Molecular characterization of a novel putative pathogen, <i>Streptococcus nakanoensis</i> sp. nov., isolated from sputum culture

The genus <i>Streptococcus</i> encompasses a wide range of bacteria with more than 60 species. Recently, there has been a notable increase in reports of novel species of α-<i>Streptococcus</i> based on genomic analysis data. However, limited information exists regarding the pathogenicity of these species. In this study, a quinolone-resistant α-hemolytic <i>Streptococcus</i> strain, MTG105, was isolated from a patient with pneumonia. Genetic analysis revealed that this species was a novel species closely related to <i>S. pseudopneumoniae</i>. In an infection assay using organotypic lung tissue models, MTG105 induced epithelial damage comparable to that caused by <i>S. pneumoniae</i> and <i>S. pseudopneumoniae</i>, strongly suggesting its potential as a pathogenic α-<i>Streptococcus</i>. The natural transformation abilities of <i>Streptococcus</i> species facilitate gene exchange within the same genus, leading to the emergence of species with increasingly diverse genome structures. Therefore, the identification of this species underscores the importance of monitoring the emergence of novel species exhibiting virulence and/or multidrug resistance.