engleski

Interaction of Legionella pneumophilla with Amoeba

The first recognized outbreak of pneumonia due to Legionella pneumophila occurred in Philadelphia, during the summer of 1976 among 180 persons attending the 56th annual American Legion Convention. Twenty nine patients died, and the disease became known as Legionnaires’ disease. In January of 1977, a Gram-negative bacterium was isolated and designated L. pneumophila. The source of the infection during the Legionnaires’ convention was later found to be the air conditioning system in the hotel. It has been documented that the hallmark of Legionnaires’ disease is the intracellular replication of L. pneumophila in the alveolar spaces. At least 48 species of legionellae have been identified, some of which are associated with disease while others are environmental isolates and whether they can cause disease is not known. L. pneumophila is responsible for more than 80% of cases of Legionnaires’ disease, and among the 13 serogroups of L. pneumophila, serogroup 1 is responsible for more than 95% of cases of Legionnaires’ disease.. Free-living amoebae are important predators controlling microbial communities. They are ubiquitous and have been isolated from various natural sources such as soil, freshwater, salt water, dust, and air. Although their abundance in soil is only limited, they have been implicated in the stimulation of phosphorus and nitrogen turnover and thus play an important role in soil ecosystems. Transient association with amoebae have been reported for a number of different bacteria including Legionella pneumophila, mycobacterium sp., Francisella tularensis, or Escherichia coli O157, among others. As most of these bacteria are human pathogens, amoebae have been suggested to represent their environmental reservoirs, acting as “ Trojan horses” of the microbial world. Intracellular multiplication of Legionella with protozoa such as Acanthamoeba polyphaga and macrophages requires the Dot/Icm secretion system for biogenesis of the phagosome and intracellular replication. The dot/icm genes are located in two different regions. The region I includes seven genes (dotA– D, icmV, W, X) and the larger region II contains the remaining 17 members (icmT, S, R, Q, P, O, N, M, L, K, E, G, C, D, J, B, F). The dot/icm genes of the Type IV secretion system are thought to encode proteins involved in the translocation of effector molecules into the host cell that will prevent the bacteria from being killed. It is not known whether the icm/dot genes derive from a single plasmid, which has been separated into the two gene clusters, or there were the result of multiple gene transfer events. However, only Coxiella burnettii has homologs of the full icm/dot genes, which are contained in a single locus. Besides the dot/icm Type IVB secretion system, L. pneumophila possess a second Type IV secretion system, the lvh/lvr genes that surprisingly are not involved in its virulence. The Type IV secretion system has been shown to be the main virulence system of L. pneumophila.

Legionella, amoeba, dot/icm secretion system

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