Beneficial microbes for the oral cavity: time to harness the oral streptococci?

Indigenous microbes are known to influence human health outcomes and various approaches are now being made to positively modulate these microbe-induced outcomes via the administration of probiotics. The application of probiotics that are specific to the oral cavity is a relatively undeveloped field, and their emergence has largely occurred as a reasoned follow-up to initial studies in which probiotics that had already been developed and obtained regulatory approval for intestinal applications were then also evaluated for their putative influence on oral microbiota functionality. These attempts to extend the application of existing probiotics were probably at least in part motivated by recognition of the substantial safety and regulatory hurdles that must be overcome prior to the introduction of a novel probiotic agent. Nevertheless, from an efficacy perspective it appears more logical to develop microbes of oral origin as the specific providers of probiotic solutions for oral diseases, rather than attempting to adapt intestinally-derived strains for this role. Oral bacteria and their bioactive molecules have evolved to operate optimally in this environment and in some cases are known to persist only in oral sites. Amongst the bacteria of more than 700 species now identified within the human oral microbiota, it is the streptococci that are numerically predominant. Although this review highlights the development of the oral cavity bacterium Streptococcus salivarius as an oral probiotic, a number of other streptococcal species have also been shown to have considerable potential as probiotic candidates.