Misidentification of alpha-hemolytic streptococci by routine tests in clinical practice

Accurate species-level identification of viridans group streptococci (VGS) is very important for understanding of their pathogenicity and virulence. However, an extremely high level of the similarity between VGS, especially Streptococcus pneumoniae, Streptococcus mitis, Streptococcus oralis and Streptococcus pseudopneumoniae, often results in misidentification of these organisms, so there is an urgent need of novel approaches to species identification. A set of 50 randomly selected clinical isolates of alpha-hemolytic streptococci from upper respiratory tract were characterized by the routine phenotypic methods (alpha-hemolysis, colony morphology, Gram stain and optochin susceptibility). Modern proteomic and genetic approaches - the direct bacterial profiling (DBP) by means of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) technique and multilocus sequence analysis (MLSA) scheme (http://viridans.emlsa.net/) - were applied for the accurate species identification. After that all isolates were stored at -70°C. Later they were re-inoculated, and a number of additional tests (bile solubility, latex agglutination by commercial "Slidex® pneumo-kit" and repeated optochin test) were performed. A considerable discrepancy was discovered in the results of the different approaches. Looking in the future, one could say that MLSA-like schemes based on the analysis of the nucleotide sequences of seven or more loci of the bacterial genome, appeared to be the most useful instrument in the VGS discrimination, in contrast to the numerous one-target identification schemes, which have been introduced into practice by now.