Which stain is used to identify acid-fast organisms in ocular specimens?

• A. Ziehl-Neelsen acid-fast stain
• B. India ink
• C. Gram stain
• D. Giemsa stain

Answer: (A)

Acid-fast staining relies on a waxy cell wall rich in mycolic acids that resists decolorization by acid alcohol, so the organisms retain the primary dye. In ocular specimens, when you suspect acid-fast bacteria—such as Mycobacterium species, and to some extent Nocardia—the Ziehl-Neelsen acid-fast stain is used because the heat-assisted staining drives the dye into these tough cell walls. After acid-alcohol decolorization, acid-fast organisms stay red from carbol fuchsin, while the surrounding non–acid-fast material takes on a contrasting blue from the counterstain. This makes acid-fast organisms stand out clearly under the microscope. The other stains aren’t designed to reveal this particular property: India ink shows encapsulated yeasts like Cryptococcus, Gram stain differentiates bacteria by general cell wall characteristics, and Giemsa is used for certain parasites and intracellular organisms.