Variants of the bile: solubility test to differentiate Streptococcus pneumoniae from other viridans group streptococci.

Aim: Bile salts promote the specific autolysis of pneumococcal cells, allowing the differentiation between Streptococcus pneumoniae and other viridans group streptococci (VGS). Material & methods: One hundred clinical VGS isolates identified by amplification and sequencing of 16S rRNA, groEL and sodA genes were analyzed with different variants of bile-solubility tests: tube testing read by naked eye; tube testing where the lysis was measured as the decrease of turbidity with a densitometer; and direct testing on blood agar plate. Results: As expected, all S. pneumoniae isolates were fully lysed in the presence of bile salts except for one isolate that partially lysate in tube testing as well as on the blood agar plate. None of the VGS were lysed by bile salts. Conclusion: Bile-solubility testing is an accurate and technically nondemanding method to discriminate between S. pneumoniae and other VGS species.