The differential staining achieved with the Gram stain is due to differences in which bacterial structure?

• A. Capsule thickness
• B. Plasma membrane composition
• C. Bacterial cell wall
• D. Nucleoid

Answer: (C)

The key idea is the cell wall, especially the peptidoglycan layer. In Gram-positive bacteria, the thick peptidoglycan layer traps the crystal violet–iodine complex and retains it after the alcohol step, so these cells stay purple. In Gram-negative bacteria, the peptidoglycan layer is thin and masked by an outer membrane; alcohol disrupts this outer membrane and washes away the dye, so the cells don’t hold the crystal violet and take up the counterstain to appear pink/red. Capsule thickness isn’t the driver of this differential staining, and the plasma membrane, nucleoid, or other features don’t determine the Gram result.