Starch Agar is basically an all-purpose medium (e.g., Nutrient Agar, Heart Infusion Agar) to which 0.4% soluble starch is added. A plate of this medium is single streaked or spot inoculated with the organism to be tested. After incubation (preferably for several days) the medium is flooded with Gram's iodine. The iodine reacts with starch to form a dark blue-colored complex. Any clear area around the growth of the culture after the addition of the iodine indicates the breakdown of starch by the organism due to its production of amylase, an extracellular enzyme.
Note the figure at the right. Two different organisms were streaked on the plate of Starch Agar. The clear (non-blue) area around the culture at left indicates the absence of intact starch – and therefore a positive reaction for the amylase test. The blue color extends up to the culture on the right which is thus seen to be unable to break down starch – a negative reaction for this test.
Many bacteria produce "extracellular enzymes" which catalyze chemical reactions outside of the cell. This way, potential nutrient sources which cannot be taken in through the cell membrane can be broken down such that the resulting products can diffuse (or be transported) into the cell. Commonly tested substances include starch, casein (milk protein) and gelatin. The test substrate does not have to be solublized in the medium, as this method can also be used to test the breakdown of fats and chitin.
Another way in which extracellular enzymes function can be seen with coagulase, an enzyme produced by Staphylococcus aureus which coagulates blood plasma around the cell, producing an "artificial capsule."
| Index of general micro lab concepts
including Differential Media and
Identification of Bacteria
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