Streptococcus mobilis sp. nov., isolated from a Helicobacter pylori-positive pre-neoplastic human stomach

A facultative anaerobic, catalase-negative, lactic acid-producing, non-endospore-forming, Gram-positive coccoid bacterium, strain MT/JULY 2010^T, was isolated from the stomach of a Helicobacter pylori-positive Japanese female patient with pre-neoplastic gastric mucosa. This bacterium can form clusters, chains or pairs or occurs singly in liquid media. The growth ranges for temperature, pH and NaCl concentration were 20-45 °C, pH 5.0-7.0 and 1-4%, respectively. Cells exhibited an unusual tumbling motion when observed under the microscope using a hanging drop assay. Observation under an electron microscope showed some unidentified structures and numerous fimbriae-like appendages. The combined results of 16S rRNA gene sequence analysis and the overall genome-related index [average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH)] classified the bacterium as a novel species of the genus Streptococcus. It forms a monophyletic cluster with Streptococcus parasanguinis ATCC 15912^T in the phylogenetic tree with 94.4% ANI. Furthermore, the conventional DNA-DNA hybridization assay and dDDH of the two strains identified their relatedness as 62.5% and 58.7% (both below the 70% threshold), respectively. The DNA G+C content, determined by nuclease P1-digested genomic DNA, was 42.6 mol%, while the digital G+C content calculated in silico was 41.8 mol%. Based on the phenotypic and genotypic results, strain MT/JULY 2010^T represents a novel species within the genus. The name Streptococcus mobilis sp. nov. is proposed. The type strain is MT/JULY 2010^T (=ATCC BAA-2258^T=NBRC 107862^T=NCBI 948105^T=V10/022878^T).