyme. These oral streptococci produce short chain fatty acids (SCFA) such as acetic acid as a by-product of sugar metabolism, and we also found that the inactivation of IAV was dependent on the mildly acidic pH (around pH 5.0) of these streptococcal cultures. Although inactivation of IAV in buffers of pH 5.0 was limited, incubation in the same buffer containing 2 mM H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> resulted in marked inactivation of IAV, which was similar to the effect of growing <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic> culture. Taken together, these results reveal that viable <jats:italic>S</jats:italic>. <jats:italic>oralis</jats:italic> can inactivate IAV via the production of SCFAs and H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub>. This finding also suggests that the combination of mildly acidic pH and H<jats:sub>2</jats:sub>O<jats:sub>2</jats:sub> at low concentrations could be an effective method to inactivate IAV.</jats:p>