Ziehl-Neelsen staining is used to identify which organism?

• A. Mycobacterium tuberculosis
• B. Escherichia coli
• C. Staphylococcus aureus
• D. Candida albicans

Answer: (A)

Ziehl-Neelsen staining detects acid-fast organisms, which have a waxy cell wall rich in mycolic acids that resists decolorization with acid alcohol. This means they retain the red primary stain (carbol fuchsin) after washing, making them stand out against a blue counterstain. Mycobacterium tuberculosis is a classic example and is identified by this stain because of its acid-fast property.

The other organisms listed do not have acid-fast cell walls. Escherichia coli is a Gram-negative rod, Staphylococcus aureus is a Gram-positive cocci, and Candida albicans is a yeast; none retain the red stain after acid-alcohol treatment, so they are not highlighted by Ziehl-Neelsen staining.