, bacteria that are still stained with red color after the decolorization step during the acid fast staining procedure are known as acid fast bacteria.
Acid fast stain (or carbolfuchsin) binds only to bacteria that have a waxy cell wall. This cell wall contains a hydrophobic waxy lipid known as mycolic acid, which occupies the 60% of the cell wall. Due to the hydrophobic property, water soluble materials are prevented from entering the cytoplasm. That is why this bacteria is unable to be stained by water soluble dyes such as methylene blue. Carbolfuchsin is composed of phenol and fuchsin so that it can be penetrated up to the cytoplasm.
During the acid alcohol decolorization step, the acid alcohol is prevented entering the cytoplasm due to the presence of hydrophobic mycolic acid, thus it is unable to remove carbolfuchsin from the bacteria cell. So the primary dye will remain in the cytoplasm even after the decolorization step.
Acid fast bacteria includes several genera such as Mycobacterium and Nocardia, which are pathogenic to human, causing tuberculosis and nocardiosis, respectively.