Reports of novel species of α-hemolytic
Streptococcus
have increased recently. However, limited information exists regarding the pathogenicity of these species, with the exception of
Streptococcus pneumoniae
and
Streptococcus pseudopneumoniae
. In this study, a quinolone-resistant α-
Streptococcus
strain, MTG105, was isolated from the sputum of a patient with pneumonia. This strain was first identified as
S. pneumoniae
at the hospital laboratory; however, it exhibited unique genetic features upon further analysis. Digital DNA-DNA hybridization and average nucleotide identity based on BLAST values from whole-genome sequencing revealed MTG105 to be a novel species closely related to
S. pseudopneumoniae
. Although MTG105 carried two copies of the pneumolysin gene, similar to
S. pseudopneumoniae
, this isolate exhibited susceptibility to optochin under both aerobic and 5% CO
2
conditions. Notably, no biochemical features could be used to definitively identify this species. In an infection assay using organotypic lung tissue models, MTG105 induced epithelial damage comparable to that of
S. pneumoniae
and
S. pseudopneumoniae
, possibly suggesting its potential as a pathogenic α-
Streptococcus
. The natural transformation abilities of
Streptococcus
species facilitate their exchange of genes within the same genus, resulting in the existence of species with increasingly more diverse genome structures. Therefore, the identification of this species highlights the importance of monitoring the emergence of novel species exhibiting virulence and/or multidrug resistance. This isolate was proposed as a novel species, designated
Streptococcus nakanoensis
sp. nov. The type strain was MTG 105
T
(= JCM 35953
T
= CCUG 76894
T
).
IMPORTANCE
The genus
Streptococcus
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-
Streptococcus
based on genomic analysis data. However, limited information exists regarding the pathogenicity of these species. In this study, a quinolone-resistant α-hemolytic
Streptococcus
strain, MTG105, was isolated from a patient with pneumonia. Genetic analysis revealed that this species was a novel species closely related to
S. pseudopneumoniae
. In an infection assay using organotypic lung tissue models, MTG105 induced epithelial damage comparable to that caused by
S. pneumoniae
and
S. pseudopneumoniae
, strongly suggesting its potential as a pathogenic α-
Streptococcus
. The natural transformation abilities of
Streptococcus
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.
