Which of the following is not a simple stain?

• A. Gram stain
• B. Methylene blue
• C. Crystal violet
• D. Safranin

Answer: (A)

Staining methods can be simple or differential. In a simple stain, you apply a single dye to fixed cells, producing a uniform color that helps you see basic features like shape and arrangement. Dyes such as methylene blue, crystal violet, and safranin are typical simple stains because one application colors the cells all the same way, giving straightforward morphology without differentiating between types of cells.

The Gram stain, in contrast, is a differential technique. It uses a sequence of steps and multiple reagents to separate bacteria based on their cell wall properties, yielding two different colors: Gram-positive organisms remain purple, while Gram-negative organisms take up the counterstain and appear pink. Because it relies on several reagents and stages to create a differential outcome, it is not a simple stain.