Aesculin

Aesculin, also called æsculin or esculin, is a coumarin glucoside that naturally occurs in the trees horse chestnut, California buckeye, and prickly box. It is also found in daphnin, dandelion coffee, and olive bark. It is reported to be present in olive bark, but not in olive leaf; therefore, identification of aesculin in abundance in an olive extract indicates that the extract has been derived from olive bark.

Uses

Aesculin is also used in a microbiology laboratory to aid in the identification of bacterial species. In fact, all strains of Group D Streptococci hydrolyze aesculin in 40% bile.

Aesculin hydrolysis test

Aesculin is incorporated into agar with ferric citrate and bile salts. Hydrolysis of aesculin forms aesculetin and glucose. Aesculetin forms dark brown or black complexes with ferric citrate, allowing the test to be read.
The bile aesculin agar is streaked and incubated at for 24 hours. The presence of a dark brown or black halo indicates that the test is positive. A positive test can occur with Enterococcus, Aerococcus, and Leuconostoc. Aesculin will fluoresce under long wave ultraviolet light and hydrolysis of aesculin results in loss of this fluorescence.
Enterococcus will often flag positive within four hours of the agar being inoculated.