Legionellosis pathophysiology

Legionella is often transmitted by aerosolized droplets produced from contaminated whirlpool spa, river, cruise ships, cooling towers, air conditioners, water supply systems . L. pneumophila usually invades the host cells and replicates intracellularly. Legionella is internalized using pseudopods and protects itself in a membrane-bound vacuole that does not fuse with lysosomes.

• Legionella is acquired by inhaling droplets from contaminated water sources such as faucets, showers, humidifiers, whirlpool spa, river, cruise ships, cooling towers, air conditioners, water supply systems ^[1]. L. longbeachae is thought to be transmitted by compost during gardening activity^[2]. However, it has also been seen in industrial coolers ^[3].
• Although is is believed that Legionella is not transmitted from one person to another person ^[4], there have been reports of person to person transmission ^[5].
• Healthcare-associated transmission of Legionella is common and has a higher mortality rate.

• Legionella are able to live within amoebic cells. Free-living amoebae may be responsible for enabling Legionella to better survive environmental hazards, adapt to living within macrophages, and may be a significant reservoir of Legionella. These amoebae may assist Legionella to remain undetected within the human environments ^[6].
• Legionella invades the host cells and replicates intracellularly.
• The internalisation of the bacteria can be enhanced by the presence of antibody and complement but is not absolutely required.
• A pseudopod coils around the bacterium in this unique form of phagocytosis.
• Once internalised, the bacteria surround themselves in a membrane-bound vacuole that does not fuse with lysosomes that would otherwise degrade the bacteria. In this protected compartment the bacteria multiply.
• The bacteria use a Type IVB secretion system known as Icm/Dot to inject effector proteins into the host.
• These effectors are involved in increasing the bacteria’s ability to survive inside the host cell. They also secrete a 39kDa metalloprotease into culture fluids, which is cytotoxic for some cultured tissue culture cells.

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