Mycobacteria can be examined by which acid-fast stain that does not require heating?

• A. Dieterle
• B. Gimenez
• C. Wright's
• D. Kinyoun

Answer: (D)

The concept tested is how acid-fast staining works and why some stains don’t require heat. Mycobacteria have a waxy, lipid-rich cell wall that holds onto dye materials even after acid-alcohol decolorization. In the hot Ziehl-Neelsen method, heating helps drive the dye into that tough cell wall. The cold, or heat-free, approach uses a higher concentration of dye with phenol so the stain can penetrate at room temperature. This non-heated acid-fast stain is the Kinyoun method. The other stains listed aren’t acid-fast stains—they’re used for different purposes (Dieterle is a silver stain for fungi and some bacteria, Gimenez for rickettsiae, and Wright’s for blood smears).