Which color do Gram-positive bacteria typically stain?

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

Answer: (A)

This question hinges on how the Gram stain distinguishes bacteria by cell wall structure. Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex during the decolorization step, so they retain the dye and appear purple under the microscope. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that doesn’t hold the dye after alcohol decolorization, so they take up the counterstain and appear pink/red. So, the color typically observed for Gram-positive organisms is purple.