Which of the following is a fluorescent stain for mycobacteria?

• A. Auromine-rhodamine
• B. Calcofluor white
• C. Fluorescein isothiocyanate
• D. Ziehl-Neelsen

Answer: (A)

Fluorescent acid-fast stains use a fluorochrome that binds to the waxy, lipid-rich cell wall of mycobacteria, making them glow brightly under fluorescence microscopy. The best-known and most widely used fluorescent stain for detecting mycobacteria is a combination of Auramine O with Rhodamine B, often referred to as auramine-rhodamine. When specimen slides, such as sputum, are examined under blue (or UV) light, the mycobacteria appear bright yellow-orange against a dark background, which improves sensitivity and speeds screening compared with traditional light-dark stains.

Calcofluor white, on the other hand, binds to polysaccharides like cellulose and chitin and is used mainly for fungi; it’s not specific for mycobacteria. Fluorescein isothiocyanate is a general fluorophore used to label antibodies or proteins in various immunofluorescence assays, but it is not by itself a standard stain for mycobacteria. Ziehl-Neelsen is the classic nonfluorescent acid-fast stain used with light microscopy, not a fluorescent method.