Acid fast staining (Ziehl-Neelsen staining)

 Some acid fast organisms such as genus Mycobacterium have nearly impermeable cell wall. They contain mycolic acid and large amounts of fatty acids and complex lipids in their cell wall and make them resistant to disinfectants and most compounds. Therefore they require a special ZN staining technique.

The primary stain used in acid fast staining is carbolfuchsin , is lipid soluble and contains phenol, which helps the stain penetrate the cell wall assisted by addition of heat. Then with the help of strong acid-alcohol decolorizer, the smear is rinsed which washes the primary stain from non-acid fast organisms but cannot do so in acid-fast organisms. For the non acid fast ones, counter stain methylene blue stains the cells.