Mycobacteria. The mycobacteria include species in the genus Mycobacterium. This group of rod‐shaped bacteria possesses large amounts of mycolic acid in the cell wall. The presence of mycolic acid makes the bacteria very difficult to stain, but when heat or other agents are used to force carbolfuchsin into the cytoplasm, the bacteria resist decolorization with a dilute acid‐alcohol solution. Therefore, they are said to be acid‐fast.
Many mycobacteria are free‐living, but two notable pathogens exist in the group: M. tuberculosis, the cause of tuberculosis in humans and cattle; and M. leprae, which causes leprosy.
Nocardioforms. Nocardioforms include nine genera of aerobic, acid‐fast rods, including members of the genus Nocardia. Nocardioforms have aerial hyphae which project above the surface of their growth medium as branching filaments. The hyphae fragment into rods and cocci. Nocardioforms are found throughout nature in many types of soil and aquatic environments. One species, N. asteroides, causes infection of the human lung.