Under a microscope, Gram-negative bacteria (when dyed with the Gram stain) appear:

• A. pink or red
• B. purple or blue
• C. colorless
• D. green

Answer: (A)

Verified step by step guidance

1. Understand the principle behind the Gram stain: it differentiates bacteria based on the structure of their cell walls, specifically the thickness of the peptidoglycan layer.
2. Recall that Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet dye, causing them to appear purple or blue under the microscope.
3. Recognize that Gram-negative bacteria have a thinner peptidoglycan layer and an outer membrane, which does not retain the crystal violet dye after the decolorization step.
4. During the staining process, Gram-negative bacteria lose the initial purple stain and take up the counterstain (usually safranin), which colors them pink or red.
5. Therefore, when observed under the microscope after Gram staining, Gram-negative bacteria appear pink or red.