Streptococcal bacteriocins and the case for Streptococcus salivarius as model oral probiotics

Members of the Gram-positive bacterial genus Streptococcus are a diverse collection of species inhabiting many body sites and range from benign, nonpathogenic species to those causing life-threatening infections. The streptococci are also prolific producers of bacteriocins, which are ribosomally synthesized proteinaceous antibiotics that kill or inhibit species closely related to the producer bacterium. With the emergence of bacterial resistance to conventional antibiotics, there is an impetus to discover, and implement, new and preferably 'natural' antibiotics to treat or prevent bacterial infections, a niche that bacterial interference therapy mediated by bacteriocins could easily fill. This review focuses on describing the diversity of bacteriocins produced by streptococci and also puts forth a case for Streptococcus salivarius, a nonpathogenic and numerically predominant oral species, as an ideal candidate for development as the model probiotic for the oral cavity. S. salivarius is a safe species that not only produces broad-spectrum bacteriocins but harbors bacteriocin-encoding (and bacteriocin-inducing) transmissible DNA entities (megaplasmids).