Legionella bacteria. Coloured transmission elec- tron micrograph (TEM) of Legionella pneumophila bacteria, the cause of Legionnaires' disease. The flagellae (light green), which enable the bacteria to be motile, are seen. These are rod-shaped Gram- negative bacilli. L. pneumophila was first identified as an agent of disease after a mysterious outbreak of pneumonia caused 29 deaths at an American Legion convention in 1976. The bacterium was found living in water tanks, showerheads and air-conditioning systems. The disease causes fatal pneumonic lung damage in the elderly and unfit. Magnification unknown.