Bile Esculin Agar Slant


What is this medium used for?  Bacteria are identified based largely on what organic compounds they can break down. The range of compounds used depends on the collection of enzymes a species of bacteria can make. Esculin is an organic compound that some bacteria can use for carbon and energy. This medium is used to determine whether the microbe can break down esculin.


Before using esculin, the microbe must first prove itself worthy by growing in the presence of bile salts. These chemicals are, as their name implies, found in bile, an important tool of the digestive system for emulsifying fats and making them digestible. Many bacteria are susceptible to bile salts and cannot grow in their presence. Incorporation of bile salts into this medium thus makes bile esculin agar inhospitable for many bacteria.


The net result is that bile esculin agar is useful for determining the ability of the microbe to grow in the presence of bile salts AND its ability to digest esculin...two distinct tests.


How is growth on bile determined?  If the microbe can grow on bile esculin agar, visible growth is seen on the slant after 24 hours.  No growth is a negative result for "growth on bile".


How is esculin hydrolysis determined?  If esculin is digested, a visible product is produced: a chocolate brown-colored compound called esculetin is present after 24 hours incubation.


What is the content of this medium?  The medium used is bile esculin agar slants. This is a nutrient agar-based medium containing 10% bile salts and allowed to solidify at a slant. It contains 0.1% esculin.


How are the two tests performed?  For information on how to determine the ability of a microbe to grow in the presence of bile salts, refer to the growth on bile salts test. For more information on how to determine the ability to digest esculin, refer to the esculin hydrolysis test.