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PathogenicityofLegionellae
All of themore than55 recognized species and70 serogroups ingenus
Legionella
are potentiallypathogenic (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-acquiredpneumonia (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 ofCAP areoftendifficult to identify,
because of both the ubiquitous nature of the bacterium and the reluctance
ofphysicians toobtain respiratory specimens tomatchwithenvironmental
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 strainsmay also occur at the
species level.Disease surveillance studies inAustralasiadetermined that
L.
longbeachae
was thecausativeagent forup tohalfof thecasesof legionellosis
diagnosed inAustralia andThailand, 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 fromAustralia versus strains that
were recovered inNorthAmerica (39).
Infectionofbothprotozoanandhumanphagocyticcellsisaccomplished
utilizing type IVb secretion system composed of 25 genes, designated the
Dot/Icm system (40).TheDot/Icm systemdelivers substrates that allow the
