Understanding the viridians group streptococci: Are we there yet?

The viridans group streptococci are a heterogeneous group of organisms which exist as commensals in the oropharynx and the gut. They cause serious infections when they gain entry into sterile sites particularly in patients with predisposing conditions. Classification and species differentiation of these organisms has always been a challenge because of phenotypic differences between strains of the same species. Facklam's typing scheme based on six metabolic properties has been the most widely used and many commercial identification systems are based on it. Due to the ambiguity in species differentiation based on phenotypic tests, nucleic acid-based methods have been developed to improve the identification of these organisms. Results using genotypic methods such as 16S rRNA and sodA gene sequencing have been promising. Multilocus sequence analysis of seven house-keeping genes map, pfl, pyk, ppaC, rpoB, soda and tuf amplified by polymerase chain reaction was found to be an accurate alternative to other methods and could be useful in the characterisation of larger collections of isolates.