than L. pneumophilia
Although more than 70 Legionella
serotypes have been identified among 50 species, L. pneumophilia causes
most legionellosis. L. pneumophilia serogroup 1 alone is responsible for
70-90% of the cases in adults. In a pediatric series, L. Pneumophilia serogroup
1 accounted for only 48% of cases, serogroup 6 accounted for 33%, and the
remaining cases involved other serotypes and species. Legionella micdadei
and L. dumoffii are the second and third most common species to cause
Legionnaires disease in children, respectively.
Legionella species associated with
human disease include L. pneumophilia, L. micdadei, L..bozemanii, L.
dumoffii, L. gormanii, L. longbeachae, among others.
Gram stain or other staining
characteristics: technically Gram-negative, although it does not retain staining well and is usually not visible on gram staining. Silver stain is typically required to demostrate in tissues. DFA is useful only for L pneumophilia type I
Morphology and structure:
Slender, pleomorphic rods
Facultative intracellular organism (inside alveolar macrophages)
Growth characteristics in the
laboratory: Require charcoal yeast extract agar/media
(with L-cysteine and iron); growth is enhanced by iron
Dot/lcm type IV secretion system
inhibits macrophage phagosome/endo/lysosome fusions
Cu-Zn and catalase-peroxidase protects
bacteria from macrophage superoxide and hydrogen peroxide oxidative burst
Pili and flagella promote attachment
Secretion of protein toxins like
RNAase, phospholipase A, and phospholipase C
Legionella spp. are transmitted via
aerosol transmission from environmental water source habitat (e.g. air
conditioning systems, hot water tanks). There is no person-to-person
They are facultative
intracellular bacteria that multiply in free-living amoebae in nature, and in
alveolar macrophages, monocytes, and alveolar epithelial cells in infected
hosts. This ability to infect and replicate in macrophages is critical.
The replicative cycle
is initiated by binding complement component C3b to an outer membrane porin
protein on the bacterial surface. The bacteria then binds to the CR3 complement
receptors on mononuclear phagocytes, after which the organisms penetrate the
cell through endocytosis. The bacteria survive intracellularly and are not killed in the cells by exposure
to toxic superoxide, hydrogen peroxide, and hydroxyl radicals because
phagolysosome fusion is inhibited.
Chemokines and cytokines
released by the infected macrophages stimulate a robust inflammatory response
that is characteristic of Legionella infections. The organisms proliferate in
their intracellular vacuole and produce proteolytic enzymes that eventually
kill the host cell when the vacuole is lysed.
Major associated diseases: pneumonia
Major medications or classes of
medications that have therapeutic efficacy: Macrolides (e.g.
azithromycin, clarithromycin) or fluoroquinolones (e.g. ciprofloxacin, levofloxacin)
are the treatment of choice
Decrease environmental exposure to reduce risk of disease; for environmental
sources associated with disease, treat with hyper-chlorination, superheating,
or copper-silver ionization
Capable of sporadic, epidemic, and nosocomial infections; commonly found in
natural bodies of water, cooling towers, condensers, and water systems
(including hospital systems); patients at risk for symptomatic disease include
patients with compromised pulmonary function and patients with decreased
cellular immunity (particularly transplant patients).