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!

Chest radiograph of patient with Legionnaire's Disease found at


For a detailed video explanation, please visit the following link:

For statistics and information about the prevalence of Legionella pneumophila infections visit the Infection Incidence page!