Which statement about Gram staining is true?

• A. Gram-positive bacteria stain pink
• B. Gram-negative bacteria stain purple
• C. Gram-positive bacteria have a thick peptidoglycan layer
• D. Gram-negative bacteria have no cell wall

Answer: (C)

Gram staining distinguishes bacteria by cell wall structure: Gram-positives have a thick peptidoglycan layer that traps the crystal violet-iodine complex, so they appear purple after staining. Gram-negatives have a thinner peptidoglycan layer and an outer membrane, which causes the dye to be washed out and a pink counterstain to be taken up. The statement that Gram-positive bacteria have a thick peptidoglycan layer is true, and that thickness is what keeps the purple color. The other ideas don’t fit: Gram-positive bacteria do not stain pink, Gram-negative bacteria do not stain purple, and Gram-negative bacteria do have a cell wall (albeit thinner with an outer membrane).