Principle

The Gram staining technique is based on the difference in cell wall structure of bacteria. Gram-positive bacteria have a thick peptidoglycan layer, which retains the primary stain (crystal violet) even after decolorization. In contrast, Gram-negative bacteria have a thin peptidoglycan layer and higher lipid content, causing them to lose the primary stain during decolorization and take up the counterstain (safranin).