Legionella. Coloured Scanning Electron Micrograph (SEM) of the bacteria Legionella pneumophila, causative agent of Legionnaires' disease. Here, a dense group of these Gram-negative rod-shaped bacilli bacteria is seen forming a network. Each bacteria has a flagellum (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, showerheads and air-conditioning systems. The disease, also called legionellosis, often causes fatal pneumonic lung damage in the elderly and unfit. Magnification: x3750 at 6x6cm size.