Why are acid-fast stains useful?

Acid-fast stains are a type of laboratory test that allows scientists to detect if a blood or tissue sample is infected with acid-fast bacteria such as Mycobacterium tuberculosis, an acid-fast bacteria that causes tuberculosis (TB). Acid fast bacteria have a wax-like cell wall that is rich in mycolic acid, a lipid that renders the cell wall resistant to most stains. These bacteria are not stained by a simple Gram staining method and can only be visualized by acid-fast staining.