Multiple Choice
In a standard Gram stain, what color do Gram-negative bacteria appear after counterstaining?
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9. Microscopes
Gram Stain
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In the Gram stain procedure, which reagent serves as the mordant (forming a crystal violet–iodine complex)?
A
Safranin
B
Gram’s iodine
C
Crystal violet
D
95% ethanol (decolorizer)
Verified step by step guidance1
Understand the role of each reagent in the Gram stain procedure: Crystal violet is the primary stain, Gram's iodine acts as the mordant, 95% ethanol is the decolorizer, and safranin is the counterstain.
Recall that a mordant is a substance that forms a complex with the primary stain to enhance its binding to the bacterial cell wall, making the stain more difficult to remove.
Identify which reagent forms a complex with crystal violet; this complex is known as the crystal violet–iodine complex.
Recognize that Gram's iodine serves as the mordant by combining with crystal violet to form this complex, which is crucial for differentiating Gram-positive and Gram-negative bacteria.
Confirm that the other reagents do not serve as mordants: safranin is a counterstain, and ethanol is used to decolorize, not to form complexes.
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