Gram staining 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 identify the structure of the bacterial cell wall. The first dye, crystal violet, stains all bacteria, while iodine acts as a mordant, fixing the dye. After washing with alcohol, Gram-positive bacteria retain the crystal violet stain, appearing purple, while Gram-negative bacteria lose the stain and are counterstained with safranin, appearing pink. This differentiation is crucial for determining appropriate antibiotic treatments and understanding bacterial characteristics.