In acid-fast staining, which dye serves as the primary stain?

In acid-fast staining, which dye serves as the primary stain?

• A. Methylene blue
• B. Carbol fuchsin
• C. Crystal violet
• D. Safranin

Answer: (B)

In acid-fast staining, you need a dye that can penetrate the waxy, mycolic acid–rich cell wall and stay bound after decolorization. Carbol fuchsin fits this role because it is a phenol-containing red dye that can penetrate the cell wall (often aided by heat or a higher phenol concentration in cold methods). It is used as the primary stain, coloring acid-fast organisms red. After acid-alcohol decolorization, non–acid-fast cells lose color and take up the counterstain (methylene blue), which makes them appear blue. The other dyes listed are used in different staining schemes or as counterstains, not as the primary stain in acid-fast procedures. So the dye that serves as the primary stain is carbol fuchsin.

In acid-fast staining, you need a dye that can penetrate the waxy, mycolic acid–rich cell wall and stay bound after decolorization. Carbol fuchsin fits this role because it is a phenol-containing red dye that can penetrate the cell wall (often aided by heat or a higher phenol concentration in cold methods). It is used as the primary stain, coloring acid-fast organisms red. After acid-alcohol decolorization, non–acid-fast cells lose color and take up the counterstain (methylene blue), which makes them appear blue. The other dyes listed are used in different staining schemes or as counterstains, not as the primary stain in acid-fast procedures. So the dye that serves as the primary stain is carbol fuchsin.