Discussion
Which one of the following statements about microbiological stains is CORRECT?
Answer: D
The correct statement is India ink is used to visualize capsules.
India ink is a negative stain: its carbon particles colour the background but cannot enter the polysaccharide capsule, which shows as a clear halo around each cell, as with Cryptococcus neoformans.
The acid-fast (Ziehl-Neelsen) stain uses a primary stain, a decolouriser and a counterstain, so it is a differential stain, not a simple one. Endospores resist the Gram stain and need a dedicated spore stain such as the Schaeffer-Fulton method with malachite green. Methylene blue is a basic (cationic) dye, which is why it binds the negatively charged surface of bacterial cells.
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