Mycolic acid

Mycolic acids are long chain saturated fatty acids (60 - 90 carbons long) and are found in the cell walls of nocardiae, corynebacteria and mycobacteria, where it makes up to 60% of the cell wall, as well as in waxes and gycolipids.

Due to the mycolic acid being a lipid any cells containing the acid do not stain well using the Gram stain techniques so instead heat and a solvent such as phenol are used. Once stained the cells hold the stain despite being washed with mineral acids and alcohols. This classies them as acid fast bacteria.