What does purple mean?

• A. Gram Positive
• B. Gram Negative
• C. Gram Neutral
• D. No stain

Answer: (A)

Purple means the bacterium retains the crystal violet-iodine complex during Gram staining, which happens because it has a thick peptidoglycan cell wall that traps the dye. In this procedure, after applying crystal violet and iodine, a decolorizer is used. Cells with a thick peptidoglycan layer hold onto the dye and stay purple, identifying them as Gram-positive. Bacteria with a thinner peptidoglycan layer and an outer membrane are decolorized and take up the counterstain, appearing pink or red, which would be Gram-negative. So, purple indicates Gram-positive. The other outcomes described don’t fit the standard results: Gram-negative would end up pink/red after the counterstain, while Gram-neutral or no stain aren’t typical results in this staining method.