Legionella bacteria. Coloured transmission electron micrograph (TEM) of four Legionella pneumophila bacteria. These bacteria (red) cause Legionnaires disease. They are rodshaped Gram negative bacilli with a single motile flagellum (green). It was named after its first recognised victims, 29 American Legion members who died at a hotel in 1976. The bacteria lives in water tanks, shower heads and air conditioning systems. The disease, also called legionellosis, causes fatal pneumonic lung damage in the elderly and unfit. It can be treated with antibiotic drugs such as erythromycin. Magnification unknown.