In Gram staining, what is the role of iodine?

In Gram staining, iodine serves as a mordant, which is a crucial step in the process of distinguishing between Gram-positive and Gram-negative bacteria. A mordant is a substance that forms a complex with the primary stain, in this case, crystal violet, enhancing its affinity for the bacterial cell wall.

When iodine is added after the application of crystal violet, it binds to the dye, forming a larger complex that is retained more effectively by Gram-positive cells. These cells possess a thick peptidoglycan layer in their cell walls that traps the crystal violet-iodine complex, making them appear purple under a microscope. Without iodine, the crystal violet would be less firmly attached and could easily be washed away in the subsequent steps of the staining process.

By acting as a mordant, iodine is essential for the proper differentiation of bacterial cells during Gram staining, allowing for the visual distinction between different types of bacteria based on their cell wall structures.