The cell walls of Mycobacterla are made up of waxy substance, Mycolic acid that Is relatively Impermeable to ordinary stainIng techniques. But, by apphcation of heat and a mordant (phenol), the cell can be stained The purpose of heating is to soften the waxy material of the cell wall and allow the stain to enter the cell. Basic fuchsin is more soluble in phenol and phenol is a better solvent for lipids and waxes.   _{Your Comment}