carbol fuchsin stain

In the classic Ziehl-Neelsen method, carbol fuchsin is driven into the waxy cell wall with heat, hence the 'hot' method; the Kinyoun modification uses a higher phenol concentration and no heat. After decolorization with acid-alcohol, only acid-fast organisms retain the red dye. Acid-fast organisms include Mycobacterium species, Nocardia (which is partially acid-fast and, unlike Mycobacterium, is aerobic, branching, filamentous, and catalase-positive), and the oocysts of Cryptosporidium, Cyclospora, and Cystoisospora, which is how they are identified in stool from a patient with AIDS and chronic watery diarrhoea. Auramine-rhodamine fluorescent staining is more sensitive and is now preferred for screening sputum, with confirmation by Ziehl-Neelsen. Acid-fastness reflects mycolic acids in the cell wall, the same lipids whose synthesis is blocked by isoniazid, and this waxy envelope also explains why mycobacteria stain poorly with Gram stain and grow slowly.