Which staining method is used to identify acid-fast bacilli?

• A. Ziehl Neelsen
• B. PAS
• C. Mucicarmine
• D. Brown and Brenn

Answer: (A)

Acid-fast staining takes advantage of the waxy, lipid-rich cell wall of mycobacteria that resists decolorization. The Ziehl-Neelsen method uses a red carbol fuchsin dye with heat to push the dye into that wall; after washing with acid alcohol, non–acid-fast organisms lose the stain while acid-fast bacilli stay red and are seen against a blue counterstain. This makes them stand out clearly and is the standard approach for identifying acid-fast bacilli like Mycobacterium species.

The other stains don’t target acid-fast properties: PAS highlights polysaccharides in fungi and some tissues; mucicarmine stains mucin; Brown and Brenn is a different histologic technique sometimes used for acid-fast organisms but is not the primary or most widely used method for detecting acid-fast bacilli.