Bile esculin agar

Bile esculin agar (BEA) is a type of agar used in microbiology to isolate and identify members of the genus Enterococcus (Streptococcus group D). It is a selective and differential medium, which can distinguish between Enterococcus species and other gram-positive bacteria based on their ability to hydrolyze esculin in the presence of bile.

Composition

Bile esculin agar contains oxgall (bile), which inhibits the growth of most gram-positive bacteria but not Enterococcus. The medium also contains esculin, a glycoside, and ferric citrate. When esculin is hydrolyzed by the enzyme beta-glucosidase, it forms esculetin and glucose. Esculetin reacts with ferric citrate to form a dark brown or black complex.

Use

In a laboratory setting, a small inoculum of the test organism is streaked onto the surface of the bile esculin agar. The plate is then incubated at 35-37°C for 18-24 hours. If the organism can hydrolyze esculin in the presence of bile, the medium in the slant will turn dark brown or black. This is a positive result, indicating the presence of Enterococcus or group D Streptococcus. If the slant remains light, this is a negative result.

Limitations

While bile esculin agar is a useful tool in identifying Enterococcus and group D Streptococcus, it is not infallible. Some strains of Staphylococcus and Aerococcus can also hydrolyze esculin and may give a false positive result. Therefore, additional tests are often required to confirm the identification of the organism.

See also

• Microbiological culture
• Selective media
• Differential media

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