Which stain is used to detect acid-fast bacteria such as Mycobacterium tuberculosis?

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

Answer: (C)

Acid-fast staining relies on the waxy, mycolic acid–rich cell wall of mycobacteria, which resists decolorization by acid. The Ziehl-Neelsen method uses hot carbol fuchsin to penetrate that wall; after acid-alcohol decolorization, acid-fast organisms retain the red dye while non–acid-fast cells do not. A blue counterstain then highlights the surrounding cells. This makes Mycobacterium tuberculosis visible as bright red against a contrasting background, which is why Ziehl-Neelsen is used to detect acid-fast bacteria in samples like sputum. Other stains don’t exploit this property: Gram stain is not reliable for mycobacteria, Giemsa is used for blood smears and parasites, and PAS highlights fungi and glycogen.