Hydrolysis of esculin forms the basis of 2 separate tests.
Esculin is a glucosidic coumarin (6-beta-glucoside-7-hydroxy-coumarin). It is fluourescent under UV light at 360 nm.
When hydrolyzed it forms esculitin, which binds with ferric ions (Fe+3) to form a black compound. Esculetin is nonfluorescent.
The bile esculin test is performed in the presence of bile or bile salts. Group D streptococci and enterococci can hydrolyze esculin in the presence of bile.
The esculin test without bile is used to identify genera of nonfermenting bacilli (Sphingobacterium, Chryseobacterium, Aeromonas and others).
Incubation is performed at 35°C for 24 to 48 hours.
Causes of a false positive result:
(1) mixed isolate
Causes of a false negative result:
(1) reading the test too soon (resulting before 48 hours)
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