Health-Associated Niche Inhabitants as Oral Probiotics: The Case of Streptococcus dentisani

Abstract

Oral diseases, including dental caries and periodontitis, are among the most prevalent diseases worldwide and develop as a consequence of a microbial dysbiosis. Several bacterial strains are being tested as potential oral health-promoting organisms, but usually they are species isolated from niches other than the site where they must exert its probiotic action, typically from fecal samples. We hypothesize that oral inhabitants associated to health conditions will be more effective than traditional, gut-associated probiotic species in key aspects such as colonization of the oral site where disease takes place or the possession of oral health promoting functions, as well as more practical issues like safety and toxicity, and establishing proper doses for administration. As an example of these active colonizers, we describe the case of Streptococcus dentisani, a new streptococcal species isolated from dental plaque of caries-free individuals. We have detected it in 98% of dental plaque samples from healthy individuals and, as expected, it does not produce any toxic secondary metabolite and does not survive a simulated stomach digestion, preventing potential secondary effects. Besides, this species has a double probiotic action, as it inhibits the growth of major oral pathogens through the production of bacteriocins, and also buffers acidic pH (the primary cause of dental caries) through an arginolytic pathway. We propose the use of S. dentisani as a promising probiotic against tooth decay.

Keywords: Streptococcus dentisani; arginolytic pathway; bacteriocins; dental caries; pH buffering; probiotics.

FIGURE 1

(A) Growth curves of the cariogenic bacteria Streptococcus mutans ATCC 25175 (squares) and S. sobrinus CECT 4034 (circles) in the presence (dotted lines) and absence (solid lines) of concentrated supernatant of S. dentisani strain 7746. (B) Growth curves of S. mutans ATCC 25175 in the presence of different size fractions of the 10× concentrated supernatant of S. dentisani 7746. Circles correspond to the fraction >10 KDa, squares to the 3–10 KDa fraction, and triangles to the fraction <3 KDa. For comparisons, S. mutans was grown in the presence of 10x concentrated BHI medium (dotted line). Means ± SD from three independent replicates are plotted.

FIGURE 2

Scanning electron microscopy (SEM) of three oral pathogens before (left) and after (right) a 30-min treatment with 10-fold concentrated supernatant of S. dentisani strain 7746. (A) Streptococcus mutans (pores are pointed with arrows). (B) Fusobacterium nucleatum. (C) Prevotella intermedia.

FIGURE 3

Inhibition of S. mutans by S. dentisani in the presence of peroxidase and proteinase. The tests were performed with S. dentisani strains 7746 and 7747, treating with the enzymes the spots where S. mutans would later be grown. The controls on the left panel show the inhibition without treatment.

FIGURE 4

Growth curves of S. dentisani strain 7746 in a medium with (triangles) and without (circles) the addition of 5 g/l of arginine. The mean ± SD of O.D. ₆₁₀ (solid lines) and pH of the culture (dotted lines) from three replicates are depicted.

FIGURE 5

Potential buffering effect in the oral cavity due to the enrichment of the dental plaque bacterial community with Streptococcus dentisani. When arginine is available (either as an added prebiotic or as a product of peptide degradation) and S. dentisani is established on the tooth surfaces, the pH drop caused by the accumulation of organic acids can be buffered by the production of NH₄⁺ through the arginolytic pathway, maintaining the pH close to neutrality (right panel). In the absence of S. dentisani, or if present at low proportions, the buffering effect would be diminished, with the consequent acidification and enamel damage. This would be the perfect scenario for the proliferation of acidogenic/cariogenic microorganisms (left panel).