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Legionella Pneumophila Life Cycle and Identifying Characteristics
Legionella pneumophila is a gram negative, non-sporulating bacterium. The microbe is non-capsulated, rod-like and is a facultative intracellular parasite. L. pneumophila is largely aquatic, inhabiting non-marine water sources that have high iron and algal contents, but has an aerobic metabolism that allows it to survive for short periods of time outside of the water (Rathore and Alvarez, 2006).
L. pneumophila has two distinct phases in its life cycle: a replicative phase followed by an infectious or transmissive phase. The replicative phase features a nonmotile bacteria with a low or nonexistent toxicity. The infectious phase occurs after a transformation that leads to a shorter, thicker rod. This new form is motile because of the growth of flagella and is much more toxic to its host. The change from the replicative to the infectious phase occurs once the bacterium has depleted local nutrient sources in the host, which triggers several specific pathway responses (Swanson, 2007).
Mode of Infection
The main hosts of L. pneumophila are amoeba and human macrophages. The bacterium begins infection by being phagocytosed by macrophages just like any other foreign microbe. Once inside the macrophage, though, L. pneumophila begins to replicate (Fraser, 1986). These microbes are able to delay their delivery to lysosomes by stunting the autophagosome maturation of their host. The importance of this step of infection is illustrated by the A/J naip5 mouse strain, which has slow autophagosome maturation and are one of the only mouse strains susceptible to L. pneumophila infection (Swanson, 2007). The bacteria also surround themselves with a thick membrane-bound vacuole, which further prevents lysis. Once the nutrients of the host macrophage are depleted, the newly replicated bacteria move into their transmissive phase, lyse the host macrophage and restart the process of infection (2007).
Replicating L. pneumophila. Image in public domain.
Two major morphological changes occur throughout L. pneumophila’s life cycle. The first change is the formation of the membrane-bound vacuole that protects the bacterium once it has been phagocytosed by the macrophage. This structural change provides a physical barrier that prevents the bacteria in its replicative phase from being presented to a lysosome (Swanson, 2007).
The second change is the microbe’s transformation from its replicative to its infectious phase, which several distinct morphological changes. These changes all increase the virulence of the bacteria and include the growth of flagella which bestows motility on L. pneumophila, allowing it to more easily make contact with host cells (Rathore and Alvarez, 2006). The sigma factor, FliA, produced in conjunction with flagellar growth, leads to the production of lysosome avoidance factors, surface modifications, and secretions of adjuvant, which lead to an acute inflammatory response (Swanson, 2007). This bolstered inflammatory response will present more macrophages to be infected.
Legionella pneumophila feature unique lipopolysaccharide structures on their outer cellular membranes. These unique chains make staining this bacteria very difficult and are also likely responsible for the many different sub-species of L. pneumophila in existence. Fourteen different serovars have been identified, which are basically different antigen types that are determined by the structures of the cell surfaces (including the LPS chains) (Swanson, 2007).
L. pneumophila is also unique in that the presence of antibodies and complement has little effect in squelching an infection because of the bacteria’s unique lifestyle and mode of infection. These unique features also lead to the formation of endosymbiotic relationships with amoebas in fresh water sources and its deadliness if not identified and properly treated immediately (Rathore and Alvarez 2007).
Pop Culture Significance
Legionella pneumophila is most well known as the microbe behind the famous outbreak of Legionella at an American Legion conference at a hotel in Philadelphia in 1976. Thirty-four elderly men eventually succumbed to the outbreak of the disease, which gained so much media attention mainly because authorities could not determine what was causing the pneumonia-like illness. Once identified, the infection can be fairly easily treated, but outbreaks of the disease usually have a 5 to 30 percent fatality rate because it is so hard to identify the microbe behind the illness (Fraser, 1986).
Fraser DW. 1986. The peculiarities of “Legionella”. Proceedings of the American Philosophical Society 130:330-335.
Rathore M and A Alvarez. 2006 May 15. eMedicine: Legionella infection. <http://www.emedicine.com/ped/topic1288.htm>. Accessed 2007 February 12.
Swanson, M. Updated 2007 February 2. Michele Swanson: University of Michigan Department of Microbiology and Immunology. <http://www.med.umich.edu/microbio/bio/swanson_m.htm>. Accessed 2007 February 12.
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