Rank: Species
Lineage: Bacteria;Proteobacteria;Gammaproteobacteria;Legionellales;Legionellaceae;Legionella;Legionella pneumophila subsp. pneumophila
Description: This organism is a non-marine bacterium usually found growing inside other organisms such as protozoans in aquatic environments. They can also be found in soil, freshwater, and in biofilms. Protozoans may act as bacterial reservoirs. The first outbreak of Legionnaire's disease occurred in 1976 at an American Legion convention and the resulting pneumonia-like disease resulted in 34 deaths. The cause of the disease was traced to Legionella bacteria. The disease greatly affects elderly patients and has a mortality rate of 5-30% of infected patients. Transmission occurs through contaminated water, sometimes in the form of aerosolized spray of standing or warm water. Once the bacteria are brought into the lungs they make contact with alveolar macrophages and are internalized where they can cause severe respiratory distress. Internalization occurs through specialized vacuoles (replicative phagosomes) that allow the bacteria to grow and replicate prior to escape from the macrophage. Formation of the replicative phagosome, which requires reprogramming of the normal phagosome maturation pathway, requires a type IV secretion system called the Dot/Icm system. This type IV system is closely related to the conjugative system of plasmid ColIb-P9, and is involved in the secretion of numerous protein components that aid in formation of the replicative phagosome. Some of these include the Sid proteins (substrates of Icm/Dot transporter), which comprise a set of three protein families encoded by the genome, and the enhanced entry proteins. Other virulence determinants include a set of multidrug transporters and other efflux pumps for toxic compounds that may allow the organism to persist in its habitat, a set of LPS phase variable genes that enhance immune evasion, and a type II secretion system for transport of hydrolases.
Reference Material:
https://www.ncbi.nlm.nih.gov/genome/?term=Legionella%20pneumophila%20subsp.%20pneumophila
