Which stain is used to detect mycobacteria in tissue?

• A. Gram stain
• B. PAS
• C. Giemsa
• D. Ziehl-Neelsen acid-fast stain

Answer: (D)

Detecting mycobacteria in tissue relies on their waxy, mycolic-acid–rich cell wall, which makes them resistant to decolorization by acid-alcohol. The Ziehl-Neelsen acid-fast stain uses heat to drive carbol fuchsin into the cells, then decolorizes with acid-alcohol and counterstains with a blue dye. The mycobacteria retain the red primary stain and stand out clearly against a blue background, allowing visualization of organisms such as Mycobacterium tuberculosis in tissue. Other stains like Gram stain often don’t reliably reveal mycobacteria due to their waxy coat, PAS highlights carbohydrates in fungi and some bacteria but isn’t specific for mycobacteria, and Giemsa is used for blood cells and certain parasites. Therefore, the acid-fast Ziehl-Neelsen stain is the appropriate choice for detecting mycobacteria in tissue.