carbolfuchsin

Carbolfuchsin is a red dye used as the primary stain in acid-fast staining procedures in microbiology. It is a preparation of basic fuchsin dissolved in phenol (carbolic acid), often with a small amount of water and glycerol, which acts as solvent and fixative to enable staining of thick, waxy cell walls.

Mechanism and procedure: Acid-fast organisms such as Mycobacteria have cell walls rich in mycolic acids that resist decolorization. When treated with carbolfuchsin (often with heat to facilitate penetration), the stain penetrates and binds to the cells. Subsequent decolorization with acid-alcohol removes the stain from non–acid-fast cells, and a counterstain (such as methylene blue or brilliant green) colors those cells blue or green, leaving acid-fast organisms bright red.

Procedure variants: In Ziehl-Neelsen staining, carbolfuchsin is applied with heat; in Kinyoun staining, a higher concentration of dye is used without heating. The result is bright red acid-fast organisms against a contrasting background.

Uses: Primary stain for identifying acid-fast bacteria in clinical specimens, including the Mycobacterium tuberculosis complex and other mycobacteria, as well as Nocardia species in some protocols. It remains a standard component of several acid-fast staining methods used in diagnostic microbiology.

Safety and handling: The preparation contains phenol and is caustic; handle in a fume hood with appropriate personal protective equipment and dispose of waste according to institutional guidelines.

History: Carbolfuchsin has been used since the late 19th century as part of acid-fast staining methods developed for rapid identification of mycobacteria.