Ziehl-Neelsen staining is used to detect acid-fast organisms (e.g., Mycobacterium). What color do acid-fast cells appear after staining?

• A. Detects Gram-positive cocci; purple against a pink counterstain.
• B. Detects acid-fast organisms such as Mycobacterium; acid-fast cells stain red/pink against a blue counterstain.
• C. Detects spores; green against a blue counterstain.
• D. Detects Gram-negative rods; pink against a yellow counterstain.

Answer: (B)

Ziehl-Neelsen staining reveals the distinctive property of acid-fast organisms: their waxy, mycolic-acid–rich cell walls resist decolorization and retain the primary stain. The red carbol fuchsin stains these cells, and after acid-alcohol decolorization, only the acid-fast cells keep that red color while non–acid-fast cells are counterstained blue by methylene blue. The result is acid-fast cells appearing red or pink against a blue background, which helps differentiate Mycobacterium from other bacteria.