Bile Esculin

The Bile Esculin Test is comprised of Escuilin, Peptone (Protein source), bile (acid salts produced by animals for digestion), Sodium Azide (a poison and inhibitor of some bacteria), and Ferric Citrate (Citrate as a carbon source and Fe3+ for it's interaction with Escuilin byproducts). Esculin is comprised of a double ring structure bound to glucose. Some organisms can cleave this glucose off for a source of energy leaving the double ring (esculetin) free in the media. Esculetin reacts with Iron III and turns the media a dark brown color for a positive result. Many organisms are capable of using Esculin like this but the bile salts and azide inhibit most growth outside of a few select groups including some Streptococcus and Enteroccocus organisms.