What does acid fast stain mean??

Answer 1

The Ziehl-Neelsen stain, also known as the acid-fast stain, was first described by two German doctors; Franz Ziehl (1859 to 1926), a bacteriologist and Friedrich Neelsen (1854 to 1894), a pathologist. It is a special bacteriological stain used to identify acid-fast mycobacteria. It can also be used to stain few other bacteria like Nocardia. The reagents used are Ziehl-Neelsen carbolfuchsin, acid alcohol and methylene blue.

Answer 2

Acid fastness is a property of certain bacteria which allows them to retain a dye in an acid medium. The word "fast" here does not refer to speed. It refers to the tenacity with which a substance binds a dye molecule, like the term "color-fast", as used in reference to fabrics. The way it works is, you use a strong solution of a powerful dye like fuchsin, and heat it so that the dye is driven into everything that is present on the slide. You then rinse the slide in an acid solution, which quickly removes the dye from everything that is not acid-fast. Very few organisms are acid-fast, but a few, like the causative organisms of tuberculosis and leprosy, are. These organisms have a thick cell wall which makes it difficult for dye molecules to enter. That's why a strong solution and heat are used to drive the dye in. But once the dye is inside the cell, that same thick cell wall prevents the dye from escaping easily, allowing these organisms to be stained selectively, and observed in the microscope.