Fig 1. Legionella pneumophila under electron microscopy. Legionella Pneumophila (L. Pneumophila) belongs to the bacteria family Legionellaceae, genus Legionella. It is a thin, Gram-negative, nonencapsulated, aerobic, and nonspore-forming bacterium with a single polar flagellum. The bacterium can be found in fresh water with the ability to contaminate hot water tanks, hot tubs, and cooling towers of large air conditioners. L. Pneumophila is a pleomorphic bacillus, sometimes also characterized as coccobacillus. It is not pigmented, neither does it autofluorescent. The colony of L. Pneumophila is colored by gray-white and its appearance is textured and cut-glass like. L. Pneumophila is aerobic, also requires iron and cysteine to thrive. Although identified as Gram-negative, L. Pneumophila stains poorly because of its unique lipopolysaccharide content in the outer leaflet of the outer cell membrane. Somatic antigen specificity of L. Pneumophila is based on the chemical component (different sugars) and arrangement of side chains of its cell wall. Up to now, at least 35 serovars have been identified.
L. Pneumophila is a primary human pathogenic bacterium that causes Legionnaires' disease in human, this disease is also known as legionellosis. Symptoms of Legionnaires' disease include ough, high fever, muscle pains, shortness of breath and headaches, sometimes may also include vomiting, nausea, and diarrhea. About 8,000 to 18,000 new infection cases were reported per year in the USA, with a mortal rate of about 10%. This disease is more common in summer and fall, although it may occur any time of a year.
Fig 2. Metabolic cues govern L. pneumophila cellular differentiation. (Maris V. Fonseca, et al. Cell. Infect. Microbiol., 2014)
L. Pneumophila invades human macrophages within which it process replications. It achieves internalization through phagocytosis. However, L. Pneumophila can also invade nonphagocytic cells, through a mechanism that still remains unclear. After the internalization, the bacteria protect themselves within a membrane-bound vacuole to prevent being fused with lysosomes, which will cause death of bacteria individuals. Then the bacteria multiply within the infected cells.
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