Bile Solubility Test Guide
An original bench reference illustrating the dissolution of Streptococcus pneumoniae colonies under 10% sodium desoxycholate.
Bench card visual
Bile Solubility plates A-B
Read first
Observe colony integrity inside the drop of 10% sodium desoxycholate after 30 minutes.
A
Soluble (Positive)Streptococcus pneumoniaeColonies completely dissolve (lyse) under the reagent drop, leaving only flat shiny imprints.
B
Insoluble (Negative)Streptococcus agalactiaeColonies remain completely intact, raised, and solid under the reagent drop.
Soluble (Positive)Streptococcus pneumoniae
Colonies completely dissolve (lyse) under the reagent drop, leaving only flat shiny imprints.
Insoluble (Negative)Streptococcus agalactiae
Colonies remain completely intact, raised, and solid under the reagent drop.
What to look for
| Visual clue | Meaning | Student shorthand |
|---|---|---|
| Colony Lysis | Bile salts lower surface tension, activating intracellular autolytic enzymes (amidase) | Shorthand: Soluble (+) |
| Intact Colonies | Lack of autolytic response; colonies remain distinct and raised | Shorthand: Insoluble (-) |
| Reagent Drop | 10% sodium desoxycholate applied directly on active growth | Requires active incubation |
Bile Solubility study readout
| Pattern | What to call it | How to think about it |
|---|---|---|
| Soluble (Positive) | Colonies disintegrate completely within 30 minutes | Confirms Streptococcus pneumoniae (pneumococci). |
| Insoluble (Negative) | Colonies remain fully intact and visible | Presumptive alpha-hemolytic viridans streptococci or enterococci. |
| Method note | Can also be run in tube broth | Broth clearing is positive; persistent turbidity is negative. |
Key takeaways
- Specifically identifies S. pneumoniae among alpha-hemolytic streptococci.
- Relies on desoxycholate-activated autolysis, not direct chemical dissolution.
- Correlate with optochin susceptibility (P-disk) for full presumptive confirmation.
Remember
S. pneumoniae is optochin susceptible (zone >= 14 mm) and bile soluble.