Gram’s iodine is the second chemical used in the Gram stain process. The first chemical used, crystal violet, binds to negatively charged parts of the bacterial cell. Iodine acts as a “mordant”, a chemical that enhances the reactivity of a stain for tissue. Thus, iodine is used to increase the tightness of bond between crystal violet and the bacterial cells. Though it has a brownish color, Gram’s iodine does not alter the color of bacteria due to its use. All bacteria stained with crystal violet and treated with iodine have a purple color.
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