What is the purpose of applying iodine during the Gram staining process?

The application of iodine during the Gram staining process serves a crucial role in enhancing the differentiation of bacterial cell types based on their cell wall composition. When iodine is applied to the bacterial smear after the initial staining with crystal violet, it acts as a mordant. A mordant is a substance that helps to fix or bind the dye to the cells more effectively.

In Gram-positive bacteria, the thick peptidoglycan layer in the cell wall retains the crystal violet dye-iodine complex, causing these cells to appear purple after the staining process. This retention is due to the structure of the Gram-positive cell wall, which prevents the dye from being washed away during subsequent steps. Therefore, the purpose of iodine is to ensure that the crystal violet dye is firmly fixed within the Gram-positive cells, allowing for clear visualization of their characteristic staining pattern under a microscope.

This function is critical for the Gram staining technique, which is widely used to classify bacteria and inform decisions regarding treatment and further investigation of microbial infections.