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Pathogenicity of Legionellae
All of the more than 55 recognized species and 70 serogroups in genus
Legionella
are potentially pathogenic (23-25). However, not all species and
serogroups are equally virulent since
L. pneumophila
serogroup 1 (Lp1)
causes more than 85% of reported disease in the United States (26, 27).
Community-acquired pneumonia (CAP) incidence studies have estimated
that there are between 8,000 and 18,000 cases of legionellosis annually in
the US, which is approximately 25-fold higher than the number of cases
reported to the Centers for Disease Control and Prevention (CDC) (4).
In fact, the majority of cases of legionellosis are sporadic, with less than
5% outbreak-related (28) The sources of CAP are often difficult to identify,
because of both the ubiquitous nature of the bacterium and the reluctance
of physicians to obtain respiratory specimens to match with environmental
isolates (29, 30). Additionally, there is a growing body of evidence that
some geographic regions may host virulent strains of Lp1 unique to that
area (31-36).Varying distribution of virulent strains may also occur at the
species level. Disease surveillance studies in Australasia determined that
L.
longbeachae
was the causative agent for up to half of the cases of legionellosis
diagnosed in Australia and Thailand, regardless of immune state or age at
onset of disease, though in the United States and Europe this species is
more often associated with disease in older, immunocompromised hosts
(37, 38). Indeed, results from one laboratory study demonstrated greater
pathogenicity in
L. longbeachae
isolates from Australia versus strains that
were recovered in North America (39).
Infectionof bothprotozoanandhumanphagocytic cells is accomplished
utilizing type IVb secretion system composed of 25 genes, designated the
Dot/Icm system (40). The Dot/Icm system delivers substrates that allow the
