Which organism is described as a fluorescent stain for mycobacteria?

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

Answer: (A)

Auramine-rhodamine is a fluorescent dye used to detect mycobacteria. It binds to the waxy, mycolic acid–rich cell walls of these bacteria and, when viewed under a fluorescence microscope with ultraviolet or blue light, the organisms appear bright yellow-orange against a dark background. This fluorescence makes screening faster and more sensitive than traditional methods.

Calcofluor white, while fluorescent, targets polysaccharides in fungal cell walls and is not specific to mycobacteria. Fluorescein isothiocyanate is a general fluorophore used to label antibodies or proteins and isn’t a standard stain for mycobacteria by itself. Ziehl-Neelsen is the classic non-fluorescent acid-fast stain, using dyes like carbol fuchsin.