Bacteria degrade macromolecules by secreting exoenzymes, enzymes that have their action outside the cytoplasm. We can investigate a bacteria's ability to degrade specific macromolecules by mixing the macromolecule into the agar and checking for degradation following exposure to the bacteria.

To ensure that we observe positive results, we inoculate these plates very heavily over a small area. Since we are not spreading bacteria over the whole plate, we can easily put two or three bacteria on a single plate. This allows us to put positive and negative controls on the plate with our unknown bacterium. See sketch below.

Starch plate. If we are looking for amylase activity breaking down starch in the media, we observe results by covering the plate (after the bacteria has grown!) with grams iodine. Iodine forms an insoluble purple complex with starch. If the starch has broken down, the area around the bacterial streak will remain clear after starch addition.