In Gram staining, which color indicates Gram-positive bacteria?

In Gram staining, which color indicates Gram-positive bacteria?

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

Answer: (B)

Purple indicates Gram-positive bacteria. The thick peptidoglycan layer in these cell walls traps the crystal violet–iodine complex and isn’t washed out by the decolorizing step, so the cells stay purple under the microscope. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that is disrupted by the decolorizer, causing the purple to be washed away; they then take up the counterstain and appear pink. So the color purple identifies Gram-positive organisms.

Purple indicates Gram-positive bacteria. The thick peptidoglycan layer in these cell walls traps the crystal violet–iodine complex and isn’t washed out by the decolorizing step, so the cells stay purple under the microscope. Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that is disrupted by the decolorizer, causing the purple to be washed away; they then take up the counterstain and appear pink. So the color purple identifies Gram-positive organisms.