Aesculin, also rendered Æsculin or Esculin, is a glucoside

Glucoside

A glucoside is a glycoside that is derived from glucose. Glucosides are common in plants, but rare in animals. Glucose is produced when a glucoside is hydrolysed by purely chemical means, or decomposed by fermentation or enzymes....

 that naturally occurs in the horse chestnut (Aesculus hippocastanum), California Buckeye (Aesculus californica

Aesculus californica

Aesculus californica is a species of buckeye that is native [ |] to California and southwest Oregon [Jackson, County], and the only buckeye native to these states.-Description:...

) and in daphnin

Daphnin

Daphnin is a plant toxin. It can be produced from dihydroxycoumarin by O-dihydroxycoumarin 7-O-glucosyltransferase....

 (the dark green resin of Daphne mezereum

Daphne mezereum

Daphne mezereum is a species of Daphne in the flowering plant family Thymelaeaceae, native to most of Europe and Western Asia, north to northern Scandinavia and Russia. In southern Europe it is confined to medium to higher elevations and in the subalpine vegetation zone, but descends to near sea...

).

Medical uses

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

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...

 and Listeria). In fact, all strains of Group D Streptococci hydrolyze æsculin in 40% bile.

Aesculin hydrolysis test

Aesculin is incorporated into agar with ferric citrate and bile salts (bile aesculin agar). Hydrolysis of the aesculin forms aesculetin (6,7-dihydroxycoumarin

Coumarin

Coumarin is a fragrant chemical compound in the benzopyrone chemical class, found in many plants, notably in high concentration in the tonka bean , vanilla grass , sweet woodruff , mullein , sweet grass , cassia cinnamon and sweet clover...

) and 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...

. 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

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...

, Aerococcus

Aerococcus

Aerococcus is a genus in the phylum Firmicutes .-Etymology:The name Aerococcus derives from:Greek masculine gender noun aer, aeros , air; New Latin masculine gender noun coccus , a berry; New Latin masculine gender noun Aerococcus, air coccus.-Species:The genus contains 7 species, namely* A...

and Leuconostoc

Leuconostoc

Leuconostoc is a genus of Gram-positive bacteria, placed within the family of Leuconostocaceae. They are generally ovoid cocci often forming chains. Leuconostoc spp. are intrinsically resistant to vancomycin and are catalase-negative . All species within this genus are heterofermentative and are...

. Aesculin will fluoresce under long wave ultraviolet

Ultraviolet

Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...

 light (360 nm): hydrolysis of aesculin results in loss of this fluorescence.

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