Starch agar contains high molecular weight starch molecules (soluable starch). When Iodine is added it reacts with starch to produces a dark brown/purple color. A zone of clearing forms around organisms that produce the exoenzyme amylase after addition of iodine. This plate was incubated for 24 hrs at 37 degree’s C then iodine added. Organism (A) Bacillus cereus shows a (+) reaction for amylase production. Organism (B) E. coli is negative.