What is the appearance of Gram-positive bacteria after the addition of the first dye (crystal violet) in the Gram stain procedure?

• A. They appear purple.
• B. They appear green.
• C. They appear colorless.
• D. They appear pink.

Answer: (A)

Verified step by step guidance

1. Understand the Gram stain procedure: It is a differential staining technique used to classify bacteria into Gram-positive and Gram-negative based on their cell wall properties.
2. Recall that the first dye used in the Gram stain is crystal violet, which is a purple dye that initially stains all bacteria.
3. Recognize that Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet dye even after the decolorization step.
4. After the addition of crystal violet (the first dye), both Gram-positive and Gram-negative bacteria will appear purple because the dye stains all cells initially.
5. Therefore, immediately after adding crystal violet, Gram-positive bacteria appear purple before any further steps like iodine treatment, decolorization, or counterstaining.