AAT Bioquest

What's the role of iodine in the Gram stain procedure?

Posted March 22, 2023


After using crystal violet dye for the slide’s initial staining, the next step is fixing the dye using iodine. Iodine forms the crystal violet-iodine complex in order for dye to easily be removed. Iodine fixes the crystal violet into the cell call of bacteria by acting as a chemical mordant. G+ bacteria remain purple due to their adherence to the iodine. If iodine has no cell wall to adhere to, the dye will rinse away and the stain color will be pink; these cells would be classified as G- bacteria. Iodine also helps to reduce the variability of the test.

Additional resources

Gram Staining

Gram Staining

MycoLight™ Rapid Fluorescence Gram-Positive Bacteria Staining Kit