Which staining method uses auramine-rhodamine to visualize acid-fast bacilli?

• A. Ziehl-Neelsen
• B. Auramine-rhodamine staining
• C. Grocott
• D. Gridley

Answer: (B)

Staining acid-fast bacilli with a fluorescent dye uses auramine-rhodamine, allowing detection under a fluorescence microscope. The auramine O and rhodamine B molecules bind to the waxy cell walls rich in mycolic acids of mycobacteria, and when viewed under UV or blue light they emit a bright yellow-orange fluorescence. This makes the organisms stand out quickly against a dark background and generally increases sensitivity for screening, such as in sputum samples. In contrast, the Ziehl-Neelsen method uses heat to drive carbol fuchsin into the cell wall and is observed with standard light, producing red rods on a pale background. Grocott’s method is a fungal silver stain, and Gridley is not used for routine visualization of acid-fast bacilli.