[Classical and new approaches in laboratory diagnosis of viridans streptococci]

Viridans group streptococci (VGS) are gram-positive microorganisms that can form alpha-hemolytic colonies on sheep blood agar. They reside as normal flora in oral cavity, respiratory, gastrointestinal, urogenital tract and on skin. They can cause bacteremia, endocarditis, meningitis and septicemia following dental procedures. The diagnosis of VGS are difficult since the taxonomic classification and species na-mes may change due in time. Viridans group streptococci are classified into 5 groups (Sanguinis, Mitis, Mutans, Salivarius, Anginosus) according to biochemical reactions and 16S rRNA sequencing. Since Streptococcus pneumoniae is a member of the Mitis group, the other important species in this group deserves investigation. Genetic exchange between Streptococcus mitis, Streptococcus oralis and S.pneumoniae by transformation and lysis mechanisms occur continously as they share the same anatomical region. These mechanisms play role in exchanging capsular and antibiotic resistance genes between these species. The cultivation of VGS usually starts with the inoculation of various patient specimens into sheep blood agar and the detection of alpha-hemolytic colonies. Observation of gram-positive cocci microscopically, the detection of optochin-resistant and bile insoluble colonies with few exceptions are the further important steps in laboratory diagnosis. VGS are then identified at species level by using biochemical reactions, automated diagnostic systems and molecular methods. The last step in the laboratory diagnosis of VGS is antibiotic susceptibility testing which is of outmost importance as penicillin and erythromycin resistance are on rise. In this review article, classification of VGS, similarities between S.pneumoniae and Mitis group streptococci and the laboratory diagnosis of VGS have been discussed.