ABSTRACTWe recently described the novel speciesStreptococcus tigurinussp. nov. belonging to theStreptococcus mitisgroup. The type strain AZ_3aTofS. tigurinuswas originally isolated from a patient with infective endocarditis. According to its phenotypic and molecular characteristics,S. tigurinusis most closely related toStreptococcus mitis,Streptococcus pneumoniae,Streptococcus pseudopneumoniae,Streptococcus oralis, andStreptococcus infantis. Accurate identification ofS. tigurinusis facilitated by 16S rRNA gene analysis. We retrospectively analyzed our 16S rRNA gene molecular database, which contains sequences of all clinical samples obtained in our institute since 2003. We detected 17 16S rRNA gene sequences which were assigned toS. tigurinus, including sequences from the 3S. tigurinusstrains described previously.S. tigurinusoriginated from normally sterile body sites, such as blood, cerebrospinal fluid, or heart valves, of 14 patients and was initially detected by culture or broad-range 16S rRNA gene PCR, followed by sequencing. The 14 patients had serious invasive infections, i.e., infective endocarditis (n= 6), spondylodiscitis (n= 3), bacteremia (n= 2), meningitis (n= 1), prosthetic joint infection (n= 1), and thoracic empyema (n= 1). To evaluate the presence ofStreptococcus tigurinusin the endogenous oral microbial flora, we screened saliva specimens of 31 volunteers. After selective growth, alpha-hemolytic growing colonies were analyzed by matrix-assisted laser desorption ionization–time of flight mass spectrometry (MALDI-TOF MS) and subsequent molecular methods.S. tigurinuswas not identified among 608 strains analyzed. These data indicate thatS. tigurinusis not widely distributed in the oral cavity. In conclusion,S. tigurinusis a novel agent of invasive infections, particularly infective endocarditis.