Pathogen Proliferation
The life
of a Legionella pneumophila
bacterium consists of two phases: A replication phase and an infection
or transmission phase. During the replication phase it is non-motile and
if it's toxic at any level during this stage, its toxicity is extremely
depleted.
Once
the bacteria enter the host human (as discussed in the
Transmission
Tactics page) they are internalized
into a phagosome by
patrolling alveolar macrophages.
Legionella pneumophila make a secretion system encoded by dot icm
genes. Signal proteins are emmited through the dot icm system and these
proteins prevent phagosome maturation.
Phagosomes containing
Legionella pneumophila enlist vesicles made from endoplasmic
reticulum. This is unique to the genus
Legionella and is called
retrograde transport. Host vesicles then contact the phagosome that
contains Legionella pneumophila and
become flat. The phagosome is covered with endoplasmic reticulum
vesicles now and effector proteins from the icm system assist in
endoplasmic reticulum vesicle recruitment.
Changes in membrane thickness occur here and ribosomes are
recruited to the phagosome containing the bacterium. Once
Legionella pneumophila is
inside an endoplasmic reticulum vesicle it is hidden from the lysosomes
and therefore cannot be destroyed by them.
It can now multiply freely within the phagosome.
Once the bacterium has depleted its nutrient sources it enters into the second phase of its life cycle and with the morphological transformation into a thicker rod, the infectious stage begins. During this time Legionella pneumophila are motile via flagella, and their toxicity level is very much increased.
They exit the host cell using a pore and spread within the cell via phagoctyosis and multiplication. An infected host with Legionnaire’s Disease is the result!
For
a detailed video explanation, please visit the following link: http://www.youtube.com/watch?v=dD4d_apLtl0&feature=related
For statistics and information about the prevalence of Legionella pneumophila infections visit the Infection Incidence page!