Aesculin (Aes)

Aesculin is a coumarin glucoside that naturally occurs in the horse chestnut, California Buckeye, Prickly Box and in daphnin. It is also found in dandelion coffee. Aesculin is used in a microbiology laboratory to aid in the identification of bacterial species. In fact, all strains of Group D Streptococci hydrolyze æsculin in 40% bile. Aesculin is incorporated into agar with ferric citrate and bile salts. Hydrolysis of the aesculin forms aesculetin (6,7-dihydroxycoumarin) and glucose. The 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 37 °C 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 (360 nm): hydrolysis of aesculin results in loss of this fluorescence.