Which counterstain in Gram staining colors Gram-negative cells pink?

Which counterstain in Gram staining colors Gram-negative cells pink?

• A. Crystal violet
• B. Safranin
• C. Iodine
• D. Methylene blue

Answer: (B)

In Gram staining, the pink color of Gram-negative cells comes from the counterstain, which is used after the cells have been decolorized. The crystal violet initially stains all cells purple, and iodine helps trap that dye in cells with thick peptidoglycan. Gram-positive cells retain the purple dye because their thick cell wall holds the crystal violet–iodine complex, even after alcohol decolorization. Gram-negative cells, with a thinner peptidoglycan layer and an outer membrane, lose the purple dye during decolorization and become colorless. The counterstain then stains these now-clear Gram-negative cells pink, providing contrast. Safranin is chosen because it yields a clear pink/red color that stands out against the purple of Gram-positive cells. Crystal violet is the primary stain, not the counterstain; iodine acts as a mordant to strengthen stain retention; methylene blue is not the standard counterstain used in this protocol.

In Gram staining, the pink color of Gram-negative cells comes from the counterstain, which is used after the cells have been decolorized. The crystal violet initially stains all cells purple, and iodine helps trap that dye in cells with thick peptidoglycan. Gram-positive cells retain the purple dye because their thick cell wall holds the crystal violet–iodine complex, even after alcohol decolorization. Gram-negative cells, with a thinner peptidoglycan layer and an outer membrane, lose the purple dye during decolorization and become colorless. The counterstain then stains these now-clear Gram-negative cells pink, providing contrast. Safranin is chosen because it yields a clear pink/red color that stands out against the purple of Gram-positive cells. Crystal violet is the primary stain, not the counterstain; iodine acts as a mordant to strengthen stain retention; methylene blue is not the standard counterstain used in this protocol.