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

A new α-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 α-galactosidase, β-galactosidase and N-acetyl-β-d-glucosaminidase and by the absence of arginine dihydrolase deiminase and IgA1-protease. It grows in a temperature range of 25 to 40 °C (optimal growth temperature at 37 °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).

Keywords: Streptococcus; dental caries; periodontal diseases; streptococcal infections; viridans streptococci.