(4). Proteolytic bacteria hydrolyze
casein to form soluble nitrogenous compounds indicated as clear zone surrounding the colonies. More clear zones are seen on
milk agar if, the bacteria produce acid from fermentable carbohydrates in the medium.
Casein enzymic hydrolysate provides amino acids and other complex nitrogenous substances. Yeast extract supplies vitamin B
complex. Addition of skim milk in the medium makes the conditions optimal for microorganisms encountered in milk. Glucose
acts as the carbon source.