acid-fast stain, a method of staining used in bacteriology in which a smear on a slide is treated with carbol-fuchsin stain or auramine-rhodamine stain, decolorized with acid alcohol, and counterstained with methylene blue or potassium permanganate to identify acid-fast bacteria. Acid-fast organisms resist decolorization and appear red or yellow against a dark background when viewed under a microscope. The stain may be performed on any clinical specimen but is most commonly used in examining sputum for Mycobacterium tuberculosis, an acid-fast bacillus. See also Ziehl-Neelsen test.
