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bacteria to subvert host cell processes for their own propagation. Over 200
different substrates are known or suspected to be translocated by the Dot/
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
the mutant compared to its isogenic wild type (42, 43). To understand the
pathogenicity of legionellae, the bacteria must be considered in the context
of their natural microbial community and not as independent inhabitants
of freshwater environments.
Inhalation of legionellae in aerosolized droplets is the primary means
of transmission for legionellosis (3, 28). The aerosolized droplets must be of
a respirable size (between 1 and 5 μm). No person-to-person transmission
of Legionnaires’ disease has been documented. The most common devices
linked to transmission fall into two broad classes: those producing aerosols
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 transmission may not be in the immediate vicinity
of affected persons, generate only small amounts of aerosols, or otherwise
not fit the canonical sources most often linked with legionellosis outbreaks
(28, 44). Thus, efficient identification of sources of transmission requires a
multidisciplinary approach that includes clinical epidemiology, traditional
microbiology, and molecular epidemiology techniques.
