Acid fast bacteria are bacteria with cell walls that contain special acids called mycolic acids. Like other bacteria they also have peptidoglycan. Mycolic acids are long chains of fatty acids that form a barrier for the bacteria against external environment. To identify these bacteria in the lab scientists use acid fast stain. Some of the examples include Mycobacterium leprae and Mycobacterium tuberculosis . Mycobacterium tuberculosis causes tuberculosis while Mycobacterium leprae causes leprosy.