Aesculin

Aesculin is a glucoside that naturally occurs in the horse chestnut Aesculus hippocastanum and in daphnin (the dark green resin of Daphne mezereum).

Medical uses

Aesculin is used in a microbiology laboratory to aid in the identification of bacterial species (especially Enterococci).

Aesculin hydrolysis test

Aesculin is incorporated into agar with ferric citrate and bile salts (bile aesculin agar). Hydrolysis of the aesculin forms aesculetin and glucose. The aesculetin forms dark brown or black complexes with ferric citrate, allowing the test to be read.

Streak the bile aesculin agar and incubate 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 (360nm): hydrolysis of aesculin results in loss of this fluorescence.

Enterococcus will often flag positive within four hours of the agar being inoculated.

References

• See National Standard Methods MSOP 48 (Bile aesculin agar) and BSOPTP 2 (Aesculin hydrolysis test).