Hydrogen Peroxide Contributes to the Epithelial Cell Death Induced by the Oral Mitis Group of Streptococci

Members of the mitis group of streptococci are normal inhabitants of the commensal flora of the oral cavity and upper respiratory tract of humans. Some mitis group species, such as Streptococcus oralis and Streptococcus sanguinis, are primary colonizers of the human oral cavity. Recently, we found that hydrogen peroxide (H₂O₂) produced by S. oralis is cytotoxic to human macrophages, suggesting that streptococcus-derived H₂O₂ may act as a cytotoxin. Since epithelial cells provide a physical barrier against pathogenic microbes, we investigated their susceptibility to infection by H₂O₂-producing streptococci in this study. Infection by S. oralis and S. sanguinis was found to stimulate cell death of Detroit 562, Calu-3 and HeLa epithelial cell lines at a multiplicity of infection greater than 100. Catalase, an enzyme that catalyzes the decomposition of H₂O₂, inhibited S. oralis cytotoxicity, and H₂O₂ alone was capable of eliciting epithelial cell death. Moreover, S. oralis mutants lacking the spxB gene encoding pyruvate oxidase, which are deficient in H₂O₂ production, exhibited reduced cytotoxicity toward Detroit 562 epithelial cells. In addition, enzyme-linked immunosorbent assays revealed that both S. oralis and H₂O₂ induced interleukin-6 production in Detroit 562 epithelial cells. These results suggest that streptococcal H₂O₂ is cytotoxic to epithelial cells, and promotes bacterial evasion of the host defense systems in the oral cavity and upper respiratory tracts.