What is this medium used for? Starch agar is a general-purpose, nutrient medium used for the cultivation of microbes. Inclusion of starch makes it a rich medium for those bacteria possessing the enzyme alpha-amylase, which breaks starch down to its component glucose molecules. This medium, therefore, is useful for the detection of alpha-amylase.
How is the presence of alpha-amylase determined? Starch combines with iodine to form a blue-brown color. If the microbe produces alpha-amylase, the medium in the vicinity of the growth remains amber, indicating that starch near the growth was digested by the alpha-amylase produced by the microbes.
What is the content of this medium? Starch agar consists of heat-stable digestive products of proteins (called peptones), as would be found in nutrient agar. These provide amino acids, minerals, and other nutrients used by a wide variety of bacteria for growth. In addition, the medium contains soluble starch.
How is the starch hydrolysis performed? For information on how to determine if a microbe produces the enzyme alpha-amylase, refer to the starch hydrolysis test.