A test for the hydrolysis of esculin in the presence of bile is especially useful in identifying species of which genus?

• A. Abiotrophia
• B. Corynebacterium
• C. Enterococcus
• D. Staphylococcus

Answer: (C)

The test for esculin hydrolysis in the presence of bile (bile esculin test) is particularly informative for organisms that can tolerate bile and break down esculin. Enterococcus species are well known for this ability. In a bile-containing medium, if the organism hydrolyzes esculin, esculetin is released and binds with ferric ions in the medium to form a dark brown-to-black precipitate, giving a positive result. The bile component helps select for bile-tolerant group D streptococci, of which Enterococcus is a major part, making this test a reliable way to identify these genera.

Other listed bacteria do not consistently show this positive bile-esculin reaction, so the dark color change is not expected with them under the same conditions. This makes the bile esculin test a useful diagnostic tool specifically for recognizing Enterococcus species.