Ziehl–Neelsen staining is a type of acid-fast stain, first introduced by Paul Ehrlich. Ziehl–Neelsen staining is a bacteriological stain used to identify acid-fast organisms, mainly Mycobacteria. It is named for two German doctors who modified the stain: the bacteriologist Franz Ziehl (1859–1926) and the pathologist Friedrich Neelsen (1854–1898). Mycobacteria In anatomic pathology specimens, immunohistochemistry and modifications of Ziehl–Neelsen staining (such as the Fite-Faraco method) have comparable diagnostic utility. Both of them are superior to traditional Ziehl–Neelsen staining. The genus Mycobacterium is a slow growing bacteria, made up of small rods that are slightly curved or straight, and are considered to be gram positive. Some mycobacteria are free-living saprophytes, but many are pathogens that cause disease in animals and humans. Mycobacterium bovis causes tuberculosis in cattle. Since tuberculosis can be spread to humans, milk is pasteurized to kill any of the bacteria. The Mycobacteria species that causes tuberculosis (TB) in humans is Mycobacterium tuberculosis, which is an airborne bacterium that typically infects the human lungs. Testing for TB includes blood testing, skin tests, and chest X-rays. When looking at the smears for TB, it is stained using an acid-fast stain. These 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 for Ziehl–Neelsen staining are carbol fuchsin, acid alcohol, and methylene blue. Acid-fast bacilli are bright red after staining. Fungi Ziehl–Neelsen staining is a type of narrow spectrum fungal stain. Narrow spectrum fungal stains are selective, and they can help differentiate and identify fungi. The results of Ziehl–Neelsen staining is variable because many fungal cell walls are not acid fast. An example of a common type of acid-fast fungus that is usually stained with Ziehl–Neelsen staining is called Histoplasma (HP). Histoplasma is found in soil and the feces of birds and bats. Humans can contract histoplasmosis by inhalation of the fungal spores. Histoplasma enters the body and goes to the lungs where the spores turn into yeast. The yeast gets into the blood stream and affects lymph nodes and other parts of the body. Usually people do not get sick from inhaling the spores, but if they do they usually have flu like symptoms. Another variation on this staining method is used in mycology to