BEA agar | Principle | Preparation | Interpretation


☰ Content :



Ⅰ. Overview

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.


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:

  1. uwyo - BILE ESCULIN AGAR
  2. Handbook of Culture Media for Food Microbiology
  3. BD - Bile Esculin Agar Slants
  4. Color Atlas of Medical Bacteriology
  5. Thermo Scientific™ - BEA AGAR
  6. neogen - BEA AGAR
  7. DALYNN - BILE ESCULIN AGAR