Acid fast stain is used to stain the bacteria of the genus Mycobacteria, because they contain large amounts of impermeable wax-like sbstances, which do not allow gram stain to penetrate the cells. Gram stain however, can penetrate the other bacteria with varying amounts of polysaccharides and lipopolysaccharadies in their cell walls, and thus can differentiate between them, For Acid fast stain, it would be difficult to differentiate between the gram positive and negative cells, and therefore, it is not used for this differential purpose, but to identify the Mycobacterim only.
