Legionella pneumophila is a Gram-negative, nonencapsulated, aerobic bacillus, which belongs to the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group. L. pneumophila is a pleomorphic bacillus, sometimes also characterized as coccobacillus. It is a primary human pathogenic bacterium and the causative agent of Legionnaires' disease (legionellosis).