Tuberculosis bacteria. Coloured scanning electron micrograph (SEM) of Mycobacterium tuberculosis bacteria, the main cause of human tuberculosis. These are Gram-positive rod-shaped bacteria. If a bacterium enters the lungs in a droplet coughed or sneezed by an infected person, it multiplies to form a localized bacterial colony (primary tubercle). Normally the tubercle is destroyed by the immune system. In a few cases, however, the bacteria then either spread through the lymphatic system to other parts of the body, which can be fatal, or lie dormant, only becoming active again many years later. Treatment is with antibiotics. Magnification unknown.