What is the mordant used in the Ziehl-Neelsen acid-fast stain?

In the Ziehl-Neelsen acid-fast stain, heat is used as a mordant to facilitate the penetration of the primary stain, carbol fuchsin, into the cell walls of acid-fast organisms, notably mycobacteria. The process involves heating the bacterial slide to help the dye penetrate the waxy lipid-rich cell walls that are characteristic of these bacteria. This heating step is crucial; without it, the primary stain would not adequately enter the cells, leading to inaccurate results in identifying acid-fast organisms.

By contrast, Gram's iodine is a mordant used in the Gram staining procedure, while auramine is a fluorescent dye more commonly associated with other types of stains, such as auramine-rhodamine stains for mycobacteria. Acid-alcohol acts as a decolorizer in the Ziehl-Neelsen method, but it is not a mordant and serves a different role in the staining process.