Which stain is best for demonstration of Mycobacterium leprae?

• A. Fite
• B. Ziehl-Neelsen
• C. PAS
• D. Gram

Answer: (A)

The essential idea is that Mycobacterium leprae is a waxy, lipid-rich organism that is best demonstrated in tissue with a stain designed to preserve weakly acid-fast bacilli. Mycobacteria resist decolorization after staining because of their mycolic acids, so acid-fast methods are used. The Fite stain is a modified, gentler acid-fast technique specifically developed for tissue sections containing mycobacteria like M. leprae. Its milder decolorizing step helps keep the bacilli visible in fixed tissue, making the organisms appear red against the counterstain. In contrast, the Ziehl-Neelsen method uses a stronger decolorizer and is great for many mycobacteria in other specimen types, but it can miss M. leprae in tissue sections. PAS and Gram stains don’t reliably highlight these organisms in tissue—the former stains polysaccharides and the latter fails to penetrate the waxy cell wall. So, for demonstrating Mycobacterium leprae in tissue, the Fite stain is the best choice.