Bile Esculin Test- Principle, Procedure and Result Interpretation
Objectives of Bile Esculin Test
This test is used for the presumptive identification of enterococci and organisms in the Streptococcus bovis group. The test differentiates enterococci and group D streptococci from non–group D viridans streptococci.
Principle of Bile Esculin Test
Gram-positive bacteria other than some streptococci and enterococci are inhibited by the bile salts in this medium. Organisms capable of growth in the presence of 4% bile and able to hydrolyze esculin to esculetin. Esculetin reacts
with Fe3+ and forms a dark brown to black precipitate.
Beef extract (11 g), enzymatic digest of gelatin (34.5 g), esculin (1 g), ox bile (2 g), ferric ammonium citrate (0.5 g), agar (15 g), per 1000 mL, pH 6.6.
Procedure of Bile Esculin Test
- Inoculate one to two colonies from an 18- to 24-hour culture onto the surface of the slant.
- Incubate at 35°-37°C in ambient air for 48 hours.
Expected Results of Bile Esculin Test
Positive: Growth and blackening of the agar slant
Negative: Growth and no blackening of medium, No growth
Limitations of Bile Esculin Test
As a result of nutritional requirements, some organisms may grow poorly or not at all on this medium.
Positive: Enterococcus faecalis (ATCC19433)—growth; black precipitate
Negative: Escherichia coli (ATCC25922)—growth; no color change Streptococcus pyogenes (ATCC19615)—no growth; no color change