Legionella is an aquatic bacterium that can be found in two distinctive shapes. The first is rod-shaped and the second is coccoid shaped, with both displaying three polar or lateral flagella. Legionella is characteristically single celled organisms that can survive in bio-films contained within man-made water sources. The legionella bacteria are gram-negative and largely aerobic and are capable of drawing energy from their hosts, therefore they are considered parasites of eukaryotic cells. Each legionella bacteria are measured to be between 300 nm-900 nm by  2000-3000 nm. Legionella are not highly motile unless they are contained within media that has a low salt concentration, in which they utilize their and are flagella. If contained within a high salt concentration media, Legionella, are found to not have any flagella present. Legionella are able to reproduce in two phases. The first is 

The genome of Legionella was sequenced and catalogued  into four strains. Each of sequences were collected from humans afflicted with Legionnaires disease. The strains were Philadelphia strain, Lens Strain, Corby strain, and the Paris strain. The Philadelphia strain was originally discovered in the original outbreak of the disease and the lens strain is responsible for 12% of cases in France. The actual genome consists of 3,259 predicted genes and 80% of these genes between the strains are the same, with 10% of genes within each strain being unique. This diversity within the genome of Legionella can be explained by its ability to perform horizontal gene transfer between each of the strains and other bacteria.