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Legionella
have been shown to stably persist for years (14, 15). Some large
outbreaksofLegionnaire’sDiseaseorPontiac fevercanbe traced tomassive
descalement episodes caused by such triggers as disruption of plumbing
service in affected buildings (16). However, the natural mechanism of
Legionella
colonization and release frombiofilms is not understood.Thus,
many cases of disease gounexplained (17).
Legionnaires’diseaseoccurswhen sufficientnumbersof legionellaeare
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 resistancewhile the
infectivephaseinvolvesincreasedvirulence,sodiumsensitivity,cytotoxicity,
osmotic resistance, andmotility (19). In the laboratory,
Legionella
exhibit
replicativephase traitswhengrowing intracellularly in free-livingprotozoa
ormacrophages while infective phase characteristics are observed during
growth under restrictive conditions such as nutrient limited brothThe
L.
pneumophila
life cycle canbemodeled inbroth cultures, wherebacteria in
the exponential growthphasehavemany attributes of the replicative form,
while stationaryphase cells have traits of the infective form (19, 20).
Recent research suggests that selective pressure from themost recent
hostmay limit thehostrangeof intracellularlyamplifieddaughtercells(12).
The host range of a given species of
Legionella
may also be determined by
genetic factors thatevolvequicklydue tohorizontal transferof 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 inhumans (3, 4, 22).
