Multiple Choice
During the Gram staining procedure, which statement correctly explains why gram-positive bacteria retain the purple color of crystal violet rather than picking up the red color of safranin?
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7. Prokaryotic Cell Structures & Functions
Gram-Positive vs. Gram-Negative Cell Walls
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What is the appearance of Gram-positive bacteria after the addition of the counterstain (safranin) during the Gram stain procedure?
A
Colorless
B
Green
C
Pink/red
D
Purple
Verified step by step guidance1
Understand the Gram stain procedure: It involves staining bacteria with crystal violet (primary stain), then adding iodine (mordant), followed by a decolorizer (usually alcohol or acetone), and finally a counterstain (safranin).
Recognize that Gram-positive bacteria have a thick peptidoglycan layer in their cell walls, which retains the crystal violet-iodine complex even after the decolorization step.
Since Gram-positive bacteria retain the crystal violet stain, they appear purple after the decolorization step and before the counterstain is added.
When the counterstain safranin is applied, it stains any cells that lost the crystal violet stain (Gram-negative bacteria) pink/red, but it does not change the color of Gram-positive bacteria because they are already stained purple.
Therefore, after the addition of safranin, Gram-positive bacteria remain purple in appearance.
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