Which color do Gram-negative bacteria typically stain?

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

Answer: (A)

Pink. Gram-negative bacteria end up pink after the Gram stain because their cell wall has a thin peptidoglycan layer plus an outer membrane. When the stain is developed, they initially take up the crystal violet-iodine complex, but the alcohol decolorizer dissolves this complex from the thin peptidoglycan layer. They then pick up the counterstain, safranin, which makes them appear pink under the microscope. In contrast, Gram-positive bacteria have a thick peptidoglycan layer that retains the purple crystal violet-iodine complex, so they appear purple. Green is not produced by this standard staining procedure, and a colorless result would indicate a staining error or an issue with the protocol.