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Rickettsia |
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 Rickettsia rickettsii
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Rickettsia felis |
Rickettsia is a genus of motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci (0.1 μm in diameter), rods (1-4 μm long) or thread-like (10 μm long). Obligate intracellular parasites, the Rickettsia depend on entry, growth, and replication within the cytoplasm of eukaryotic host cells (typically endothelial cells).1 Because of this, Rickettsia cannot live in artificial nutrient environments and are grown either in tissue or embryo cultures (typically, chicken embryos are used). In the past they were regarded as microorganisms positioned somewhere between viruses and true bacteria. The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.
Rickettsia species are carried as parasites by many ticks, fleas, and lice, and cause diseases such as typhus, rickettsialpox, Boutonneuse fever, African Tick Bite Fever, Rocky Mountain spotted fever, Australian Tick Typhus, Flinders Island Spotted Fever and Queensland Tick Typhus 2 in human beings. They have also been associated with a range of plant diseases. Like viruses, they only grow inside living cells. The name rickettsia is often used for any member of the Rickettsiales. They are thought to be the closest living relatives to bacteria that were the origin of the mitochondria organelle that exists inside most eukaryotic cells.
The method of growing Rickettsia in chicken embryos was invented by Ernest William Goodpasture and his colleagues at Vanderbilt University in the early 1930s.
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The Rickettsia human pathogens are generally grouped as follows:
The following plant diseases have been associated with Rickettsia-like organisms3.
Certain segments of Rickettsial genomes resemble that of mitochondria.4 The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding genes.5 Unlike free-living bacteria, it contains no genes for anaerobic glycolysis or genes involved in the biosynthesis and regulation of amino acids and nucleosides. In this regard it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used. ATP production in Rickettsia is the same as that in mitochondria. In fact, of all the microbes known, the Rickettsia is probably the closest "relative" (in phylogenetic sense) to the mitochondria. Unlike the latter, the genome of R. prowazekii, however, contains a complete set of genes encoding for the tricarboxylic acid cycle and the respiratory chain complex. Still, the genomes of the Rickettsia as well as the mitochondria are frequently said to be "small, highly derived products of several types of reductive evolution".
The recent discovery of another parallel between Rickettsia and viruses may become a basis for fighting HIV infection.6 Human immune response to the scrub typhus pathogen, Orientia tsutsugamushi rickettsia, appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. A probable reason for this actively studied phenomenon is a certain degree of homology between the rickettsia and the virus - namely, common epitope(s) due to common genome fragment(s) in both pathogens. Surprisingly, the other infection reported to be likely to provide the same effect (decrease in viral load) is the virus-caused illness dengue fever.
The genus Rickettsia is named after Howard Taylor Ricketts (1871–1910), who studied and eventually died of typhus. Despite the similar name, Rickettsia bacteria do not cause rickets, which is a result of vitamin D deficiency.
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