Legionella. Coloured transmission electron micrograph of the bacteria Legionella pneumophila, causative agent of Legionnaires' disease. Here, a cluster of these rod-shaped Gram- negative bacilli bacteria are seen; the single flagellum occurring on each bacterium is not seen. Legionella 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 bacteria was found living in water- tanks, shower- heads and air-conditioning systems. The disease, also called legionellosis, often causes fatal pneumonic lung damage in the elderly & unfit. Magnification: x8640 6x4.5cm size.