Competition and Caries on Enamel of a Dual-species Biofilm Model of Streptococcus mutans and Streptococcus sanguinis.

Imbalances within the dental biofilm trigger dental caries, currently considered a dysbiosis and the most prevalent non-communicable disease. There is still a gap in knowledge about the dynamics of enamel colonization by bacteria from the dental biofilm in caries. The aim, therefore, was to test whether the sequence of enamel colonization by a typically commensal and a cariogenic species modifies biofilm's cariogenicity. Dual-species biofilms of Streptococcus mutans (Sm) and Streptococcus sanguinis (Ss) on saliva-coated enamel slabs were inoculated in different sequences: Sm followed by Ss (Sm-Ss), Ss followed by Sm (Ss-Sm), Sm and Ss inoculated at the same time (Sm=Ss) and the single-species controls Sm followed by Sm (Sm-Sm) and Ss followed by Ss (Ss-Ss). Biofilms were exposed to 10% sucrose, 3x/day for 5 days and the slabs/biofilms were retrieved to assess demineralization, viable cells, biomass, proteins, polysaccharides and H2O2 production. When compared with Sm-Sm, primary inoculation with Ss reduced demineralization (p<0.05). Both Ss-Sm and Sm=Ss sequences showed reduction in biomass, protein and polysaccharide content (p<0.05). The highest S.sanguinis viable cells and H2O2 production and the lowest acidogenicity were observed when Ss colonized enamel before Sm (p<0.05). Initial enamel adherence with commensal biofilms seems to induce more intense competition against more typically cariogenic species, reducing cariogenicity.Importance The concept of caries as an ecological disease implies the understanding of the intricate relationships among the populating microorganisms. Under frequent sugars exposure, some bacteria from the dental biofilm develop pathogenic traits that lead to imbalances (dysbiosis). Depending on which microorganism colonizes the dental surface first, different competition strategies may be developed. Studying the interactions in the entire dental biofilm is not an easy task. In this article, therefore, we modeled the interplay among these microorganisms using a caries-inducing (S. mutans) and a health-associated species (S. sanguinis). Initial enamel adherence with S. sanguinis seems to induce more intense competition against typically caries-inducing species. Besides continuous exposure with sugars, early colonization of the enamel by highly cariogenic species, like S. mutans, appears to be needed to develop caries lesions, as well. Promoting early colonization by health-associated bacteria, such as S. sanguinis, could help to maintain oral health, delaying dysbiosis.