Rickettsia

From Free net encyclopedia

{{Taxobox | color = lightgrey | name = Rickettsia | regnum = Bacteria | phylum = Proteobacteria | classis = Alpha Proteobacteria | ordo = Rickettsiales | familia = Rickettsiaceae | genus = Rickettsia | genus_authority = da Rocha-Lima, 1916 | subdivision_ranks = Species | subdivision = Rickettsia felis
Rickettsia prowazekii
Rickettsia rickettsii
Rickettsia typhi
Rickettsia conorii
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 cells. The name rickettsia is often used for any member of the Rickettsiales.

Rickettsia are non-motile, Gram-negative, non-sporeforming, highly pleomorphic organisms 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 eukaryotic host cells (typically endothelial cells).<ref name=Barron>Template:Cite book</ref> 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 true bacteria. The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.

1 Pathogenesis
2 Genomics
3 Historical
4 References

[edit]

Pathogenesis

The Rickettsia human pathogens are generally grouped as follows:

[edit]

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. sibirica (Siberia, Mongolia, northern China)

Siberian tick typhus

R. australis (Australia)

Australian tick typhus

R. japonica (Japan)

Oriental spotted fever

[edit]

Typhus group

R. prowazekii (Worldwide)

Epidemic, recrudescent and sporadic typhus

R. typhi (Worldwide)

Murine typhus

[edit]

Scrub typhus group

R. tsutsugamushi (SW Asia, Northern Australia, Pacific Islands)

(now a separate genus, Orientia)

Scrub typhus

[edit]

Genomics

Certain segments of Rickettsial genomes resemble that of mitochondria.<ref name=Emelyanov_2003>Template:Cite journal }}</ref> The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding genes.<ref name=Andersson_1998>Template:Cite journal }}</ref> Unlike free-living bacteria, it contains no genes for anaerobic glycolysis or genes involved in the biosynthesis and regulation 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.<ref>Template:Cite journal }}</ref> 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.

[edit]

Historical

The genus Rickettsia is named after 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 word for spine, rhakhis, and is a vitamin deficiency disease, not an infectious disease.