Which staining method identifies acid-fast bacteria?

Which staining method identifies acid-fast bacteria?

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

Answer: (B)

Acid-fast bacteria have a waxy cell wall rich in mycolic acids, which makes them resistant to decolorization by acid-alcohol. The Ziehl-Neelsen stain is designed to exploit that property. It uses carbol fuchsin as the primary stain and heat to drive it into the cell wall; after washing, an acid-alcohol decolorizer is applied. Acid-fast organisms retain the red color, while non–acid-fast cells are decolorized and then counterstained (usually blue). This makes acid-fast organisms like Mycobacterium tuberculosis or Mycobacterium leprae stand out as red against a contrasting background. Gram stain differentiates bacteria by general cell wall structure but doesn’t specifically identify acid-fast organisms, so it won’t reliably reveal them. Giemsa and Wright stains are used for blood cells and certain parasites, not for detecting acid-fast bacteria.

Acid-fast bacteria have a waxy cell wall rich in mycolic acids, which makes them resistant to decolorization by acid-alcohol. The Ziehl-Neelsen stain is designed to exploit that property. It uses carbol fuchsin as the primary stain and heat to drive it into the cell wall; after washing, an acid-alcohol decolorizer is applied. Acid-fast organisms retain the red color, while non–acid-fast cells are decolorized and then counterstained (usually blue). This makes acid-fast organisms like Mycobacterium tuberculosis or Mycobacterium leprae stand out as red against a contrasting background.

Gram stain differentiates bacteria by general cell wall structure but doesn’t specifically identify acid-fast organisms, so it won’t reliably reveal them. Giemsa and Wright stains are used for blood cells and certain parasites, not for detecting acid-fast bacteria.