Mycobacteria are slender rod-shaped bacteria that can be stained using special techniques. The most important mycobacteria that cause disease are M. tuberculosis, which causes tuberculosis (TB), and M. leprae, which causes leprosy. Both bacteria are acid-fast rods that form colonies when cultured. M. tuberculosis can be grown on nutrient-rich substrates but M. leprae cannot be cultured. Both bacteria cause disease by forming granulomas in the body and can be detected microscopically using staining or molecular techniques. TB and leprosy are treated with antibiotics but leprosy can also cause nerve damage and mutilations if untreated.