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bacteria to subvert host cell processes for their ownpropagation.Over 200
different substrates are knownor suspected tobe translocatedby theDot/
Icm system.These substrates vary considerably not only between species
but also between strains of the same species (40, 41). Deletion of any
single or a few substrates does not result in complete inability to replicate
intracellularly but does sometimes constrict the effective host range of
themutant compared to its isogenicwild type (42, 43). Tounderstand the
pathogenicityof legionellae, thebacteriamust beconsidered in thecontext
of their naturalmicrobial community andnot as independent inhabitants
of freshwater environments.
Inhalation of legionellae in aerosolized droplets is the primarymeans
of transmission for legionellosis (3, 28).Theaerosolizeddropletsmustbeof
a respirable size (between1 and5μm).Noperson-to-person transmission
of Legionnaires’ diseasehas beendocumented.Themost commondevices
linked to transmission fall into twobroadclasses: thoseproducingaerosols
of contaminated potable water, such as showers and faucets; and those
from non-potable water, such as cooling towers and whirlpool spas.
However, the source of transmissionmaynot be in the immediate vicinity
of affectedpersons, generate only small amounts of aerosols, or otherwise
not fit the canonical sourcesmost often linkedwith legionellosisoutbreaks
(28, 44).Thus, efficient identificationof sources of transmission requires a
multidisciplinary approach that includes clinical epidemiology, traditional
microbiology, andmolecular epidemiology techniques.
