The Ziehl–Neelsen stain, also known as the acid-fast stain, was first described by two German doctors: the bacteriologist Franz Ziehl (1859–1926) and the pathologist Friedrich Neelsen (1854–1898). It is a special bacteriological stain used to identify acid-fast organisms, mainly Mycobacteria. Mycobacterium tuberculosis is the most important of this group because it is responsible for tuberculosis (TB). Other important Mycobacterium species involved in human disease are Mycobacterium kansasii, Mycobacterium marinum, and members of the Mycobacterium avium complex. Acid fast organisms like Mycobacterium contain large amounts of lipid substances within their cell walls called mycolic acids. These acids resist staining by ordinary methods such as a Gram stain. It can also be used to stain a few other bacteria, such as Nocardia. The reagents used are Ziehl–Neelsen carbolfuchsin, acid alcohol, and methylene blue. Acid-fast bacilli will be bright red after staining.