Mycobacterium tuberculosis. Coloured transmission electron micrograph (TEM) of the bacteria Mycobacterium tuberculosis. This is a Gram positive rodshaped bacillus bacterium that causes tuberculosis (TB) in humans. The outer surface of the cell is covered by a complex of proteins, fats and lipids. These surface proteins stimulate an immune response from an infected patient. Mycobacterium tuberculosis is spread by coughing or sneezing, and is inhaled into the lungs. It forms a primary tubercle during initial infection, which develops into pulmonary TB. TB is still a fatal disease. Magnification unknown.