Esculin Azide Broth

Principle And Interpretation

Esculin Azide Broth is prepared as per the modification of original formula of Isenberg (3). Enterococci are able to hydrolyze esculin whereas other streptococci are not able to do so. Swan (7) used an esculin medium containing 40% bile salts and reported that a positive reaction on bile esculin medium could be correlated with a serological group D precipitin reaction. Further studies by Facklam and Moody presumptively identified group D Streptococci and found that the bile esculin test provided a reliable means of identifying group D Streptococci and differentiating them from non-group D Streptococci (2). The present formulation is a modification of Bile Esculin Agar formulated by Isenberg et al in which bile concentration was 40 g/l. In Esculin Azide Broth, the concentration of bile was reduced to 10 g/l and also additional sodium azide was added (3).

The broth is selective due to presence of bile salts and sodium azide and provides rapid growth of Streptococci. Peptone and yeast extract provide nitrogenous nutrients to the organisms. Bile salts inhibit other gram-positive bacteria while sodium azide inhibits gram-negative bacteria. Streptococci hydrolyze esculin to esculetin and dextrose. Esculetin and ferric ammonium citrate forms dark brown to black complex, imparting dark brown colour to the broth.