Bile esculin agar is a medium used to identify group D streptococci. This group of bacteria have the ability to grow in the presence of bile, an emulsifying agent produced in the liver. Group D streptococci also have the ability to hydrolyze esculin. This hydrolysis of esculin turns the medium black and denotes a positive test. Other bacteria capable of growing in the presence of bile do not turn the medium black. A variation of this medium uses sodium azide to inhibit the growth of all other Gram-positive bacteria and Gram-negative bacteria.
