In Gram staining, which type typically shows a pink/red color due to a thin peptidoglycan layer and the presence of an outer membrane?

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

Answer: (A)

The staining pattern hinges on the structure of the bacterial cell wall. Gram-negative bacteria have a thin peptidoglycan layer plus an outer membrane. When you apply alcohol during the decolorization step, that outer membrane is disrupted and the thin peptidoglycan can’t retain the crystal violet-iodine complex, so the stain is washed away. Then a counterstain (safranin) is applied, and the cells appear pink or red. That pink/red color is a hallmark of Gram-negative bacteria.

In contrast, Gram-positive bacteria have a thick peptidoglycan layer and no outer membrane, which traps the crystal violet-iodine complex during decolorization, so they stay purple. The term Gram-variable describes inconsistent results in some cases, and Gram-neutral isn’t a standard category.