Streptococcus downii sp. nov., isolated from the oral cavity of a teenager with Down syndrome.

A new alpha-haemolytic streptococcal strain has been isolated from the dental plaque of a teenager with Down syndrome. Genetic and taxonomic analyses place this Streptococcus within the oralis group. It is a Gram-stain-positive, non-motile, non-spore-forming spherical alpha-haemolytic coccus arranged in chains, and it ferments a large number of monosaccharides and disaccharides, as well as polymeric carbohydrates. It differs biochemically from closely related species of Streptococcus due to its production of alpha-galactosidase, beta-galactosidase and N-acetyl-beta-d-glucosaminidase and by the absence of arginine dihydrolase deiminase and IgA1-protease. It grows in a temperature range of 25 to 40 degrees C (optimal growth temperature at 37 degrees C) and in a pH range of 4.5 to 8 (optimal pH at 7.0). A phylogenetic analysis based on its 16S and 23S rRNA gene sequences placed it close to Streptococcus dentisani CECT 7747(T). The ANIb and ANIm values were 93.19 and 93.61 %, respectively, both below the accepted threshold to designate it as a new species of bacteria. A phylogenetic tree based on its core genome placed it close to Streptococcus oralis subsp. dentisani strain CECT 7747(T) with a distance in the expanded core phylogeny of 0.1298. The in silico DNA-DNA hybridization value was 52.2 % with respect to the closest species, S. oralis subsp. dentisani CECT 7747(T). Based on these data, a new species of bacteria within the genus Streptococcus, family Streptococcaceae and order Lactobacillales is described, for which the name of Streptococcus downii sp. nov. is proposed (type strain CECT 9732(T)=CCUG 73139(T)).