Rickettsia
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{{Taxobox
| color = lightgrey
| name = ''Rickettsia''
|image = [[Image:Rickettsia rickettsii.jpg|240px]]
|image_caption = ''Rickettsia rickettsii''
|brown = [[Bacterium|Bacteria]]
| phylum = [[Proteobacteria]]
| classis = Alpha Proteobacteria
| ordo = [[Rickettsiales]]
| familia = [[Rickettsiaceae]]
| genus = '''''Rickettsia'''''
| genus_authority = da Rocha-Lima, 1916
| subdivision_ranks = Species
| subdivision =
''[[Rickettsia felis]]''<br/>
''[[Rickettsia prowazekii]]''<br/>
''[[Rickettsia rickettsii]]''<br/>
''[[Rickettsia typhi]]''<br/>
''[[Rickettsia conorii]]''<br/>
''[[Rickettsia africae]]''<br/>
etc.
}}
'''''Rickettsia''''' is a [[genus]] of [[Motility|non-motile]], [[Gram-negative]], [[Endospore|non-sporeforming]], highly [[pleomorphic]] [[Bacterium|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 [[eukaryote|eukaryotic]] host cells (typically endothelial cells).<ref name=Barron>{{cite book | author = Walker DH | title = Rickettsiae. ''In:'' Barron's Medical Microbiology ''(Barron S ''et al'', eds.)| edition = 4th ed. | publisher = Univ of Texas Medical Branch | year = 1996 | id = [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.2078 (via NCBI Bookshelf)] ISBN 0-9631172-1-1 }}</ref> Because of this, ''Rickettsia'' cannot live in artificial nutrient environments and are grown either in [[Biological tissue|tissue]] or [[embryo]] cultures (typically, chicken embryos are used). In the past they were regarded as microorganisms positioned somewhere between viruses and true [[bacterium|bacteria]]. The majority of ''Rickettsia'' bacteria are susceptible to [[antibiotic]]s of the [[tetracycline]] group.
''Rickettsia'' species are carried as [[parasite]]s by many [[tick]]s, [[flea]]s, and [[lice]], and cause [[disease]]s 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]] <ref>[http://www.ncbi.nlm.nih.gov/sites/entrez?cmd=Retrieve&db=pubmed&dopt=AbstractPlus&list_uids=17553271&tool=MedlinePlus Flinders Island spotted fever rickettsioses caused...[Emerg Infect Dis. 2007] - PubMed Result<!-- Bot generated title -->]</ref> in human beings. They have also been associated with a range of plant diseases. Like [[virus]]es, they grow only in 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 exist in most [[eukaryotic]] cells.
The method of growing Rickettsia in chicken embryos was invented by [[Ernest William Goodpasture]] and colleagues at [[Vanderbilt University]] in the early 1930s.
==Human pathogenesis==
The ''Rickettsia'' human pathogens are generally grouped as follows:
===Spotted fever group===
:*''R. rickettsii'' (Western hemisphere)
::[[Rocky Mountain spotted fever]]
:*''R. akari'' (USA, former Soviet Union)
::[[Rickettsialpox]]
:*''R. conorii'' (Mediterranean countries, Africa, Southwest Asia, India)
::[[Boutonneuse fever]]
:*''R. siberica'' (Siberia, Mongolia, northern China)
::Siberian tick typhus
:*''R. australis'' (Australia)
::[[Australian tick typhus]]
:*''R. japonica'' (Japan)
::Oriental spotted fever
:*''R. africae'' (South Africa)
:: African Tick Bite Fever
===Typhus group===
:*''[[Rickettsia prowazekii|R. prowazekii]]'' (Worldwide)
::Epidemic, recrudescent and sporadic [[typhus]]
:*''R. typhi'' (Worldwide)
::Murine (endemic) typhus
===Scrub typhus group===
:*The causative agent of [[scrub typhus]] formerly known as ''R. tsutsugamushi'' has been reclassified into the genus ''[[Orientia]]''.
==Plant pathogenesis==
The following plant diseases have been assocaited with Rickettsia-like organisms<ref name=Smith>{{cite book | author = Smith IM, Dunez J, Lelliot RA, Phillips DH, Archer SA| title = European Handbook of Plant Diseases. | publisher = Blackwell Scientific Publications| year = 1988 | id = ISBN 0-632-01222-6}}</ref>.
:* Beet latent Rosette RLO
:* Citrus Greening bacterium
:* Clover leaf RLO
:* Grapevine infectious necrosis RLO
:* Grapevine Pierce's RLO
:* Grapevine yellos RLO
:* Larch witche's broom disease
:* Peach phoney RLO
==Genomics==
Certain segments of Rickettsial [[genome]]s resemble that of [[mitochondria]].<ref name=Emelyanov_2003>{{cite journal | author=Emelyanov VV | title=Mitochondrial connection to the origin of the eukaryotic cell | journal=Eur J Biochem | year=2003 | pages=1599–618 | volume=270 | issue=8 | pmid=12694174 | doi = 10.1046/j.1432-1033.2003.03499.x}}</ref> The deciphered genome of ''R. prowazekii'' is 1,111,523 [[base pair|bp]] long and contains 834 protein-coding [[genes]].<ref name=Andersson_1998>{{cite journal | author=Andersson SG, ''et al'' | title=The genome sequence of Rickettsia prowazekii and the origin of mitochondria | journal=Nature | year=1998 | pages=133–40 | volume=396 | issue=6707 | pmid=9823893 | doi=10.1038/24094 }}</ref> Unlike free-living bacteria, it contains no genes for [[anaerobic]] [[glycolysis]] or genes involved in the biosynthesis and regulation of [[amino acid]]s and [[nucleoside]]s. In this regard it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used. [[Adenosine triphosphate|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.<ref>{{cite journal | author=Kannangara S, DeSimone JA, Pomerantz RJ | title=Attenuation of HIV-1 infection by other microbial agents | journal=J Infect Dis | year=2005 | pages=1003–9 | volume=192 | issue=6 | pmid=16107952 | doi = 10.1086/432767}}</ref> 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 (biology)|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]].
==Naming==
The genus ''Rickettsia'' is named after [[Howard T. Ricketts|Howard Taylor Ricketts]] ([[1871]]–[[1910]]), who worked on and eventually died of typhus. Despite the similar name, ''Rickettsia'' bacteria do not cause [[rickets]]. The disease of rickets takes its name from the [[Greek language|Greek]] word for [[vertebral column|spine]], ''hrake'', and is a [[Avitaminosis|vitamin D deficiency disease]], not an infectious disease.
==References==
<references />
==External links==
* [http://patric.vbi.vt.edu/organism/overview.php?organismId=3 Rickettsia] (from [http://patric.vbi.vt.edu/ PATRIC] the PathoSystems Resource Integration Center, a [http://www3.niaid.nih.gov/ NIAID] Bioinformatics Resource Center)
* African Tick Bite Fever [http://www.cdc.gov/ncidod/EID/vol12no09/05-1540.htm] from Centre for Disease Control and Prevention [http://www.cdc.gov/ncidod/eid/]
[[Category:Rickettsiales]]
[[Category:Bacteriology]]
[[Category:Microbiology]]
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