Color enhanced TEM of Legionnaires' disease bacteria (Legionella pneumophila) on bacteriologic medium with vacuoles present. TEM of Legionella pneumophila. Legionella pneumophila is a thin, aerobic, pleomorphic, flagellated, nonspore-forming, gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of Legionnaires' disease, also known as legionellosis. The disease got its name in 1976 when an outbreak of pneumonia occurred at an American Legion convention in Philadelphia. Magnification @ 90,000x.