Identification of clinically relevant viridans group streptococci by phenotypic and genotypic analysis

Two phenotypic and three molecular methods were assessed for their ability to identify viridans group streptococci (VGS) to the species level. A panel of 23 clinical isolates, comprising strains isolated from infective endocarditis, blood cultures, pleural and peritoneal fluid, and 19 type/reference strains were analyzed. Identification was performed using two conventional phenotypic methods: APIA (R) rapid ID 32 Strep and the VITEKA (R) 2 system, and genotypic analysis of the nucleotide sequence of the housekeeping gene sodA, restriction patterns generated by restriction fragment length polymorphism (RFLP) of the 16S rRNA gene and multilocus sequence analysis (MLSA) of seven housekeeping genes. The APIA (R) rapid ID 32 Strep accurately speciated 79% of the strains assessed, while the VITEKA (R) 2 generated a successful identification for 55%, presenting limitations particularly with regard to species belonging to the mitis group. RFLP of the 16S rRNA gene correctly speciated 24% of the strains, having failed to allocate a species for 36% of the isolates examined. In contrast, sequence analysis of the sodA gene provided a correct identification for 95% of the strains assessed, while identification using the MLSA technique was unsuccessful due to practical limitations. The results generated herein indicate that no single methodology can be used to provide an accurate identification to the species level of all VGS, although nucleotide sequence analysis of the sodA gene proved to be useful in providing reliable speciation.