The acid-fast staining property relies on which characteristic of the organism?

• A. Ability to resist decolorization with dilute acids
• B. Ability to be counterstained with methylene blue
• C. Presence of spores
• D. Gram reaction only in tissues

Answer: (A)

Acid-fast staining hinges on the organism’s waxy, lipid-rich cell wall, rich in mycolic acids. This thick, hydrophobic barrier traps the primary dye so tightly that it is not washed out by dilute acid during decolorization. That resistance to decolorization is what makes an organism acid-fast. After the stain, non–acid-fast cells are decolorized and then counterstained to be seen, typically with methylene blue. The presence of spores is unrelated to acid-fast staining, and the Gram reaction in tissues doesn’t define acid-fast properties. So the defining feature is the ability to resist decolorization with dilute acids.