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 orderRickettsiales. Beingobligate intracellular bacteria, rickettsias depend on entry, growth, and replication within thecytoplasm of livingeukaryotic host cells (typicallyendothelial cells).[9] Accordingly,Rickettsia species cannot grow in artificial nutrient culture; they must be grown either intissue orembryo cultures. Mostly chicken embryos are used, following a method developed byErnest William Goodpasture and his colleagues atVanderbilt University in the early 1930s. Many new strains or species ofRickettsia are described each year.[10][11] SomeRickettsia species are pathogens of medical and veterinary interest, but manyRickettsia are non-pathogenic to vertebrates, including humans, and infect only arthropods, often non-hematophagous, such as aphids or whiteflies.[12][13][14] ManyRickettsia species are thus arthropod-specific symbionts, but are often confused with pathogenicRickettsia (especially in medical literature), showing that the current view in rickettsiology has a strong anthropocentric bias.[15]
The classification ofRickettsia into three groups (spotted fever, typhus, andscrub typhus) was initially based onserology. This grouping has since been confirmed byDNA sequencing. All three of these groups include humanpathogens. 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]
Rickettsias are more widespread than previously believed and are known to be associated witharthropods,leeches, andprotists. Divisions have also been identified in the spotted fever group and this group likely should be divided into twoclades.[18] Arthropod-inhabiting rickettsiae are generally associated with reproductive manipulation (such asparthenogenesis) to persist in host lineage.[16]
Infection occurs in nonhuman mammals; for example, species ofRickettsia have been found to afflict theSouth Americanguanaco,Lama guanacoe[24] potentially marsupials[25][26] and reptiles.[27]
Certain segments of rickettsialgenomes resemble those ofmitochondria.[29] The deciphered genome ofR. prowazekii is 1,111,523bp long and contains 834genes.[30] Unlike free-living bacteria, it contains no genes foranaerobicglycolysis or genes involved in the biosynthesis and regulation ofamino acids andnucleosides. In this regard, it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used.
ATP production inRickettsia is the same as that in mitochondria. In fact, of all the microbes known, theRickettsia is probably the closest relative (in aphylogenetic sense) to the mitochondria. Unlike the latter, the genome ofR. prowazekii, however, contains a complete set of genes encoding for thetricarboxylic acid cycle and therespiratory chain complex. Still, the genomes of theRickettsia, 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 betweenRickettsia and viruses may become a basis for fightingHIV infection.[31] Human immune response to thescrub 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 ofhomology between the rickettsiae and the virus, namely, commonepitope(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 illnessdengue fever.
Comparative analysis of genomic sequences have also identified fiveconserved signature indels in important proteins, which are uniquely found in members of the genusRickettsia. These indels consist of a four-amino-acid insertion intranscription repair coupling factor Mfd, a 10-amino-acid insertion in ribosomal protein L19, a one-amino-acid insertion inFtsZ, a one-amino-acid insertion in majorsigma factor 70, and a one-amino-acid deletion inexonuclease VII. These indels are all characteristic of the genus and serve as molecular markers forRickettsia.[32]
Bacterial small RNAs play critical roles in virulence and stress/adaptation responses. Although their specific functions have not been discovered inRickettsia, few studies showed the expression of novel sRNA in human microvascularendothelial cells (HMEC) infected withRickettsia.[33][34]
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.[35]
The genusRickettsia is named afterHoward Taylor Ricketts (1871–1910), who studied Rocky Mountain spotted fever in theBitterroot 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.
^"Rickettsia".NCBI taxonomy. Bethesda, MD: National Center for Biotechnology Information. Retrieved8 January 2019.
^Walker DH (1996). Baron S, et al. (eds.).Rickettsiae.In: Barron's Medical Microbiology (4th ed.). Univ of Texas Medical Branch.ISBN978-0-9631172-1-2.(via NCBI Bookshelf).
^Himler AG, Adachi-Hagimori T, Bergen JE, Kozuch A, Kelly SE, Tabashnik BE, et al. (April 2011). "Rapid spread of a bacterial symbiont in an invasive whitefly is driven by fitness benefits and female bias".Science.332 (6026):254–256.Bibcode:2011Sci...332..254H.doi:10.1126/science.1199410.PMID21474763.S2CID31371994.
^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;[1], accessed 16 July 2010.
^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
^Vilcins IE, Old JM, Deane EM (2008). Detection of a spotted fever group Rickettsia in the tickIxodes 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
^Vilcins I, Fournier P, Old JM, Deane EM (2009). Evidence for the presence of Francisella and spotted fever group Rickettsia DNA in the TickAmblyomma fimbriatum (Acari: Ixodidae), Northern Territory, Australia. Journal of Medical Entomology. 46(4), 926-933. doi: 10.1186/s13071-015-0719-3
^Gupta RS (January 2005). "Protein signatures distinctive of alpha proteobacteria and its subgroups and a model for alpha-proteobacterial evolution".Critical Reviews in Microbiology.31 (2):101–135.doi:10.1080/10408410590922393.PMID15986834.S2CID30170035.
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