Mycobacterium tuberculosis

Mycobacterium tuberculosis is the rod-shaped bacterium that causes tuberculosis (TB). It has an unusually thick, waxy cell wall that is rich in lipids (mycolic acids). It grows very slowly, dividing only about once every 15 to 20 hours.Transmission. TB spreads by airborne droplets released when an infected person coughs or sneezes. It spreads most easily in crowded, poorly ventilated living conditions.Infection. In the lungs the bacteria are engulfed by macrophages. The waxy wall protects them from the lysosomal enzymes, so they survive and even multiply inside the macrophages. The immune system responds by walling off the infected cells: macrophages, T cells and other cells gather round to form a mass called a tubercle (granuloma), in which the centre dies. The bacteria can remain dormant inside tubercles for years. This is latent TB: the person has no symptoms and is not infectious.Active TB. If the immune system is weakened, for example by HIV infection, malnutrition or old age, the bacteria become active again. They multiply and destroy lung tissue, and can spread to other organs. Symptoms include a persistent cough (sometimes with blood), fever, night sweats and severe weight loss.Treatment and prevention. TB needs a long course, usually at least 6 months, of several antibiotics taken together. This is partly because the bacteria grow slowly and live inside cells, and partly to stop resistance developing. The BCG vaccine gives some protection, especially to children.