What is the function of the mordant in the Gram staining process?

In the Gram staining process, the mordant plays a crucial role by binding the primary dye, crystal violet, to the peptidoglycan layer of the bacterial cell wall. This staining method differentiates between Gram-positive and Gram-negative bacteria based on their cell wall composition. The mordant, commonly iodine, forms a complex with the crystal violet dye, which makes it more difficult for the dye to be washed out during subsequent steps, particularly when the decolorizer is applied.

This binding strengthens the interaction between the dye and the cell wall, ensuring that Gram-positive bacteria retain the purple color while undergoing the decolorization step. In contrast, Gram-negative bacteria, with their thinner peptidoglycan layers and outer membrane structure, do not hold onto the dye as effectively after being treated with the decolorizer, leading them to take up the counterstain instead. Understanding the function of the mordant is essential in recognizing how the Gram staining technique effectively distinguishes bacterial types.