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Legionella
have been shown to stably persist for years (14, 15). Some large
outbreaks of Legionnaire’s Disease or Pontiac fever can be traced to massive
descalement episodes caused by such triggers as disruption of plumbing
service in affected buildings (16). However, the natural mechanism of
Legionella
colonization and release from biofilms is not understood. Thus,
many cases of disease go unexplained (17).
Legionnaires’ disease occurs when sufficient numbers of legionellae are
aerosolized and inhaled by a susceptible host (3). In addition to human
host factors, transmission of legionellosis from the environment may be
influenced by
Legionella
growth phase: replicative or infective (18). The
replicative phase is characterized by acid and sodium resistance while the
infective phase involves increased virulence, sodiumsensitivity, cytotoxicity,
osmotic resistance, and motility (19). In the laboratory,
Legionella
exhibit
replicative phase traits when growing intracellularly in free-living protozoa
or macrophages while infective phase characteristics are observed during
growth under restrictive conditions such as nutrient limited broth The
L.
pneumophila
life cycle can be modeled in broth cultures, where bacteria in
the exponential growth phase have many attributes of the replicative form,
while stationary phase cells have traits of the infective form (19, 20).
Recent research suggests that selective pressure from the most recent
host may limit the host range of intracellularly amplified daughter cells (12).
The host range of a given species of
Legionella
may also be determined by
genetic factors that evolve quickly due to horizontal transfer of foreignDNA
(21). Several studies have described the relationship between legionellae
and protozoa in aquatic environments as well as the accidental nature of
legionellae extending their host range to alveolar macrophages to cause
legionellosis in humans (3, 4, 22).
