We isolated 13 BLIS strains of oral streptococci, with only four strains belonging to BAY 57-1293 in vitro the S. salivarius

species. Among them, one strain, S. salivarius DSM 23307, isolated from nasal swabs, possessed the main characteristics that make it suitable to be used as a potential oral probiotic and was further characterized. It is well known that the α-hemolytic streptococci – such as S. salivarius, S. mitis, S. mutans, and S. sanguis – isolated from the human pharynx have been the target of many studies because of their ability to interfere with respiratory pathogens (Book, 1999; Roos et al., 2000; Power et al., 2008). They are predominant in the oral cavity, being the main producers of antimicrobial peptides such as bacteriocins and for this reason they could be good candidates for oral probiotics (Wescombe et al., 2009; Guglielmetti et al., 2010), even if some species such as S. mitis have been associated, in some cases, with infections, resulting in their exclusion for their potential pathogenicity (Johnston et al., 2010). On the other hand, in the oral microbiome, S. salivarius, a primary

and predominant colonizer of oral mucosal surfaces in humans, is characterized by low pathogenic potential and is able to persist as a dominant species in the oral find more cavity (Horz et al., 2007). The safety of probiotics has been the subject of active discussion and, to date, there have not been any clear general guidelines for all strains: S. salivarius is a typical example, in fact, this species, in other parts of the world but not in Europe, has been included in the GRAS status (Burton et al., 2005, 2006a, b). For this reason, the safety assessment

of each strain that could be used as a probiotic represents the fundamental step for a good commercial product. The report of the FAO/WHO Working Group (Food and Drug Administration 2008) recommended the need to determine: (1) the genus and species of the probiotic strain; (2) antibiotic resistance patterns, in particular, for resistance genes associated with mobile elements; (3) virulence determinants; (4) metabolic activity that could be harmful for the host; and (5) hemolytic activity if the strain belongs to species that can have hemolytic potential. Streptococcus salivarius, even if Reverse transcriptase it does not have the GRAS status yet, is closely related to Streptococcus thermophilus, a species belonging to the salivarius group with major economic importance because of its wide use for production of yoghurt and cheese. Many comparative genomic studies regarding taxonomy and phylogeny among dairy streptococci have demonstrated that Streptococcus spp. are clustered in two main groups: one comprising S. macedonicus, and S. bovis species and the other S. thermophilus and S. salivarius: the species in each group show strong similarities in the DNA sequence of the ribosomal locus (Facklam, 2002; Mora et al., 2003). For all these reasons, S.