What color do Gram-positive bacteria appear after staining with crystal violet and a mordant?

What color do Gram-positive bacteria appear after staining with crystal violet and a mordant?

• A. Purple (blue/purple)
• B. Pink
• C. Green
• D. Yellow

Answer: (A)

This question hinges on how the initial crystal violet–iodine stain interacts with bacterial cell walls. The crystal violet, with the mordant iodine, forms a large complex that sticks to cell walls. Gram-positive bacteria have a thick peptidoglycan layer rich in teichoic acids, which traps this crystal violet–iodine complex and keeps the cells purple. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, so the dye–iodine complex is washed out during subsequent steps, and they take up the counterstain later. So after staining with crystal violet and the mordant, Gram-positive organisms appear purple.

This question hinges on how the initial crystal violet–iodine stain interacts with bacterial cell walls. The crystal violet, with the mordant iodine, forms a large complex that sticks to cell walls. Gram-positive bacteria have a thick peptidoglycan layer rich in teichoic acids, which traps this crystal violet–iodine complex and keeps the cells purple. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, so the dye–iodine complex is washed out during subsequent steps, and they take up the counterstain later. So after staining with crystal violet and the mordant, Gram-positive organisms appear purple.