![Acid-fast bacilli acid-fast bacteria, Ziehl-Neelsen stain](mycobacterium-tuberculosis-acid-fast-rods.jpg)
Computer-generated image of Mycobacterium tuberculosis bacteria. Illustration of the Ziehl-Neelsen stain result. The reagents used are carbol fuchsin, acid alcohol, and the malachite green counterstain (quite toxic, nowadays replaced by methylene blue). Acid-fast bacilli stain red and the background is green.