Bile Esculin Agar is a selective and differential culture medium used primarily to isolate and identify Enterococcus and Streptococcus species. It contains bile salts, which inhibit the growth of most Gram-positive bacteria, allowing only certain organisms to thrive. The medium also includes esculin, which some bacteria can hydrolyze to produce a dark brown complex when combined with iron salts. When bacteria capable of hydrolyzing esculin grow on this medium, they produce a darkening of the agar, indicating a positive result. This characteristic helps microbiologists differentiate between Enterococcus and other non-groupable Streptococcus species, aiding in clinical diagnostics.