What distinguishes acid-fast staining from Gram staining, and what color indicates an acid-fast organism after counterstain?

• A. It uses a carbol fuchsin stain with high lipid content cell walls; acid-fast organisms remain red/pink after counterstain with methylene blue or brilliant green.
• B. It uses crystal violet only; acid-fast become purple
• C. It uses safranin only; acid-fast become green
• D. It uses methylene blue primary stain; acid-fast become blue

Answer: (A)

Acid-fast staining relies on the waxy, lipid-rich cell walls (rich in mycolic acids) that require a strong dye to penetrate. The primary stain used is carbol fuchsin, often with heat to help entry into those lipid barriers. Because these walls resist decolorization by acid-alcohol, acid-fast organisms keep the stain and then take on the counterstain color only in the background. As a result, acid-fast cells appear red or pink after the counterstain with a dye like methylene blue or brilliant green, while non–acid-fast cells take on the counterstain color. This red/pink outcome is what identifies acid-fast organisms in this staining method.