During the Gram staining procedure, which statement correctly explains why gram-positive bacteria retain the purple color of crystal violet rather than picking up the red color of safranin?

• A. Their outer membrane dissolves, allowing safranin to replace the crystal violet.
• B. Their thin peptidoglycan layer allows the crystal violet to be washed out easily.
• C. Their thick peptidoglycan layer retains the crystal violet-iodine complex, preventing safranin from staining the cells.
• D. They lack peptidoglycan, so only safranin can stain them.

Answer: (C)

Verified step by step guidance

1. Understand the Gram staining procedure involves applying crystal violet dye, iodine (which forms a complex with crystal violet), alcohol decolorization, and counterstaining with safranin.
2. Recognize that gram-positive bacteria have a thick peptidoglycan layer in their cell walls, while gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane.
3. During the alcohol decolorization step, the thick peptidoglycan layer in gram-positive bacteria dehydrates and traps the crystal violet-iodine complex inside the cell wall.
4. In contrast, the thinner peptidoglycan layer in gram-negative bacteria cannot retain the crystal violet-iodine complex after alcohol treatment, causing it to be washed out.
5. Because the crystal violet-iodine complex remains in gram-positive bacteria, they retain the purple color and do not take up the red safranin counterstain, which only colors the decolorized gram-negative cells.