title: Legionella pneumophila pathogesesis: a fateful journey from amoebae to macrophages.
abstract: Legionella pneumophila first commanded attention in 1976, when investigators from the Centers for Disease Control and Prevention identified it as the culprit in a massive outbreak of pneumonia that struck individuals attending an American Legion convention (). It is now clear that this gram-negative bacterium flourishes naturally in fresh water as a parasite of amoebae, but it can also replicate within alveolar macrophages. L. pneumophila pathogenesis is discussed using the following model as a framework. When ingested by phagocytes, stationary-phase L. pneumophila bacteria establish phagosomes which are completely isolated from the endosomal pathway but are surrounded by endoplasmic reticulum. Within this protected vacuole, L. pneumophila converts to a replicative form that is acid tolerant but no longer expresses several virulence traits, including factors that block membrane fusion. As a consequence, the pathogen vacuoles merge with lysosomes, which provide a nutrient-rich replication niche. Once the amino acid supply is depleted, progeny accumulate the second messenger guanosine 3',5'-bispyrophosphate (ppGpp), which coordinates entry into the stationary phase with expression of traits that promote transmission to a new phagocyte. A number of factors contribute to L. pneumophila virulence, including type II and type IV secretion systems, a pore-forming toxin, type IV pili, flagella, and numerous other factors currently under investigation. Because of its resemblance to certain aspects of Mycobacterium, Toxoplasma, Leishmania, and Coxiella pathogenesis, a detailed description of the mechanism used by L. pneumophila to manipulate and exploit phagocyte membrane traffic may suggest novel strategies for treating a variety of infectious diseases. Knowledge of L. pneumophila ecology may also inform efforts to combat the emergence of new opportunistic macrophage pathogens.
authors.affiliation: Department of Microbiology and Immunology, University of Michigan Medical School, Ann Arbor, Michigan 48109, USA. mswanson@umich.edu,bhammer@umich.edu
authors.forename: M S
authors.initials: MS
authors.lastname: Swanson
authors.forename: B K
authors.initials: BK
authors.lastname: Hammer
mesh_terms: D000656:Amoeba / Q000382:microbiology*; D000818:Animals; D004197:Disease Reservoirs; D016952:Legionella pneumophila / Q000472:pathogenicity*; D007877:Legionnaires' Disease / Q000209:etiology* / Q000635:transmission; D016676:Macrophages, Alveolar / Q000382:microbiology*; D014617:Vacuoles / Q000382:microbiology