Bile Solubility

Question 1

Purpose of bile solubility:

Answer 1

This test differentiates Streptococcus pneu moniae (positive-soluble) from alpha-hemolytic streptococci (negative-insoluble

Question 2

Principle of bile solubility:

Answer 2

Bile or a solution of a bile salt (e.g., sodium desoxycholate) rapidly lyses pneumococcal colonies. Lysis depends on the presence of an intracellular autolytic enzyme, amidase. Bile salts lower the surface tension between the bacterial cell membrane and the medium, thus accelerating the organism’s natural autolytic process.

Question 3

Method of bile solubility:

Answer 3

1. After 12 to 24 hours of incubation on 5% sheep blood agar, place 1 to 2 drops of 10% sodium desoxycholate on a well- isolated colony. Note: A tube test is performed with 2% sodium desoxycholate.
2. Gently wash liquid over the colony without dislodging the colony from the agar.
3. Incubate the plate at 35°-37°C in ambient air for 30 minutes.
4. Examine for lysis of colony

Question 4

Expected results of bile solubility:

Answer 4

Positive: Colony disintegrates; an imprint of the lysed colony may remain in the zone.
Negative: Intact colonies

Question 5

Limitation of bile solubility:

Answer 5

Enzyme activity may be reduced in old cultures. Therefore, negative results with colonies resembling S. pneumoniae should be further tested for identification with alternate methods

Question 6

Quality control of bile solubility:

Answer 6

Positive: Streptococcus pneumoniae (ATCC49619) bile soluble

Negative: Enterococcus faecalis (ATCC29212) bile insoluble