In a completed Gram stain procedure, what is the final color of Gram-negative bacteria when observed under a microscope?

• A. Pink or red
• B. Colorless
• C. Blue
• D. Purple

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 Gram-positive bacteria have a thick peptidoglycan layer that retains the crystal violet-iodine complex, appearing 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 stain after the decolorization step.
4. During the staining process, after decolorization, Gram-negative bacteria lose the crystal violet stain and are then counterstained with safranin, a red or pink dye.
5. Therefore, the final color observed for Gram-negative bacteria under the microscope is pink or red due to the safranin counterstain.