Bile Esculin Agar (BEA) is a selective differential agar used to isolate and identify members of the genus Enterococcus, also known as "group D streptococci".
Bile esculin agar contains oxgall (bile salts) to inhibit the growth of gram positive organisms other than enterococci and group D streptococci. It also contains nutrients, esculin, and ferric citrate. When an organism hydrolyzes the glycoside esculin to form esculetin and dextrose, the esculetin reacts with the ferric citrate to produce a dark brown or black phenolic iron complex.
If an organism can hydrolyze esculin, the media will turn dark brown or black. However, the test is interpreted as a positive result only if more than half the medium is dark brown or black after incubation.
Bile-esculin agar medium is prepared as agar slants or plates.
The constituent of Bile-esculin agar medium are:
- Peptone
- Beef extract
- Oxgall (Bile)
- Esculin
- Ferric citrate
- Agar
Bile esculin medium contains esculin and peptone for nutrition and bile to inhibit Gram-positive bacteria other than Group D streptococci and enterococci. Ferric citrate is added as a color indicator.
Principle: Bile-esculin test is based on the ability of certain bacteria, notably the group D streptococci and Enterococcus species, to hydrolyze esculin in the presence of bile (4% bile salts or 40% bile).
Note: Many bacteria can hydrolyze esculin, but few can do so in the presence of bile
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