In a standard Gram stain, what color do Gram-negative bacteria appear after counterstaining?

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

Answer: (A)

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 or blue.
3. Know that the decolorization step with alcohol or acetone removes the crystal violet stain from Gram-negative bacteria due to their thinner peptidoglycan layer.
4. Recognize that after decolorization, Gram-negative bacteria are colorless and need to be counterstained to be visible under the microscope.
5. Identify that the counterstain used is safranin, which stains Gram-negative bacteria pink to red, while Gram-positive bacteria remain purple or blue.