A Gram stain is a laboratory technique used to classify bacteria into two groups: Gram-positive and Gram-negative. This method involves applying a series of dyes to a bacterial sample, which helps to visualize the cell wall structure. The primary stain, crystal violet, is followed by iodine, which forms a complex with the dye. After washing with alcohol, the cells are counterstained with safranin. The key difference between the two groups lies in their cell wall composition. Gram-positive bacteria retain the crystal violet stain, appearing purple, due to a thick peptidoglycan layer. In contrast, Gram-negative bacteria do not retain the primary stain and appear pink, as they have a thinner peptidoglycan layer and an outer membrane.