The Human Microbiome Project states that an understanding of human health and disease is impossible without understanding the human microbiome (Dewhirst et al., 2010). More than 700 bacterial species are present in the oral cavity and, maintaining the bacterial

communities unaltered, has a significant impact on general health by either preventing or causing infections. It has been suggested that changes in the structure of this complex community could contribute to a shift in the balance of the resident microflora to a disease-associated species composition (Marsh, 1991; Aas et al., 2005; Caglar et al., 2005). Bacterial interference, such as antagonism, has a fundamental role in keeping the balance of the microbial ecology associated with the ability of bacterial species to interfere during surface

colonization. This phenomenon represents an interesting mechanism of defense because of AP24534 mw the capability of endogenous microflora to interfere or inhibit the growth of potential pathogens (Falagas et al., 2008). Clinical evidence of bacterial interference in the treatment of halitosis and/or Streptococcus pyogenes infection has been reported by J. R. Tagg and co-workers, attributing this ability to the presence of Streptococcus salivarius K12 belonging to the normal commensal flora of the nasopharynx as it is a salA bacteriocin producer strain able to interfere with S. pyogenes species (Burton et al., 2006a, b; buy PD0332991 Power et al., 2008). Streptococcus salivarius, a non-pathogenic species and predominant colonizer in the oral microbiome, is one of the

major producers of a variety of bacteriocin-like inhibitory substances (BLISs), which are active against other microorganisms, reducing the frequency of colonization of the main pathogens involved in upper respiratory tract infections (URTIs) (Wescombe et al., 2009). For this reason, S. salivarius is a good candidate for oral probiotics in humans. Probiotics are traditionally associated with gut health, in fact, many Tryptophan synthase probiotics are used to prevent or treat several diseases mainly in the intestinal tract (Gareau et al., 2010), and recently many studies have been involved in the development of oral probiotic applications. Many of them, now, have the GRAS (generally regarded as safe) status, a designation generally used by the Food and Drug Administration (FDA) to indicate that these products can be used without any demonstrable harm to consumers. Some streptococci have a GRAS status for their virtuous nature, and among these S. salivarius, even if it is not yet included in the GRAS status, is most closely related to Streptococcus themophilus, used by yogurt manufactures, than to other oral species in which the virtuous nature is controversial. (Food & Drug Administration, 2005; EFSA, 2005). Oral probiotic applications of S. salivarius are commercially available: BLIS K12™ Throat Guard that contains S.