- Bile-esculin test is based on the ability of certain bacteria, notably the group D streptococci and Enterococcus species, to hydrolyze esculin in the presence of bile (4% bile salts or 40% bile).
- Bacteria that are bile-esculin positive are, first of all, able to grow in the presence of bile salts.
- Hydrolysis of the esculin in the medium results in the formation of glucose and a compound called esculetin.
- Esculetin, in turn, reacts with ferric ions (supplied by the inorganic medium component ferric citrate) to form a black diffusible complex.
- Group D streptococci and enterococci include opportunistic pathogens such as Enterococcus faecalis, Enterococcus faecium, and Streptococcus bovis.
Results and Interpretation:
- Diffuse blackening of more than half of the slant within 24-48 hours indicates esculin hydrolysis. On plates, black haloes will be observed around isolated colonies and any blackening is considered positive. All group D streptococci will be bile-esculin positive within 48 hours.
Bile-Esculin test for Enterococcus species
