Bile solubility test

Principle

Bile salts (sodium deoxycholate) activate autolytic enzymes in Streptococcus pneumoniae, which lyses the cells. Viridans streptococci do not lyse.

Procedure

• Tube: mix a dense suspension with 10% deoxycholate; a control tube gets saline. Incubate at 35–37 °C for up to 3 h.
• Plate: drop 10% deoxycholate on a colony and incubate for about 15 minutes.

Results

• Positive: suspension clears (or colony disappears) = S. pneumoniae
• Negative: stays turbid = viridans streptococci