Respiratory CAS Practice Exam

Acid-fast staining is used because Mycobacterium tuberculosis and Nocardia have cell walls rich in mycolic acids, a waxy lipid layer that resists decolorization by acid alcohol. This lets them retain the red dye (carbol fuchsin) after the acid-alcohol step, making them visible under the microscope. TB bacteria are strongly acid-fast, while Nocardia is variably acid-fast and is often demonstrated with a modified (weaker decolorizer) acid-fast stain to highlight its partial acid-fastness. Other stains aren’t as reliable for these organisms: Gram stain can show them as Gram-positive, but their waxy walls hamper reliable Gram staining; silver stain is more for fungi and certain other organisms; Giemsa is used for blood cells and parasites. So the technique that best reveals both is acid-fast staining.