Editing Rickettsia

{{Short description|Genus of bacteria}}
{{Distinguish|rickets}}
{{Use dmy dates|date=December 2019}}
{{Automatic taxobox
| image = Rickettsia rickettsii.jpg
| image_alt = Red-stained ''[[Rickettsia rickettsii]]'' visible in the cell of an [[Ixodidae|Ixodid]] [[Vector (epidemiology)|vector]] tick
| image_caption = Red-[[Staining|stained]] ''[[Rickettsia rickettsii]]'' visible in cells of an [[Ixodidae|Ixodid]] [[Vector (epidemiology)|vector]] [[tick]]
| taxon = Rickettsia
| authority = [[Henrique da Rocha Lima|da Rocha-Lima]], 1916
| synonyms =
| synonyms_ref =
| subdivision_ranks = [[Species group]]s and [[species]]
| subdivision_ref = <ref name=NCBI>{{cite web|author1=<!-- not stated -->|title=''Rickettsia''|url=https://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode=Undef&id=780&lvl=3&lin=f&keep=1&srchmode=1&unlock|website=NCBI taxonomy|publisher=National Center for Biotechnology Information|access-date=8 January 2019|location=Bethesda, MD|language=en}}</ref>
| subdivision =
* [[belli group]] 
** ''[[Candidatus]] [[Rickettsia angustus]]'' 
** ''[[Candidatus]] [[Rickettsia kingi]]'' 
** ''[[Candidatus]] [[Rickettsia mendelii]]'' 
** ''[[Candidatus]] [[Rickettsia tarasevichiae]]'' 
** ''[[Rickettsia bellii]]'' 
** ''[[Rickettsia canadensis]]'' <ref name=list/><ref>{{cite journal | vauthors = Truper HG, De' Clari L | year = 1997 | title = Taxonomic note: Necessary correction of specific epithets formed as substantives (nouns) 'in apposition' | url = http://ijs.sgmjournals.org/content/47/3/908.full.pdf+html | journal = Int J Syst Bacteriol | volume = 47 | issue = 3| pages = 908–909 | doi=10.1099/00207713-47-3-908| doi-access = free }}</ref>
** ''[[Rickettsia monteiroi]]'' 
* [[spotted fever group]] 
** ''[[Candidatus]] [[Rickettsia andeanae]]'' 
** ''Candidatus [[Rickettsia antechini]]'' 
** ''Candidatus [[Rickettsia barbariae]]'' 
** ''Candidatus [[Rickettsia goldwasserii]]'' 
** ''Candidatus [[Rickettsia hungarica]]'' 
** ''Candidatus [[Rickettsia jingxinensis]]'' 
** ''Candidatus [[Rickettsia kellyi]]'' 
** ''Candidatus [[Rickettsia kotlanii]]'' 
** ''Candidatus [[Rickettsia kulagini]]'' 
** ''Candidatus [[Rickettsia leptotrombidium]]'' 
** ''Candidatus [[Rickettsia longicornii]]'' 
** ''Candidatus [[Rickettsia nicoyana]]'' 
** ''Candidatus [[Rickettsia principis]]'' 
** ''Candidatus [[Rickettsia rara]]'' 
** ''Candidatus [[Rickettsia rioja]]'' 
** ''Candidatus [[Rickettsia senegalensis]]'' 
** ''Candidatus [[Rickettsia siciliensis]]'' 
** ''Candidatus [[Rickettsia tasmanensis]]'' 
** ''Candidatus [[Rickettsia uralica]]'' 
** ''Candidatus [[Rickettsia wissemanii]]'' 
** ''[[Rickettsia aeschlimannii]]'' <ref>Beati, L.; Meskini, M., et al. (1997), [http://ijs.sgmjournals.org/content/47/2/548.long "''Rickettsia aeschlimannii'' sp. nov., a new spotted fever group rickettsia associated with ''Hyalomma marginatum'' ticks"], ''Int J Syst Bacteriol'' '''47''' (2): 548-55s4</ref>
** ''[[Rickettsia africae]]'' <ref>{{cite journal | vauthors = Kelly PJ, Beati L, Mason PR, Matthewman LA, Roux V, Raoult D | title = Rickettsia africae sp. nov., the etiological agent of African tick bite fever | journal = International Journal of Systematic Bacteriology | volume = 46 | issue = 2 | pages = 611–614 | date = April 1996 | pmid = 8934912 | doi = 10.1099/00207713-46-2-611 | doi-access = free }}</ref>
** ''[[Rickettsia akari]]'' <ref name=list>{{Cite book | veditors = Skerman VB, McGowan V, Sneath PH | date = 1989 | title = Approved Lists of Bacterial Names | edition = amended | publisher = American Society for Microbiology | location = Washington, DC | url = https://www.ncbi.nlm.nih.gov/books/NBK835/ }}</ref>
** ''[[Rickettsia amblyommatis]]'' 
** ''[[Rickettsia argasii]]'' 
** ''[[Rickettsia asembonensis]]'' 
** ''[[Rickettsia asiatica]]'' <ref>Fujita, H.; Fournier, P.-E., et al. (2006), [http://ijs.sgmjournals.org/content/56/10/2365.long "''Rickettsia asiatica'' sp. nov., isolated in Japan"], ''Int J Syst Evol Microbiol'' '''56''' (Pt 10): 2365–2368</ref>
** ''[[Rickettsia australis]]'' <ref name=list/>
** ''[[Rickettsia buchneri]]'' 
** ''[[Rickettsia conorii]]'' 
*** ''R. conorii'' subsp. ''caspia'' 
*** ''R. conorii'' subsp. ''conorii'' 
*** ''R. conorii'' subsp. ''indica'' 
*** ''R. conorii'' subsp. ''israelensis'' 
** ''[[Rickettsia cooleyi]]'' <ref>{{cite journal | vauthors = Billings AN, Teltow GJ, Weaver SC, Walker DH | title = Molecular characterization of a novel Rickettsia species from Ixodes scapularis in Texas | journal = Emerging Infectious Diseases | volume = 4 | issue = 2 | pages = 305–309 | year = 1998 | pmid = 9621204 | pmc = 2640119 | doi = 10.3201/eid0402.980221 | url = http://ftp.cdc.gov/pub/EID/vol4no2/adobe/bil.pdf | url-status = dead | archive-url = https://web.archive.org/web/20170808141339/http://ftp.cdc.gov/pub/EID/vol4no2/adobe/bil.pdf | archive-date = 2017-08-08 }}</ref>
** ''[[Rickettsia felis]]'' <ref>La Scola, B.; Meconi, S., et al. (2002), [http://ijs.sgmjournals.org/content/52/6/2035.long "Emended description of ''Rickettsia felis'' (Bouyer ''et al.'' 2001), a temperature-dependent cultured bacterium"]{{Dead link|date=March 2022 |bot=InternetArchiveBot |fix-attempted=yes }}, ''Int J Syst Evol Microbiol'' '''52''' (Pt 6): 2035–2041</ref>
** ''[[Rickettsia fournieri]]'' 
** ''[[Rickettsia gravesii]]'' 
** ''[[Rickettsia heilongjiangensis]]'' 
** ''[[Rickettsia helvetica]]'' 
** ''[[Rickettsia honei]]'' 
*** [[Rickettsia honei subsp. marmionii|''R. honei ''subsp. ''marmionii'']] 
** ''[[Rickettsia hoogstraalii]]'' 
** ''[[Rickettsia hulinensis]]'' 
** ''[[Rickettsia hulinii]]'' 
** ''[[Rickettsia japonica]]'' 
** ''[[Rickettsia lusitaniae]]'' 
** ''[[Rickettsia marmionii]]'' 
** ''[[Rickettsia martinet]]'' 
** ''[[Rickettsia massiliae]]'' 
** ''[[Rickettsia monacensis]]'' 
** ''[[Rickettsia montanensis]]'' 
** ''[[Rickettsia moreli]]'' 
** ''[[Rickettsia parkeri]]'' <ref name=list/>
** ''[[Rickettsia peacockii]]'' 
** ''[[Rickettsia philipii]]'' 
** ''[[Rickettsia raoultii]]'' 
** ''[[Rickettsia rhipicephali]]'' 
** ''[[Rickettsia rickettsii]]'' <ref name=list/>
** [[Rickettsia sibirica subgroup]] 
*** ''[[Rickettsia sibirica]]'' <ref name=list/>
** ''[[Rickettsia slovaca]]'' 
** ''[[Rickettsia tamurae]]'' 
** ''[[Rickettsia vini]]'' 
* [[typhus group]] 
** ''[[Rickettsia prowazekii]]'' <ref name=list/>
** ''[[Rickettsia typhi]]'' <ref name=list/>
* ''Rickettsia'' ''[[incertae sedis]]''
** ''[[Rickettsia limoniae]]'' 
** [[Candidatus]] ''[[Rickettsia colombianensi]]'' 
** [[Candidatus]] ''[[Rickettsia laoensis]]'' 
** [[Candidatus]] ''[[Rickettsia liberiensis]]'' 
** [[Candidatus]] ''[[Rickettsia mahosotii]]'' 
}}

'''''Rickettsia''''' is a [[genus]] of [[Motility|nonmotile]], [[gram-negative]], [[Endospore|nonspore-forming]], highly [[pleomorphism (microbiology)|pleomorphic]] [[bacteria]] that may occur in the forms of [[cocci]] (0.1 μm in diameter), [[Bacillus (shape)|bacilli]] (1–4 μm long), or threads (up to about 10 μm long). The genus was named after [[Howard T. Ricketts|Howard Taylor Ricketts]] in honor of his pioneering work on tick-borne [[spotted fever]].

Properly, ''Rickettsia'' is the name of a single genus, but the informal term "rickettsia", plural "rickettsias," usually not capitalised, commonly applies to any members of the order [[Rickettsiales]]. Being [[Intracellular parasites#Obligate|obligate intracellular bacteria]], rickettsias depend on entry, growth, and replication within the [[cytoplasm]] of living [[eukaryote|eukaryotic]] host cells (typically [[Endothelium|endothelial]] cells).<ref name=Barron>{{Cite book| vauthors = Walker DH | title = Rickettsiae. ''In:'' Barron's Medical Microbiology| veditors = Baron S| display-editors = etal| edition = 4th | publisher = Univ of Texas Medical Branch | year = 1996 | id = [https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.2078 (via NCBI Bookshelf)]| isbn = 978-0-9631172-1-2 }}</ref> Accordingly, ''Rickettsia'' species cannot grow in artificial nutrient culture; they must be grown either in [[Biological tissue|tissue]] or [[embryo]] cultures. Mostly chicken embryos are used, following a method developed by [[Ernest William Goodpasture]] and his colleagues at [[Vanderbilt University]] in the early 1930s. Many new strains or species of ''Rickettsia'' are described each year.<ref>{{cite journal | vauthors = Binetruy F, Buysse M, Barosi R, Duron O | title = Novel Rickettsia genotypes in ticks in French Guiana, South America | journal = Scientific Reports | volume = 10 | issue = 1 | pages = 2537 | date = February 2020 | pmid = 32054909 | pmc = 7018960 | doi = 10.1038/s41598-020-59488-0 | bibcode = 2020NatSR..10.2537B }}</ref><ref>{{cite journal | vauthors = Buysse M, Duron O | title = Two novel Rickettsia species of soft ticks in North Africa: 'Candidatus Rickettsia africaseptentrionalis' and 'Candidatus Rickettsia mauretanica' | journal = Ticks and Tick-Borne Diseases | volume = 11 | issue = 3 | pages = 101376 | date = May 2020 | pmid = 32005627 | doi = 10.1016/j.ttbdis.2020.101376 | s2cid = 210997920 | doi-access = free }}</ref> Some ''Rickettsia'' species are pathogens of medical and veterinary interest, but many ''Rickettsia'' are non-pathogenic to vertebrates, including humans, and infect only arthropods, often non-hematophagous, such as aphids or whiteflies.<ref>{{cite journal | vauthors = Sakurai M, Koga R, Tsuchida T, Meng XY, Fukatsu T | title = Rickettsia symbiont in the pea aphid Acyrthosiphon pisum: novel cellular tropism, effect on host fitness, and interaction with the essential symbiont Buchnera | journal = Applied and Environmental Microbiology | volume = 71 | issue = 7 | pages = 4069–4075 | date = July 2005 | pmid = 16000822 | pmc = 1168972 | doi = 10.1128/AEM.71.7.4069-4075.2005 | bibcode = 2005ApEnM..71.4069S }}</ref><ref>{{cite journal | vauthors = Himler AG, Adachi-Hagimori T, Bergen JE, Kozuch A, Kelly SE, Tabashnik BE, Chiel E, Duckworth VE, Dennehy TJ, Zchori-Fein E, Hunter MS | display-authors = 6 | title = Rapid spread of a bacterial symbiont in an invasive whitefly is driven by fitness benefits and female bias | journal = Science | volume = 332 | issue = 6026 | pages = 254–256 | date = April 2011 | pmid = 21474763 | doi = 10.1126/science.1199410 | bibcode = 2011Sci...332..254H | s2cid = 31371994 }}</ref><ref>{{cite journal | vauthors = Giorgini M, Bernardo U, Monti MM, Nappo AG, Gebiola M | title = Rickettsia symbionts cause parthenogenetic reproduction in the parasitoid wasp Pnigalio soemius (Hymenoptera: Eulophidae) | journal = Applied and Environmental Microbiology | volume = 76 | issue = 8 | pages = 2589–2599 | date = April 2010 | pmid = 20173065 | pmc = 2849191 | doi = 10.1128/AEM.03154-09 | bibcode = 2010ApEnM..76.2589G }}</ref> Many ''Rickettsia'' species are thus arthropod-specific symbionts, but are often confused with pathogenic ''Rickettsia'' (especially in medical literature), showing that the current view in rickettsiology has a strong anthropocentric bias.<ref>{{cite journal | vauthors = Labruna MB, Walker DH | title = Rickettsia felis and changing paradigms about pathogenic rickettsiae | journal = Emerging Infectious Diseases | volume = 20 | issue = 10 | pages = 1768–1769 | date = October 2014 | pmid = 25271441 | pmc = 4193273 | doi = 10.3201/eid2010.131797 }}</ref>

Pathogenic ''Rickettsia'' species are transmitted by numerous types of [[arthropod]]s, including [[chiggers]], [[tick]]s, [[flea]]s, and [[lice]], and are associated with both human and plant diseases.<ref name="Perlman2006">{{cite journal | vauthors = Perlman SJ, Hunter MS, Zchori-Fein E | title = The emerging diversity of Rickettsia | journal = Proceedings. Biological Sciences | volume = 273 | issue = 1598 | pages = 2097–2106 | date = September 2006 | pmid = 16901827 | pmc = 1635513 | doi = 10.1098/rspb.2006.3541 }}</ref> Most notably, ''Rickettsia'' species are the pathogens responsible for [[typhus]], [[rickettsialpox]], [[boutonneuse fever]], [[African tick-bite fever]], [[Rocky Mountain spotted fever]], [[Flinders Island spotted fever]], and [[Queensland tick typhus]] ([[Ixodes holocyclus|Australian tick]] typhus).<ref>{{cite journal | vauthors = Unsworth NB, Stenos J, Graves SR, Faa AG, Cox GE, Dyer JR, Boutlis CS, Lane AM, Shaw MD, Robson J, Nissen MD | display-authors = 6 | title = Flinders Island spotted fever rickettsioses caused by "marmionii" strain of Rickettsia honei, Eastern Australia | journal = Emerging Infectious Diseases | volume = 13 | issue = 4 | pages = 566–573 | date = April 2007 | pmid = 17553271 | pmc = 2725950 | doi = 10.3201/eid1304.050087 }}</ref> The majority of pathogenic ''Rickettsia'' bacteria are susceptible to [[antibiotic]]s of the [[Tetracycline antibiotics|tetracycline]] group.

==Classification==
The classification of ''Rickettsia'' into three groups (spotted fever, typhus, and [[scrub typhus]]) was initially based on [[serology]]. This grouping has since been confirmed by [[DNA sequencing]].  All three of these groups include human [[pathogen]]s. The scrub typhus group has been reclassified as a related new genus,  ''[[Orientia]]'', but they still are in the order Rickettsiales and accordingly still are grouped with the rest of the rickettsial diseases.{{citation needed|date=April 2022}}

Rickettsias are more widespread than previously believed and are known to be associated with [[arthropod]]s, [[leech]]es, and [[protist]]s. Divisions have also been identified in the spotted fever group and this group should probably be divided into two [[clade]]s.<ref name="Gillespie2007">{{cite journal | vauthors = Gillespie JJ, Beier MS, Rahman MS, Ammerman NC, Shallom JM, Purkayastha A, Sobral BS, Azad AF | display-authors = 6 | title = Plasmids and rickettsial evolution: insight from Rickettsia felis | journal = PLOS ONE | volume = 2 | issue = 3 | pages = e266 | date = March 2007 | pmid = 17342200 | pmc = 1800911 | doi = 10.1371/journal.pone.0000266 | bibcode = 2007PLoSO...2..266G | doi-access = free }} {{open access}}.</ref> Arthropod-inhabiting rickettsiae are generally associated with reproductive manipulation (such as [[parthenogenesis]]) to persist in host lineage.<ref name="Perlman2006"/>

In March 2010, Swedish researchers reported a case of [[bacterial meningitis]] in a woman caused by ''[[Rickettsia helvetica]]'' previously thought to be harmless.<ref>[https://www.cdc.gov/eid/content/16/3/490.htm "Rickettsia helvetica in Patient with Meningitis, Sweden, 2006" ''Emerging Infectious Diseases'', Volume 16, Number 3 – March 2010]</ref>

===Spotted fever group===
:* ''[[Rickettsia rickettsii]]'' (Western Hemisphere)
:: [[Rocky Mountain spotted fever]]

:* ''[[Rickettsia akari]]'' (USA, former Soviet Union)
:: [[Rickettsialpox]]

:* ''[[Rickettsia conorii]]'' (Mediterranean countries, Africa, Southwest Asia, India)
:: [[Boutonneuse fever]]

:* ''[[Rickettsia sibirica]]'' (Siberia, Mongolia, northern China)
:: [[North Asian tick typhus|Siberian tick typhus or North Asian tick typhus]]

:* ''[[Rickettsia australis]]'' (Australia)
:: [[Australian tick typhus]]

:* ''[[Rickettsia felis]]'' (North and South America, Southern Europe, Australia)
:: [[Flea-borne spotted fever]]

:* ''[[Rickettsia japonica]]'' (Japan)
:: [[Oriental spotted fever]]

:* ''[[Rickettsia africae]]'' (South Africa)
:: [[African tick bite fever]]

:* ''[[Rickettsia hoogstraalii]]'' (Croatia, Spain and Georgia USA)<ref>Duh, D., V. Punda-Polic, T. Avsic-Zupanc, D. Bouyer, D.H. Walker, V.L. Popov, M. Jelovsek, M. Gracner, T. Trilar, N. Bradaric, T.J. Kurtti and J. Strus. (2010) Rickettsia hoogstraalii sp. nov., isolated from hard- and soft-bodied ticks. ''International Journal of Systematic and Evolutionary Microbiology'', 60, 977–984; [https://archive.today/20120710185809/http://ijs.sgmjournals.org/cgi/content/abstract/60/4/977], accessed 16 July 2010.</ref>
:: Unknown pathogenicity

===Typhus group===
:* ''[[Rickettsia prowazekii]]'' (worldwide)
:: [[Epidemic typhus]], recrudescent typhus, and sporadic typhus

:* ''[[Rickettsia typhi]]'' (worldwide)
:: [[Murine typhus]] (endemic typhus)

===Scrub typhus group===

:* The causative agent of [[scrub typhus]] formerly known as ''R. tsutsugamushi'' has been reclassified into the genus ''[[Orientia]]''.{{Rickettsialesphylogeny}}

==Flora and fauna pathogenesis==
Plant diseases have been associated with these ''Rickettsia''-like organisms (RLOs):<ref name=Smith>{{Cite book|vauthors=Smith IM, Dunez J, Lelliot RA, Phillips DH, Archer SA | title = European Handbook of Plant Diseases. | publisher = Blackwell Scientific Publications| year = 1988 | isbn = 978-0-632-01222-0}}</ref>

:* Beet latent rosette RLO
:* Citrus greening bacterium possibly this [[Orange (fruit)#Citrus greening disease|citrus greening disease]]
:* Clover leaf RLO
:* Grapevine infectious necrosis RLO
:* Grapevine Pierce's RLO
:* Grapevine yellows RLO
:* [[Witch's broom]] disease on ''[[Larix]]'' spp.
:* Peach phony RLO
:* [[Papaya Bunchy Top Disease]]<ref>[https://www.apsnet.org/publications/phytopathology/backissues/Documents/1996Abstracts/Phyto_86_102.htm Davis, M. J. 1996]</ref>

Infection occurs in nonhuman mammals; for example, species of ''Rickettsia'' have been found to afflict the [[South American]] [[guanaco]], ''Lama guanacoe''<ref>C. Michael Hogan. 2008. [http://globaltwitcher.auderis.se/artspec_information.asp?thingid=42654 ''Guanaco: Lama guanicoe'', GlobalTwitcher.com, ed. N. Strömberg] {{webarchive |url=https://web.archive.org/web/20110304004355/http://globaltwitcher.auderis.se/artspec_information.asp?thingid=42654 |date=4 March 2011 }}</ref> potentially marsupials<ref>Vilcins IE, Old JM, Deane EM (2009). Molecular detection of Rickettsia, Coxiella and Rickettsiella in three Australian native tick species. Experimental and Applied Acarology. 49(3), 229-242. DOI: 10.1007/s10493-009-9260-4</ref><ref>Vilcins IE, Old JM, Deane EM (2008). Detection of a spotted fever group Rickettsia in the tick ''Ixodes tasmani'' collected from koalas (''Phascolarctos cinereus'') in Port Macquarie, N.S.W. Journal of Medical Entomology. 45(4), 745-750. DOI: 10.1016/j.vetpar.2009.02.015</ref> and reptiles.<ref>Vilcins I, Fournier P, Old JM, Deane EM (2009). Evidence for the presence of Francisella and spotted fever group Rickettsia DNA in the Tick ''Amblyomma fimbriatum'' (Acari: Ixodidae), Northern Territory, Australia. Journal of Medical Entomology. 46(4), 926-933. doi:  10.1186/s13071-015-0719-3</ref>

==Pathophysiology==
{{expand section|date=August 2013}}
{{main|Typhus}}
Rickettsial organisms are [[Intracellular parasites#Obligate|obligate intracellular parasites]] and invade [[Blood vessel|vascular]] [[endothelial]] cells in target organs, damaging them and producing increased [[vascular permeability]] with consequent [[oedema]], [[hypotension]], and [[hypoalbuminaemia]].<ref name="medscape-reference">{{cite web |url= https://reference.medscape.com/article/968385-overview#showall|title= Rickettsial infection: Overview| vauthors = Rathore MH |date= 14 June 2016|website= Medscape|access-date= 16 November 2017}}</ref>

==Genomics==
Certain segments of rickettsial [[genome]]s resemble those of [[Mitochondrial DNA|mitochondria]].<ref name=Emelyanov_2003>{{cite journal | vauthors = Emelyanov VV | title = Mitochondrial connection to the origin of the eukaryotic cell | journal = European Journal of Biochemistry | volume = 270 | issue = 8 | pages = 1599–1618 | date = April 2003 | pmid = 12694174 | doi = 10.1046/j.1432-1033.2003.03499.x | doi-access = free }}</ref> The deciphered genome of ''R. prowazekii''  is 1,111,523 [[base pair|bp]] long and contains 834 [[genes]].<ref name=Andersson_1998>{{cite journal | vauthors = Andersson SG, Zomorodipour A, Andersson JO, Sicheritz-Pontén T, Alsmark UC, Podowski RM, Näslund AK, Eriksson AS, Winkler HH, Kurland CG | display-authors = 6 | title = The genome sequence of Rickettsia prowazekii and the origin of mitochondria | journal = Nature | volume = 396 | issue = 6707 | pages = 133–140 | date = November 1998 | pmid = 9823893 | doi = 10.1038/24094 | doi-access = free | bibcode = 1998Natur.396..133A | author1-link = Siv G. E. Andersson }}</ref> Unlike free-living bacteria, it contains no genes for [[Anaerobic respiration|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 a [[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 | vauthors = Kannangara S, DeSimone JA, Pomerantz RJ | title = Attenuation of HIV-1 infection by other microbial agents | journal = The Journal of Infectious Diseases | volume = 192 | issue = 6 | pages = 1003–1009 | date = September 2005 | pmid = 16107952 | doi = 10.1086/432767 | doi-access = free }}</ref> Human immune response to the [[scrub typhus]] pathogen, ''[[Orientia tsutsugamushi]]'', 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 rickettsiae 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]].

Comparative analysis of genomic sequences have also identified five [[conserved signature indels]] in important proteins, which are uniquely found in members of the genus ''Rickettsia''. These indels consist of a four-amino-acid insertion in [[transcription repair coupling factor]] Mfd, a 10-amino-acid insertion in ribosomal protein L19, a one-amino-acid insertion in [[FtsZ]], a one-amino-acid insertion in major [[sigma factor]] 70, and a one-amino-acid deletion in [[exonuclease VII]]. These indels are all characteristic of the genus and serve as molecular markers for ''Rickettsia''.<ref>{{cite journal | vauthors = Gupta RS | title = Protein signatures distinctive of alpha proteobacteria and its subgroups and a model for alpha-proteobacterial evolution | journal = Critical Reviews in Microbiology | volume = 31 | issue = 2 | pages = 101–135 | date = January 2005 | pmid = 15986834 | doi = 10.1080/10408410590922393 | s2cid = 30170035 }}</ref>

[[Bacterial small RNA]]s play critical roles in virulence and stress/adaptation responses. Although their specific functions have not been discovered in ''Rickettsia'', few studies showed the expression of novel sRNA in human microvascular [[Endothelium|endothelial cells]] (HMEC) infected with ''Rickettsia''.<ref>{{cite journal | vauthors = Schroeder CL, Narra HP, Rojas M, Sahni A, Patel J, Khanipov K, Wood TG, Fofanov Y, Sahni SK | display-authors = 6 | title = Bacterial small RNAs in the Genus Rickettsia | journal = BMC Genomics | volume = 16 | pages = 1075 | date = December 2015 | pmid = 26679185 | pmc = 4683814 | doi = 10.1186/s12864-015-2293-7 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Schroeder CL, Narra HP, Sahni A, Rojas M, Khanipov K, Patel J, Shah R, Fofanov Y, Sahni SK | display-authors = 6 | title = Identification and Characterization of Novel Small RNAs in Rickettsia prowazekii | journal = Frontiers in Microbiology | volume = 7 | pages = 859 | date = 2016 | pmid = 27375581 | pmc = 4896933 | doi = 10.3389/fmicb.2016.00859 | doi-access = free }}</ref>

Genomes of intracellular or parasitic bacteria undergo massive reduction compared to their free-living relatives. Examples include Rickettsia for alpha proteobacteria, T. whipplei for Actinobacteria, Mycoplasma for Firmicutes (the low G+C content Gram-positive), and Wigglesworthia and Buchnera for gamma proteobacteria.<ref>{{cite journal | vauthors = Raoult D, Ogata H, Audic S, Robert C, Suhre K, Drancourt M, Claverie JM | title = Tropheryma whipplei Twist: a human pathogenic Actinobacteria with a reduced genome | journal = Genome Research | volume = 13 | issue = 8 | pages = 1800–1809 | date = August 2003 | pmid = 12902375 | pmc = 403771 | doi = 10.1101/gr.1474603 }}</ref>

==Naming==
The genus ''Rickettsia'' is named after [[Howard Taylor Ricketts]] (1871–1910), who studied Rocky Mountain spotted fever in the [[Bitterroot Valley]] of Montana, and eventually died of typhus after studying that disease in Mexico City.

In his early part of career, he undertook research at Northwestern University on blastomycosis. He later worked on Rocky Mountain spotted fever at the University of Chicago and Bitterroot Valley of Montana. He was so devoted  to his research that on several occasions, he  injected himself with pathogens to study their effects. On account of the apparent similarity between Rocky Mountain fever and typhus fever, he became occupied in investigating the latter in Chicago where the disease was epidemic, and became a victim of the epidemic in 1910. His investigations and discoveries added materially to the sum of medical knowledge.

== References ==
{{Reflist}}

== External links ==
* [https://patricbrc.org/view/Taxonomy/780#view_tab=overview Rickettsia] genomes and related information at [http://patricbrc.org/ PATRIC], a Bioinformatics Resource Center funded by [http://www.niaid.nih.gov/ NIAID]
* [https://www.cdc.gov/ncidod/EID/vol12no09/05-1540.htm African Tick Bite Fever] from the [https://www.cdc.gov/ncidod/eid/ Centers for Disease Control and Prevention]

{{Taxonbar|from=Q627992}}
{{Authority control}}

[[Category:Rickettsiaceae]]
[[Category:Bacteria genera]]
[[Category:Pathogenic bacteria]]