Which of the following is a fluorescent stain for mycobacteria?

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

Answer: (D)

Staining mycobacteria can be done with acid-fast methods and with fluorescent fluorochrome stains. The fluorescent stain used for mycobacteria is auramine-rhodamine. It binds to the lipid-rich mycobacterial cell wall and, under fluorescence microscopy, produces a bright yellow-orange signal that makes acid-fast bacilli stand out against a dark background. This approach is more sensitive and faster for screening many specimens than traditional bright-field acid-fast staining. Calcofluor white targets polysaccharides in fungal cell walls, not mycobacteria. The Ziehl-Neelsen stain is the classic bright-field acid-fast method, which is not fluorescent. Fluorescein isothiocyanate is a general fluorescent tag used to label antibodies or other molecules, not a specific mycobacterial stain by itself.