Legionella pneumophila, the primary causative agent for Legionnaire's disease, is an aerobic, pleomorphic, flagellated, non-spore-forming, Gram-negative bacterium.

[4] Infected humans may display symptoms such as fever, delirium, diarrhea, and decreased liver and kidney function.

[10] L. pneumophila colony morphology is gray-white with a textured, cut-glass appearance; it also requires cysteine and iron to thrive.

[11] It grows on buffered charcoal yeast extract agar as well as in moist environments, such as tap water, in "opal-like" colonies.

[18] Of note, this bacterium can form and reside in biofilms within water system pipes, allowing it be aerosolized through fixtures such as faucets, showers, and sprinklers.

[19] Biofilms are specialized, surface attachment communities that can consist of one or multiple microbes, ranging from bacteria, algae, and protozoa.

[17] These protective matrixes enable the microbe to live for extended periods of time in low-nutrient environments and in the presence of biocides.

This bacterium can infect and survive within protozoa genera such as Acanthamoeba, Vermamoeba, and Naegleria which often feed on bacteria in biofilms.

Cyst-forming protozoans allow L. pneumophila to survive harsh environmental conditions such as chlorine, UV, ozonisation, and thermal treatments.

[25] Even though L. pneumophila primarily uses amino acids as a carbon source, the bacteria does contain multiple amylases, such as LamB which hydrolyzes polysaccharides into glucose monomers for metabolism.

[23] Protein degradation to recycle amino acids and hydrolyzing polysaccharides are not the only methods by which L. pneumophila obtains carbon and energy sources from the host.

The genome is relatively large of about 3.5 mega base pairs (mbp) which reflects a higher number of genes, corresponding with the ability of Legionella to adapt to different hosts and environments.

Other genes of L. pneumophila encode for Legionella-specific vacuoles, efflux transporters, ankyrin-repeat proteins, and many other virulence related characteristics.

[29] Horizontal gene transfer allows L. pneumophila to evolve at a rapid pace and commonly is associated with drug resistance.

Once internalized, the Dot/Icm system begins secreting bacterial effector proteins that recruit host factors to the Legionella containing vacuole (LCV).

In the cytoplasm, L. pneumophila inhibit organelle and plasma membrane function and structure which ultimately leads to osmotic lysis of the host cell.

[35] L. pneumophila exhibits a unique lipopolysaccharide (LPS) structure that is highly hydrophobic due to its being densely packed with branched fatty acids, and elevated levels of O-acetyl and N-acetyl groups.

[37] L. pneumophila possesses a singular, polar flagellum that is used for cell motility, adhesion, host invasion, and biofilm formation.

MIP displays peptidyl–prolyl cis/trans isomerase (PPIase) activity which is crucial for survival within the macrophage, along with transmigration across the lung epithelial barrier.

[7] Effective antibiotic treatment for Legionella pneumonia includes fluoroquinolones (levofloxacin or moxifloxacin) or, alternately, azithromycin.

Combination treatments with rifampicin are being tested as a response to antibiotic resistance during mono-treatments, though its effectiveness remains uncertain.

Approximately 20% of reported Legionnaires disease cases come from healthcare, senior living, or travel facilities that have been exposed to water contaminated with L. pneumophila.

[43] There may also be an increased risk of contracting L. pneumophila from private wells, as they are often unregulated and not as rigorously disinfected as municipal water systems.

[45] Several large outbreaks of Legionnaire's Disease have come from public hot tubs due to the temperature range of the water being ideal for the bacteria's growth.

Hot tubs were identified as the likely source and the cruise lines modified their operation by increasing frequency of cleaning and hyperchlorination among other changes.