The principle behind Ziehl-Neelsen staining is based on the high lipid content of the cell walls of acid-fast bacteria. These lipids make the cell wall impermeable to most stains, but the use of a carbol fuchsin solution, containing phenol, allows the dye to penetrate these walls. The dye is retained even after treatment with acid-alcohol, which decolorizes non-acid-fast cells. A counterstain, typically methylene blue, is then applied to stain the background tissue and non-acid-fast organisms.
