Which Gram designation is associated with a thicker peptidoglycan layer?

• A. Gram-positive
• B. Gram-negative
• C. Acid-fast
• D. Gram-neutral

Answer: (A)

During Gram staining, the thickness of the peptidoglycan layer determines how well the crystal violet-iodine complex is retained. Gram-positive bacteria have a thick, multilayered peptidoglycan wall without an outer membrane, so the dye is trapped and the cells stay purple after the decolorization step. Gram-negative bacteria possess a thinner peptidoglycan layer and an outer membrane; the dye is washed out more easily and the cells take up the counterstain, appearing pink. Acid-fast organisms have a waxy, lipid-rich wall due to mycolic acids, which resists decolorization by a different mechanism, not simply a thick peptidoglycan layer. So the designation associated with a thicker peptidoglycan layer is Gram-positive.