Enterococcus faecalis colonies on bile esculin azide agar. The ability to hydrolyze esculin in the presence of bile is a characteristic of enterococci and group D streptococci. Bile esculin azide agar is a modification of bile esculin agar with the addition of sodium azide as an inhibitor of Gram-negative bacteria and with the reduction of the bile concentration. The resulting medium is more selective. Esculin hydrolysis is resulting in the blackening of the medium around the colony. Bile salts inhibit gram-positive bacteria other than enterococci.