pH buffering activity of the oral probiotic strain Streptococcus dentisani through arginine and agmatine metabolism

In the last years, microbial arginine metabolism has been extensively investigated to develop novel anti-caries strategies. Such approaches are based on the ammonia-generating capacity of some oral bacterial species to cope with the acidification cycles that continuously occur in the oral environment. Streptococcus dentisani was proposed as a promising probiotic against tooth decay because it is able to inhibit the growth of major oral pathogens through the production of bacteriocins, efficiently colonizes human dental plaque, and it improves several clinical features associated with oral health, including pH. In the current manuscript, we have located and annotated the genes involved in extracellular pH buffering by arginine and agmatine degradation in the genome of the probiotic strain 7746. In addition, the expression of key genes in the pathways have been studied in S. dentisani 7746 and in the type strain 7747 in order to evaluate how they could promote a beneficial effect in pH homeostasis. We analyzed the regulation of the arginolytic pathway by transcriptomics and quantitative PCR, demonstrating that the activation of the pathway requires both an acidic pH and presence of arginine. When co-cultured with the acidogenic species S. mutans, S dentisani was able to buffer culture pH only when inoculated at higher relative proportions. Finally, the addition of the probiotic strain to in vitro oral biofilms was proven to counteract the acid produced by the sugar metabolism of the bacterial community, improving the pH buffering effect of fluoride and arginine. The data support the use of S. dentisani, specially the strain 7746, as an efficient oral probiotic to reduce caries risk, by acting not only through bacteriocin production against acidogenic species but also in the pH homeostasis of the oral environment.