Which type of bacteria stains blue/purple with a standard Gram stain?

• A. Gram-negative
• B. Gram-positive after acid-fast staining
• C. Gram-positive
• D. Gram-neutral

Answer: (C)

When you see a blue/purple color with a standard Gram stain, you’re looking at Gram-positive bacteria. This happens because their cell walls have a thick layer of peptidoglycan that retains the crystal violet-iodine complex even after the alcohol decolorization step, so the stain remains purple. In contrast, Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane that washes out the purple dye, leaving them to take up the counterstain and appear pink. Acid-fast staining is a different technique used for organisms with mycolic acids, so it isn’t what gives the blue/purple result in a standard Gram stain. Therefore, the blue/purple result points to Gram-positive.