Multiple Choice
In the figure showing different bacterial cell wall structures labeled A, B, and C, which diagram represents a Gram-negative cell wall?
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7. Prokaryotic Cell Structures & Functions
Gram-Negative Cell Walls
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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
Verified step by step guidance1
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.
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.
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.
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.
Therefore, when observed under the microscope after Gram staining, Gram-negative bacteria appear pink or red.
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