Acid-fast Stain

The lipoid capsule of the acid-fast organism takes up carbol-fuchsin resists decolorization (dilute mineral acid). Carbol-fuchsin is more soluble in the lipids of the cell wall than in acid-alcohol, but is readily removed from bacteria that lack the waxy capsule. Staining is enhanced by the phenol and the alcohol, and both of these chemicals also aid in dissolving the basic fuchsin. Alcoholic, rather than aqueous, solutions of acid are used because more uniform decolorization is obtained with alcoholic solutions. The lipoid capsule of mycobacteria is of such high molecular weight that it is waxy at room temperature and successful penetration by the aqueous-based staining solutions used in Gram's staining procedures is prevented.