Acid-fast bacilli retain which dye?

Acid-fast bacilli retain carbolfuchsin due to the unique structure of their cell walls, which contain high amounts of mycolic acids. This waxy layer makes them resistant to decolorization by acids during the staining process. When performing a Ziehl-Neelsen stain, for example, carbolfuchsin is applied and then heated, allowing it to penetrate the cell wall. Once cooled, acid-fast bacilli will retain the bright red carbolfuchsin dye even after being treated with acid-alcohol, which would wash out other dyes. This property is crucial for the identification of pathogens like Mycobacterium tuberculosis in clinical microbiology.

In contrast, methylene blue, malachite green, and gentian violet do not penetrate the cell wall of acid-fast bacilli effectively or remain within them after the decolorization step, making them unsuitable for specifically identifying these organisms.