Gram's Iodine Solution

Gram′s iodine solution acts as a mordant for Gram staining technique. It forms an insoluble crystal violet-iodine complex, which imparts purple color to the cells. The Gram staining allows a fast differentiation of bacteria in Gram-positive and Gram-negative. The mureine structure of the bacteria walls is the basis of the color affinity. Bacteria will be stained with Gram's crystal violet solution - an aniline dye - in the first step. After the treatment with iod solution, a dye-iod complex will form. During the decolorizing step, this complex stays in the multilayer mureine structures of the Gram-positive bacteria and they will appear blue. Gram-negative bacteria have a monolayer mureine structure only, the dye-iod complex does not stay bound to the cellwall, they will be decolorized. Gram-negative bacteria will be counterstained by safranin solution and appear orange.