Friend or Foe? Studying Cariogenic Non-cariogenic Bacterial Interaction

Objectives: Biofilm formation represents the first and main step in the development and progression of the majority of oral disorders. Streptococcus mutans and Bacillus subtilis have been both extensively studied individually as model bacteria for robust biofilm formation. While S. mutans is considered a leading cause of dental disorders, B. subtilis is a non-pathogenic food borne bacterium, which recently has been isolated from the oral cavity though its role in dental biofilm remains unknown.
The aim of this research is to investigate the effect of the non-pathogenic bacterium B. subtilis on virulence factors of S. mutans.
Methods: S. mutans and B. subtilis cells were grown in co-culture under several environmental conditions such as different dietary sugars (sucrose vs. alcoholic sugars) and presence of hydrogen peroxide. The effect of the different conditions on bacterial interactions was determined by examining the generated biofilm using SEM, CLSM and biofilm biomass quantification. Furthermore, the adhesion force between the bacteria was tested using single cell force spectroscopy by AFM. Mutant strains in key enzymes were used to deepen our understanding on the mutual interaction.
Results: Visualization of the formed biofilm showed that sucrose promotes developed dual-species biofilm structure. A higher adhesion force between B. subtilis and S. mutans biofilm of 5-fold was found in comparison to interaction with B. subtilis biofilm. Furthermore, in the presence of hydrogen peroxide, B. subtilis contributed to the long-term endurance of the dual-species biofilm. However, when sucrose was replaced by alcoholic sugar, B. subtilis significantly decreased S. mutans' biofilm formation.
Conclusions: Overall, our results suggest that non-cariogenic bacteria may have significant role in biofilm formation and carcinogenicity of oral biofilm.