Acidfast

Acid-fast describes a staining property of certain bacteria that enables them to retain a primary stain after decolorization with an acid-alcohol. The best-known examples are mycobacteria, including Mycobacterium tuberculosis and Mycobacterium leprae, as well as some Nocardia species that are classified as weakly acid-fast. The term is used in clinical microbiology to distinguish organisms with waxy, lipid-rich cell walls from non-acid-fast bacteria.

The classic laboratory methods are the Ziehl-Neelsen stain and the cold Kinyoun stain. In both, carbol fuchsin is applied as the primary stain, the slide is washed with an acid-alcohol decolorizer, and a counterstain (such as methylene blue) is used. Acid-fast organisms retain the red or pink color, while non-acid-fast cells take up the counterstain and appear blue. Nocardia can be variably acid-fast and may require modified or additional staining procedures (e.g., Fite-Faraco) for reliable detection.

Biologically, acid-fastness is largely due to the high lipid content of the cell wall, especially mycolic acids, which form a waxy barrier. This structure reduces staining washout and contributes to resistance to desiccation and some chemical agents, aiding environmental persistence and, to some extent, intrinsic resistance mechanisms.

Clinically, acid-fast staining is a rapid, initial diagnostic tool used to screen specimens such as sputum for tuberculosis and other mycobacterial infections. A positive stain prompts further testing, including culture and molecular assays. The technique is not species-specific and must be complemented by additional tests. Some non-mycobacterial organisms and certain parasites can also show variable acid-fast staining under specific protocols.