Streptococcus strain D19^T as a probiotic candidate to modulate oral health

As probiotics protect host cells, they are used to treat bacterial infections. It has been indicated that probiotics may prevent or reduce the attachment of pathogens to host cells. In this study, Streptococcus strain D19^T was isolated from the oropharynx of a healthy child, and its adhesion performance and Staphylococcus aureus adhesion inhibition effect were analysed using human bronchial epithelial (16-HBE) cells, as an in vitro cell model. We evaluated the probiotic properties of the D19^T strain based on its acid-base, bile salt, and lysozyme tolerance; antibacterial activity; cytotoxicity; antibiotic sensitivity; in vitro adhesion to 16-HBE cells; and competitive, exclusion, and displacement effects against S. aureus.

Streptococcus strain D19^T showed tolerance to a PH range of 2-5 and 0.5-1% bile. However, it was more tolerant to 0.5% bile than to 1% bile. The strain also demonstrated an ability to adapt to maladaptive oropharyngeal conditions (i.e., tolerating 200 µg/mL lysozyme). It was resistant to 0.8 mM H₂O₂. The results also demonstrated that D19^T exhibited inhibitory activities against various common pathogenic bacteria. Furthermore, D19^T was not toxic to 16-HBE cells at different multiplicities of infection and was sensitive to most antibiotics tested. The adhesion rate of D19^T cells to 16-HBE cells was 47% ± 1.2%, which was significantly higher than that of S. aureus to 16-HBE cells. The competition, exclusion, and displacement assay results showed that D19^T has good inhibitory effect against S. aureus adhesion.

The present study revealed that Streptococcus strain D19^T has the potential to be developed as a respiratory microbiota preparations.