After the Gram stain procedure, what color do Gram-negative cells appear due to the counterstain?

After the Gram stain procedure, what color do Gram-negative cells appear due to the counterstain?

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

Answer: (B)

Gram staining differentiates bacteria by cell wall structure. After the primary stain and mordant, Gram-negative cells are decolorized by alcohol because of their thin peptidoglycan layer and outer membrane, then take up the counterstain, typically safranin, which is pink-red. That pink color is what you see in Gram-negative cells under the microscope. In contrast, Gram-positive cells retain the purple crystal violet-iodine complex due to their thick peptidoglycan layer. So the pink appearance reflects the effect of the counterstain on decolorized cells.

Gram staining differentiates bacteria by cell wall structure. After the primary stain and mordant, Gram-negative cells are decolorized by alcohol because of their thin peptidoglycan layer and outer membrane, then take up the counterstain, typically safranin, which is pink-red. That pink color is what you see in Gram-negative cells under the microscope. In contrast, Gram-positive cells retain the purple crystal violet-iodine complex due to their thick peptidoglycan layer. So the pink appearance reflects the effect of the counterstain on decolorized cells.