Mechanism of Action of Streptococcus downii, a New Bacterial Species with Probiotic Potential

Abstract

Streptococcus downii is a recently reported bacterial species of oral origin, with inhibitory capacity against Streptococcus mutans, Actinomyces naeslundii, Veillonella parvula and Aggregatibacter actinomycetemcomitans, which confers upon it the potential of being an oral probiotic. The aim of the present study was to identify the potential mechanisms by which S. downii exerts its inhibitory effect on S. mutans. To this end, the study assessed the consumption of glucose and proteins available in the culture medium, the modification of the pH, the production of short-chain fatty acids, the changes in the protein panel of the inhibition halo, the production of hydrogen peroxide and the effect of proteinase K. There were no differences in the glucose values or in the protein content of the medium, but there was a reduction in pH (with no effect on the growth of S. mutans). Significant increases were detected in the levels of lactic and formic acid (with no effect on the growth of S. mutans), as well as changes in the peptide panel (with no effect on the growth of S. mutans). The inhibitory effect was maintained in the presence of peroxidase but disappeared after adding proteinase K. Based on these results, it is suggested that the main mechanism of inhibition of S. downii against S. mutans is the production of bacteriocins.

Keywords: Streptococcus downii; Streptococcus mutans; bacteriocin; dental caries; probiotic.

Figure 1

Inhibition test for S. downii versus S. mutans in the presence of peroxidase (A) and proteinase K (B). Hydrogen peroxide production is not responsible for the inhibition, given that after adding peroxidase, the inhibitory capacity of S. downii versus S. mutans is maintained (A); however, the presence of proteinase K eliminates the inhibitory effect, which confirms the molecule’s protein nature with inhibitory capacity produced by S. downii (B).

Figure 2

Growth curves of S. mutans in the presence of concentrated S. downii supernatant (SP S. downii) and concentrated BHI medium (BHI × 10). The addition of the concentrated S. downii supernatant inhibits the growth of S. mutans (blue line), while S. mutans preserves its growth capacity after the addition of the concentrated BHI medium (red line). Mean and standard deviation of three replicates.

Figure 3

Summary of the experiments performed to study the mechanism of S. downii inhibition of S. mutans.