Which type of bacteria stains pinkish/red with a standard Gram stain?

• A. Gram-positive
• B. Gram-neutral
• C. Both
• D. Gram-negative

Answer: (D)

The pinkish/red result in a standard Gram stain points to Gram-negative bacteria. This staining pattern comes from the cell wall structure: Gram-negative cells have a thin peptidoglycan layer and an outer membrane. During the decolorization step, this thin layer and outer membrane don’t retain the crystal violet-iodine complex, so the cells are decolorized. When the counterstain is applied, these cells take up the red dye and appear pink or red. In contrast, Gram-positive bacteria have a thick peptidoglycan layer that traps the crystal violet-iodine complex, so they remain purple after decolorization. The idea of Gram-neutral isn’t part of the standard Gram-stain results, and “both” isn’t correct because the two groups yield distinct colors.