In Gram staining, a result showing both Gram-positive and Gram-negative bacteria remaining purple is most commonly caused by which issue?

• A. Underdecolorized in the acetone step
• B. Overdecolorized with alcohol
• C. Inadequate crystal violet application
• D. Excess counterstain applied

Answer: (A)

The key idea is that the decolorization step in Gram staining determines whether Gram-negative cells lose the crystal violet-iodine complex or not. Gram-positive cells retain it because of their thick peptidoglycan layer, while Gram-negative cells, with a thinner wall and outer membrane, release it when the decolorizer is applied properly. If the decolorizer is used too lightly or briefly, the crystal violet-iodine complex isn’t fully removed from the Gram-negative cells, so they stay purple just like Gram-positive cells. That creates a mixed appearance where both types appear purple.

If the stain were decolorized too much, Gram-positive cells would lose their purple color and take up the counterstain, making them pink, and Gram-negative cells would also be pink, so you wouldn’t see both types purple. Inadequate crystal violet application or excessive counterstain can lead to weak, inconsistent, or confusing staining, but they don’t produce the specific scenario of both organisms remaining purple.