Introduction

The human oral cavity is a complex and dynamic ecosystem teeming with microorganisms, including bacteria, fungi, protozoa, archaea, and viruses. This microbial community, organized in biofilms or plaques, plays a crucial role in maintaining oral health and contributing to oral diseases. Among these microorganisms, streptococci are particularly significant due to their dominant presence and diverse roles.

Taxonomy of Oral Streptococci: Classification and Diversity

Streptococci are ubiquitous in the human body, predominantly colonizing the oral cavity and upper respiratory tract. Historically, the classification of streptococci, including oral streptococci, was based on hemolysis patterns on blood agar plates. These classifications included β-hemolysis (complete lysis), α-hemolysis (partial lysis), and γ-hemolysis (non-hemolytic). Further differentiation of streptococci required biochemical and physiological tests. Initial classification methods led to the identification of seven streptococcal groups based on carbohydrate fermentation, chain length, and growth on gelatin: S. salivarius, S. mitis, S. anginosus, S. equinus, S. pyogenes, S. faecalis, and S. pneumoniae. S. faecalis was later reclassified into the genus Enterococcus.

With the advent of DNA sequencing, bacterial identification at the species level became more precise. Sequencing of the 16S rRNA region allowed the determination of phylogenetic relationships, resulting in the oral streptococci being separated into four initial groups: anginosus, mitis, mutans, and salivarius. Streptococci isolated from outside the oral cavity were classified into either the bovis or pyogenic groups. A more robust phylogenetic approach later identified eight distinct streptococcal groups: mitis, sanguinis, anginosus, salivarius, downei, mutans, pyogenic, and bovis. Currently, oral streptococci are found in all groups except pyogenic and bovis.

Fig. 1 Phylogeny of the indicated streptococcal species derived from a core set of 136 concatenated genes (Abranches J.; et al. 2018).

Colonization Mechanisms of Oral Streptococci

Adhesion and Biofilm Formation

Oral streptococci are primary colonizers of the mouth, possessing multiple high-affinity adhesins that mediate binding to tooth surfaces via interactions with components of the acquired salivary pellicle. These adhesins include proteins from the antigen I/II family, serine-rich repeat glycoproteins, and glucosyltransferases (Gtfs). The production of extracellular polysaccharides and extracellular DNA further facilitates bacterial colonization and biofilm formation.

Coaggregation and Community Formation

Late colonizers, both Gram-positive and Gram-negative species, build upon the initial streptococcal foundation through coaggregation and coadhesion events until a mature plaque is established. Once established, the plaque community remains relatively stable, with different species existing in homeostasis under healthy conditions. Oral streptococci also engage in interactions with other microbial species and respond to environmental cues, including the presence of other microorganisms.

Dental Caries and Oral Streptococci

Dental caries, commonly known as cavities, is a multifactorial disease involving a carbohydrate-rich diet and oral microbial composition. Cariogenic bacteria like Streptococcus mutans and Streptococcus sobrinus play key roles in the development and progression of dental caries. In healthy individuals, commensal bacteria keep cariogenic populations in check through the production of alkali, hydrogen peroxide, and other inhibitory substances. However, frequent exposure to dietary carbohydrates creates a dysbiotic environment, leading to enamel demineralization and cavitation of the tooth surface.

The Good Streptococci: Commensal Oral Flora

In a healthy individual, the oral biofilm is dominated by commensal bacteria that engage in a dialog with the host immunity, maintaining homeostasis integral to human health. Commensal communities provide additional benefits by fending off pathogenic species. Dental health is often associated with a greater proportion of commensals like S. gordonii, S. sanguinis, and S. parasanguinis, which are associated with lower proportions of cariogenic S. mutans.

Conclusion

Understanding the interplay between oral microbiota and environmental factors is essential for advancing our approach to oral health and disease management. Streptococcal species, with their metabolic versatility and role in maintaining oral homeostasis, are central players in this complex ecosystem. Further research into the intricate dynamics of the oral microbiome will provide valuable insights into the prevention and treatment of oral diseases, ultimately contributing to improved overall health.