What color do Gram-positive bacteria stain with the Gram stain?

What color do Gram-positive bacteria stain with the Gram stain?

• A. Pink
• B. Purple
• C. Green
• D. Yellow

Answer: (B)

Gram staining differentiates bacteria by cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex during staining, so that complex remains in place during the decolorization step. Because it isn’t washed away, these cells retain the purple dye and appear purple under the microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; alcohol decolorization removes the purple dye from them, and they take up the pink counterstain, appearing pink. Therefore, Gram-positive bacteria stain purple.

Gram staining differentiates bacteria by cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex during staining, so that complex remains in place during the decolorization step. Because it isn’t washed away, these cells retain the purple dye and appear purple under the microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane; alcohol decolorization removes the purple dye from them, and they take up the pink counterstain, appearing pink. Therefore, Gram-positive bacteria stain purple.