Bile Esculin Agar (or Enterococcal Agar or BEA) is prepared according to the formula specified by Swan, and subsequently evaluated by Facklam and Moody. It is a selective medium for the isolation of Enterococci and Group D Streptococci. It was concluded that the biliary esculin test was a reliable means of identifying enterococci and differentiating them from non-enterococci. This medium contains bile to inhibit the growth of gram-positive bacteria as well as many gram-negative bacteria. Sodium azide also suppresses the growth of gram-negative bacteria and Streptococci except those of group D. Enterococci hydrolyze esculin to esculetin, which forms a complex with ferric citrate to produce insoluble iron salts, causing the medium to turn brown-black or black. Enterococci therefore form small translucent colonies surrounded by a black halo.