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Anginosus and Salivarius Group 8.2.4.1 Streptococcus salivarius

A group K Streptococcus, Streptococcus salivarius is a primary colonizer of the oral mucosa and is a permanent member of the oral flora [68]. The anginosus (milleri) group of streptococcal species are commensal organisms found in the oral cavity and pharynx that are frequent agents of human disease. Members of the group can differ in Lancefield classification while being related at the genetic level. Streptococcus intermedius (group F, C, or not typeable) has been found associated with abscesses of the brain and liver, while Streptococcus anginosus (group G or C) and Streptococcus constellatus subsp. constellatus (group F mostly) are isolated from infections in a wider range of anatomical sites [69]. In 1999, a new member of the anginosus group, S. constellatus subsp. pharynges, was isolated from cases of pharyngitis and was serologically a member of Lancefield group C [70]. The toxin intermedilysin is produced by S. intermedius, distinguishing it from the other members of this group who all lack this gene. Further, intermedilysin production level in isolates from brain and liver abscesses is higher than in strains from normal habitats, such as dental plaque, or from peripheral infection sites, suggesting that this protein is a key factor in inducing or maintaining deep-seated infections [71]. The potential genetic relationships to other pathogenic streptococci of this group of species is poorly characterized and should be clarified by future genome analysis.

8.2.4.2 Streptococcus thermophilus

Streptococcus thermophilus is a thermophilic lactic acid bacterium used for the manufacture of dairy products. On the basis of phylogenetic sequence data of the RNase P RNA gene, rnpB, it is part of the salivarius group [72]. The complete sequence of two different strains of S. thermophilus has been reported (Table 8.1) [73]. Both strains have a genome size of 1.8 Mbp with 42 regions of sequence differences and a remarkably high level (10%) of gene decay. Horizontal gene transfer is evident in the genomes with the presence of numerous phages, transposons, and integrons. One region identified as a hotspot of IS sequences contains a mosaic of fragments with high identity to gene regions from Lactobacillus bulgari-cus and L. lactis, organisms commonly found in milk. Interestingly, many of the genes involved in carbohydrate metabolism are decayed or missing. However a specific symporter for lactose (the main carbohydrate of milk) is present in S. ther-mophilus, but absent in other streptococci. The significant differences in gene content from other streptococci most likely have occurred by horizontal gene transfer and gene decay as the organism evolved into its unique ecological niche.

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