Which stain is best known for highlighting fungal cell walls in tissue by producing a black deposit?

• A. Grocott methenamine silver stain
• B. Periodic acid–Schiff (PAS)
• C. Gram stain
• D. Wright-Giemsa

Answer: (A)

Fungi have walls rich in polysaccharides that can be selectively targeted to create a strong, dark contrast in tissue. The Grocott methenamine silver stain uses a silver impregnation step after oxidation, so the fungal cell walls deposit metallic silver that appears as a black or very dark deposit. This produces high contrast against the lighter green counterstain in the slide, making fungal elements easy to see and identify in tissue sections. It’s especially reliable for detecting a wide range of fungi, including yeasts and molds, such as Aspergillus, Candida, and Pneumocystis jirovecii.

Other stains don’t highlight fungal walls with a black deposit. PAS stains fungal walls magenta due to polysaccharides but does not produce the characteristic black deposit; Gram stain can be used for certain specimens but is not routinely reliable for fungi in tissue; and Wright-Giemsa is a cytology/hematology stain not used for visualizing fungi in tissue.