286
Legionella
bacteria were first identified during investigation of an
explosive, common-source outbreak of pneumonia among attendees of
anAmericanLegion convention inPhiladelphia in 1976 (1, 2).The highly
publicized original reports of the epidemic identified 182 cases of disease,
29 of whichwere fatal. Legionellae are now associatedwith two forms of
respiratory illness, collectively referred to as legionellosis (3) Legionnaires’
disease is the pneumonic andmore severe form of illness.The other form
of respiratory illness is named Pontiac fever after the first documented
outbreak,whichoccurredat ahealthdepartment inPontiac,Michigan (4).
The twodifferentpresentationsof legionellosishavenot yet beenexplained
but theories includedifferences in innatebacterialvirulence, restrictedhost
range, decreased viability, and/or host susceptibility (4-6). A few limited
studies suggestPontiac fevermaybeassociatedwithexposure to
Legionella
endotoxin, rather than intracellular replication (7, 8).
EcologyofLegionellae
Legionella
bacteria are facultative intracellular parasites of free living
protozoa and are ubiquitous in the natural, freshwater environment.
Legionellaehave been found inponds, rivers, soil, and even as far afield as
theMir space stationandAntarctic lakes (4, 9, 10).Theyhaveanextremely
broad host range, demonstrated to multiply in 14 species of amoebae, 2
speciesof ciliatedprotozoa, andone speciesof slimemoldwhile
Legionella
growth in theabsenceofprotozoahasonlybeendocumentedon laboratory
media (11, 12). Biofilms in building water systems are reservoirs for
the transmission of
Legionella
to humans (13). The complex association
of heterogeneous bacteria, protozoa, and fungi in the biofilm provides
nutrients and protection from adverse environmental conditions where
