Safety assessment of Streptococcus salivarius DB-B5 as a probiotic candidate for oral health.

Streptococcus salivarius DB-B5 was previously isolated from the supragingival plaque of a healthy female adult and selected for development as a probiotic candidate for oral health. Probiotics are an important emerging therapeutic method for preventing, treating, and maintaining oral health. Although S. salivarius is a predominant member of the commensal oral microbiota and generally regarded as a safe species, it is recognized that each strain needs to be comprehensively assessed for safety. This study describes the in silico, in vitro, and clinical testing that were conducted to evaluate the safety of S. salivarius DB-B5. Both 16S rRNA and multi-gene phylogenetic reconstruction was used to confirm the taxonomic identity of this strain. Bioinformatic analysis of the genome demonstrated the absence of transmissible antibiotic resistance genes or virulence factors. Phenotypic testing further showed S. salivarius DB-B5 to be susceptible to clinically relevant antibiotics. S. salivarius DB-B5 displayed weak alpha-hemolysis, and does not produce biogenic amines. In a randomized, double-blind, placebo-controlled clinical study, consumption of S. salivarius DB-B5 at 10 billion CFU/day for 4 weeks by healthy adults was safe and well-tolerated (ClinicalTrials.gov registry number NCT04492631). This work has indicated that S. salivarius DB-B5 is a safe probiotic candidate.

Three glycosylated serine-rich repeat proteins play a pivotal role in adhesion and colonization of the pioneer commensal bacterium, Streptococcus salivarius.

Bacterial adhesion is a critical step for colonization of the host. The pioneer colonizer and commensal bacterium of the human gastrointestinal tract, Streptococcus salivarius, has strong adhesive properties but the molecular determinants of this adhesion remain uncharacterized. Serine-rich repeat (SRR) glycoproteins are a family of adhesins that fulfil an important role in adhesion. In general, Gram-positive bacterial genomes have a unique SRR glycoprotein-encoding gene. We demonstrate that S. salivarius expresses three large and glycosylated surface-exposed proteins - SrpA, SrpB and SrpC - that show characteristics of SRR glycoproteins and are secreted through the accessory SecA2/Y2 system. Two glycosyltransferases - GtfE/F - encoded outside of the secA2/Y2 locus, unusually, perform the first step of the sequential glycosylation process, which is crucial for SRR activity. We show that SrpB and SrpC play complementary adhesive roles involved in several steps of the colonization process: auto-aggregation, biofilm formation and adhesion to a variety of host epithelial cells and components. We also show that at least one of the S. salivarius SRR glycoproteins is important for colonization in mice. SrpA, SrpB and SrpC are the main factors underlying the multifaceted adhesion of S. salivarius and, therefore, play a major role in host colonization.