In Gram staining, after applying the counterstain, what color do Gram-negative bacteria appear?

In Gram staining, after applying the counterstain, what color do Gram-negative bacteria appear?

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

Answer: (A)

The stained result hinges on cell wall structure. After the initial crystal violet stain and iodine, the alcohol decolorizer washes away the dye from cells with a thin peptidoglycan layer and an outer membrane. That makes Gram-negative bacteria colorless. When the counterstain is applied, these decolorized cells take up the pink/red dye and appear pink under the microscope. In contrast, Gram-positive bacteria retain the purple crystal violet-iodine complex because their thick peptidoglycan wall traps it. So, after the counterstain, Gram-negative bacteria appear pink.

The stained result hinges on cell wall structure. After the initial crystal violet stain and iodine, the alcohol decolorizer washes away the dye from cells with a thin peptidoglycan layer and an outer membrane. That makes Gram-negative bacteria colorless. When the counterstain is applied, these decolorized cells take up the pink/red dye and appear pink under the microscope. In contrast, Gram-positive bacteria retain the purple crystal violet-iodine complex because their thick peptidoglycan wall traps it. So, after the counterstain, Gram-negative bacteria appear pink.