Streptococcus oralis maintains homeostasis in supragingival biofilms by antagonizing cariogenic pathogen Streptococcus mutans

Summary

Bacteria residing in oral biofilms live in a state of dynamic equilibrium with one another. The intricate synergistic or antagonistic interactions between them are crucial for determining this balance. Using the 6-species Zürich "supragingival" biofilm model, this study aimed to investigate interactions regarding growth and localization of the constituent species. As control, an inoculum containing all six strains was used, whereas in each of the further five inocula one of the bacterial species was absent, and in the last both streptococci were absent. Biofilms were grown anaerobically on hydroxyapatite discs, and after 64 h they were harvested and quantified by culture analyses. For visualization, fluorescence in situ hybridization and confocal laser scanning microscopy was used. Compared to the control, no statistically significant difference of total CFU was observed in the absence of any of the biofilm species, except for F. nucleatum whose absence caused a significant decrease in total bacterial numbers. Absence of S. oralis resulted in a significant decrease in A. oris, and increase in S. mutans (p<0.001). Absence of A. oris, V. dispar or S. mutans did not cause any changes. The structure of the biofilm with regards to the localization of the species did not result in observable changes. In summary, the most striking observation was that absence of S. oralis resulted in limited growth of commensal A. oris and overgrowth of S. mutans. This data establishes S. oralis as commensal keeper of homeostasis in the biofilm by antagonizing S. mutans, thus preventing a caries-favoring dysbiotic state.

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