Wikipedia

Rickettsia

Rickettsiais agenusofnonmotile,gram-negative,nonspore-forming,highlypleomorphicbacteriathat may occur in the forms ofcocci(0.1 μm in diameter),bacilli(1–4 μm long), or threads (up to about 10 μm long). The genus was named afterHoward Taylor Rickettsin honor of his pioneering work on tick-bornespotted fever.

Rickettsia
Red-stained Rickettsia rickettsii visible in the cell of an Ixodid vector tick
Red-stainedRickettsia rickettsiivisible in cells of anIxodidvectortick
Scientific classificationEdit this classification
Domain: Bacteria
Phylum: Pseudomonadota
Class: Alphaproteobacteria
Order: Rickettsiales
Family: Rickettsiaceae
Tribe: Rickettsieae
Genus: Rickettsia
da Rocha-Lima,1916
Species groupsandspecies[8]

Properly,Rickettsiais 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 thecytoplasmof livingeukaryotichost cells (typicallyendothelialcells).[9]Accordingly,Rickettsiaspecies cannot grow in artificial nutrient culture; they must be grown either intissueorembryocultures. Mostly chicken embryos are used, following a method developed byErnest William Goodpastureand his colleagues atVanderbilt Universityin the early 1930s. Many new strains or species ofRickettsiaare described each year.[10][11]SomeRickettsiaspecies are pathogens of medical and veterinary interest, but manyRickettsiaare non-pathogenic to vertebrates, including humans, and infect only arthropods, often non-hematophagous, such as aphids or whiteflies.[12][13][14]ManyRickettsiaspecies 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]

PathogenicRickettsiaspecies are transmitted by numerous types ofarthropods,includingchiggers,ticks,fleas,andlice,and are associated with both human and plant diseases.[16]Most notably,Rickettsiaspecies are the pathogens responsible fortyphus,rickettsialpox,boutonneuse fever,African tick-bite fever,Rocky Mountain spotted fever,Flinders Island spotted fever,andQueensland tick typhus(Australian ticktyphus).[17]The majority of pathogenicRickettsiabacteria are susceptible toantibioticsof thetetracyclinegroup.

Classification

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The classification ofRickettsiainto 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]

In March 2010, Swedish researchers reported a case ofbacterial meningitisin a woman caused byRickettsia helveticapreviously thought to be harmless.[19]

Spotted fever group

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Rocky Mountain spotted fever
Rickettsialpox
Boutonneuse fever
Siberian tick typhus or North Asian tick typhus
Australian tick typhus
Flea-borne spotted fever
Oriental spotted fever
African tick bite fever
Unknown pathogenicity

Typhus group

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Epidemic typhus,recrudescent typhus, and sporadic typhus
Murine typhus(endemic typhus)

Scrub typhus group

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  • The causative agent ofscrub typhusformerly known asR. tsutsugamushihas been reclassified into the genusOrientia.
Schematic ribosomal RNA phylogeny ofAlphaproteobacteria
Magnetococcidae

Magnetococcus marinus

The cladogram of Rickettsidae has been inferred by Ferlaet al.[21]from the comparison of16S+23Sribosomal RNA sequences.

Flora and fauna pathogenesis

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Plant diseases have been associated with theseRickettsia-like organisms (RLOs):[22]

Infection occurs in nonhuman mammals; for example, species ofRickettsiahave been found to afflict theSouth Americanguanaco,Lama guanacoe[24]potentially marsupials[25][26]and reptiles.[27]

Pathophysiology

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Main article:Typhus

Rickettsial organisms areobligate intracellular parasitesand invadevascularendothelialcells in target organs, damaging them and producing increasedvascular permeabilitywith consequentoedema,hypotension,andhypoalbuminaemia.[28]

Genomics

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Certain segments of rickettsialgenomesresemble those ofmitochondria.[29]The deciphered genome ofR. prowazekiiis 1,111,523bplong and contains 834genes.[30]Unlike free-living bacteria, it contains no genes foranaerobicglycolysisor genes involved in the biosynthesis and regulation ofamino acidsandnucleosides.In this regard, it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used.

ATPproduction inRickettsiais the same as that in mitochondria. In fact, of all the microbes known, theRickettsiais probably the closest relative (in aphylogeneticsense) to the mitochondria. Unlike the latter, the genome ofR. prowazekii,however, contains a complete set of genes encoding for thetricarboxylic acid cycleand therespiratory chaincomplex. 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 betweenRickettsiaand viruses may become a basis for fightingHIVinfection.[31]Human immune response to thescrub typhuspathogen,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 ofhomologybetween 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 indelsin important proteins, which are uniquely found in members of the genusRickettsia.These indels consist of a four-amino-acid insertion intranscription repair coupling factorMfd, a 10-amino-acid insertion in ribosomal protein L19, a one-amino-acid insertion inFtsZ,a one-amino-acid insertion in majorsigma factor70, 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 RNAsplay 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]

Naming

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The genusRickettsiais named afterHoward Taylor Ricketts(1871–1910), who studied Rocky Mountain spotted fever in theBitterroot Valleyof 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

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