Bile esculin agar
Encyclopedia
Agar
Agar or agar-agar is a gelatinous substance derived from a polysaccharide that accumulates in the cell walls of agarophyte red algae. Throughout history into modern times, agar has been chiefly used as an ingredient in desserts throughout Asia and also as a solid substrate to contain culture medium...
used to isolate and identify members of the genus Enterococcus
Enterococcus
Enterococcus is a genus of lactic acid bacteria of the phylum Firmicutes. Enterococci are Gram-positive cocci that often occur in pairs or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the...
, also known as "group D streptococci".
Composition and process
Bile salts are the selective ingredient, while esculin is the differential component. EnterococcusEnterococcus
Enterococcus is a genus of lactic acid bacteria of the phylum Firmicutes. Enterococci are Gram-positive cocci that often occur in pairs or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the...
hydrolyze esculin to products that react with ferric citrate in the medium to produce insoluble iron salts, resulting in the blackening of the medium.
Test results must be interpreted in conjunction with gram stain morphology.
Uses
Bile Esculin Agar is used primarily to differentiate EnterococcusEnterococcus
Enterococcus is a genus of lactic acid bacteria of the phylum Firmicutes. Enterococci are Gram-positive cocci that often occur in pairs or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the...
from Streptococcus
Streptococcus
Streptococcus is a genus of spherical Gram-positive bacteria belonging to the phylum Firmicutes and the lactic acid bacteria group. Cellular division occurs along a single axis in these bacteria, and thus they grow in chains or pairs, hence the name — from Greek στρεπτος streptos, meaning...
. Members of the genus Enterococcus
Enterococcus
Enterococcus is a genus of lactic acid bacteria of the phylum Firmicutes. Enterococci are Gram-positive cocci that often occur in pairs or short chains, and are difficult to distinguish from streptococci on physical characteristics alone. Two species are common commensal organisms in the...
are capable of growing in the presence of 4% bile (oxgall) and hydrolyzing esculin to glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
and esculetin
Esculetin
Aesculetin is a derivative of coumarin. It is a natural lactone that derives from the intramolecular cyclization of a cinnamic acid derivative....
. Esculetin combines with ferric ions to produce a black complex.
For some purposes, certain bacteria are able to hydrolyze aesculin
Aesculin
Aesculin, also rendered Æsculin or Esculin, is a glucoside that naturally occurs in the horse chestnut , California Buckeye and in daphnin ....
. A plate containing aesculin will fluoresce a pale blue under UV radiation. Some bacteria can hydrolise this, leading to UV dark colonies, as opposed to UV light ones.
When new techniques are produced to identify enterococci, they are often compared to the use of bile esculin agar.