The extracellular enzyme alpha amylase catalyzes the breakdown of starch to maltose. The smaller maltose molecules can enter the cell to be used for energy. Iodine interacts with starch and makes it dark blue. If starch has been removed from a culture medium by the action of the bacterial enzyme no blue color appears in the presence of iodine. If iodine stains the medium blue in the presence of growing bacterial colonies then no amylase is produced by that species