Rickettsia

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Rickettsia

Scientific classification

Kingdom:	Bacteria
Phylum:	Proteobacteria
Class:	Alpha Proteobacteria
Order:	Rickettsiales
Family:	Rickettsiaceae
Genus:	Rickettsia da Rocha-Lima, 1916

Species

Rickettsia felis
Rickettsia prowazekii
Rickettsia rickettsii
Rickettsia typhi
etc.

Bacteria of the genus Rickettsia are carried as parasites by many ticks, fleas, and lice, and cause diseases such as typhus, rickettsialpox, Brill-Zinsser disease, Boutonneuse fever, Rocky Mountain spotted fever, and endemic typhus in human beings. Like viruses, they grow only in living tissue. The name rickettsia is often used for any member of the Rickettsiales.

The genus Rickettsia is named after Howard Taylor Ricketts (1871–1910). Despite the similar name, Rickettsia bacteria do not cause rickets. The disease of rickets takes its name from the Greek word for spine, rhakhis, and is a vitamin deficiency disease, not an infectious disease.

The Rickettsia human pathogens are usually grouped as follows:

Spotted Fever group

Organism name	Disease caused	Distribution
R. rickettsii	Rocky Mountain spotted fever	Western hemisphere
R. akari	Rickettsialpox	USA, former Soviet Union
R. conorii	Boutonneuse fever	Mediterranean countries, Africa, Southwest Asia, India
R. sibirica	Siberian tick typhus	Siberia, Mongolia, nothern China
R. australis	Australian tick typhus	Australia
R. japonica	Oriental spotted fever	Japan

Typhus group

Organism name	Disease caused	Distribution
R. prowazekii	Epidemic, Recrudescent and Sporadic typhus	Worldwide
R. typhi	Murine typhus	Worldwide

Scrub typhus group

Organism name	Disease caused	Distribution
R. tsutsugamushi (now a separate genus, Orientia)	Scrub typhus	SW Asia, Northern Australia, Pacific Islands

Rickettsia are non-motile, gram-negative, non-sporeforming highly pleomorphic organisms that could be shaped as cocci (0.1 μm in diameter), rods (1-4 μm long) or thread-like (10 μm long). Being obligate intracellular pathogens, the Rickettsia depend on entry, growth, and replication within the cytoplasm of a eukaryotic host cell (typically endothelial cells). 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 real bacteria. The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.

Certain places of Rickettsia genome are often said to resemble that of mitochondria. The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding genes. Unlike free-living bacteria, it contains no genes for anaerobic glycolysis or genes involved in the biosynthesis and regulation of biosynthesis 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. Human immune response to the scrub typhus pathogen, Orientia tsutsugamushi rickettsia, appears to provide a beneficial effect on 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 a virus-caused fever - Dengue.