Final answer:
In Gram staining, iodine serves as the mordant that complexes with the crystal violet dye to help it bind to the bacterial cells, particularly those with thick peptidoglycan layers, resulting in gram-positive bacteria appearing purple and gram-negative bacteria appearing pink after counterstaining with safranin.
Step-by-step explanation:
The mordant used in the Gram staining process is iodine. A mordant is a substance that helps to fix or stabilize stains and dyes on specimens. In Gram staining, after applying the primary stain, which is crystal violet, Gram's iodine is added. This mordant forms a complex with the crystal violet dye, making it more difficult to wash out of cells with thicker peptidoglycan layers, typically gram-positive bacteria. These cells will appear purple after the staining procedure. Conversely, gram-negative bacteria, which have thinner peptidoglycan layers, do not retain the dye-mordant complex as well and are decolorized when the ethanol decolorizing agent is added, making them susceptible to counterstaining by safranin, which dyes them pink.