In a standard Gram stain, what color do Gram-negative bacteria appear after the safranin counterstain step?

• A. Green
• B. Pink to red
• C. Colorless (unstained)
• D. Purple

Answer: (B)

Verified step by step guidance

1. Understand the purpose of the Gram stain: it differentiates bacteria into Gram-positive and Gram-negative based on their cell wall structure.
2. Recall that the primary stain, crystal violet, initially stains all bacteria purple.
3. During the decolorization step, Gram-negative bacteria lose the crystal violet stain because their thinner peptidoglycan layer cannot retain it, while Gram-positive bacteria retain the purple color.
4. The safranin counterstain is then applied, which stains the now colorless Gram-negative bacteria a pink to red color.
5. Therefore, after the safranin counterstain step, Gram-negative bacteria appear pink to red, while Gram-positive bacteria remain purple.