BEA agar | Principle | Preparation | Interpretation
☰ Content :
Ⅰ. Overview
Bile Aesculin Agar, BEA Agar, is a selective and differential medium. This medium tests the ability of organisms to hydrolyze esculin in the presence of bile. It is commonly used to identify members of the genus group D streptococci and enterococci.
BEA medium contains bile to inhibit the growth of gram-positive bacteria as well as many gram-negative bacteria, and esculin which is the differential component.
In some preparations, the medium may contain sodium azide (increases the selectivity)
BEA agar
Ⅱ. Preparation / Composition
Suspend the components, dehydrated powder, in water (64g in 1000ml of purified/distilled water). Bring the medium to a boil with constant stirring for at least 1 minute.
Sterilize in autoclave at 121°C for 15 minutes, cool to 45-50°C.
BEA agar composition | |||
|---|---|---|---|
| Ingredients | Gram/liter | ||
| Beef extract | 11g | ||
| Enzymatic digestion of gelatin | 34,5 g/L | ||
| Esculin | 1g | ||
| manganese sulfate | 0.05g | ||
| Ox bile | 2g | ||
| Ferric Ammonium Citrate | 0,5 g/L | ||
| Agar | 15,0 g/L | ||
◈ Note : the medium may contain sodium azide which increases selectivity, but sodium azide is extremely dangerous.
Ⅲ. Principle of BEA agar
◈ Gelatin peptone and beef extract provide the essential elements needed for growth.
◈ Differentiation : If an organism can hydrolyze aesculin in the presence of bile, the product esculetin is formed. Esculetin reacts with ferric citrate (in the middle), forming a phenolic iron complex that transforms the middle from dark brown to black .
◈ Selectivity : Selectivity is obtained by the addition of bile (oxgall), which inhibits the growth of most Gram-positive cocci other than group D streptococci and enterococci.
Ⅳ. Interpretation
◈ Examine after 18 to 24 hours for esculinase positive colonies. Wait up to 72 hours before reporting as negative.
| Bacteria | Growth |
|---|---|
| Enterococcus | Good growth. Positive esculin hydrolysis resulting in darkening of the medium around the growth. |
| Escherichia coli | Good growth. No darkening of media around colonies |
| Streptococcus pyogenes | No growth |
References:
- uwyo - BILE ESCULIN AGAR
- Handbook of Culture Media for Food Microbiology
- BD - Bile Esculin Agar Slants
- Color Atlas of Medical Bacteriology
- Thermo Scientific™ - BEA AGAR
- neogen - BEA AGAR
- DALYNN - BILE ESCULIN AGAR