Which staining technique is used to identify acid-fast bacteria such as Mycobacterium tuberculosis?

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

Answer: (D)

Acid-fast staining targets bacteria with waxy, mycolic-acid–rich cell walls that resist decolorization by acid alcohol. The standard method to identify Mycobacterium tuberculosis is the Ziehl-Neelsen acid-fast stain. It uses carbol fuchsin and heat to force the dye into the waxy wall; after applying acid-alcohol, acid-fast organisms keep the red color, while other cells take up a contrasting counterstain. A cold alternative is the Kinyoun stain, which yields a similar result without heating. In contrast, Gram stain is a general method that doesn’t specifically highlight acid-fast organisms, and Wright-Giemsa is used for blood smears and parasites rather than detecting acid-fast bacteria.