Rickettsia

Related Topics:
Rickettsial :: Rickettsialpox

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. They have also been associated with a range of plant diseases. Like viruses, 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 relative to the mitochondria which inhabit each cell of all Eukaryotes, including humans, working as the energy-conversion organ for that cell.

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). 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.

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. sibirica (Siberia, Mongolia, northern China)

Siberian tick typhus

R. australis (Australia)

Australian tick typhus

R. japonica (Japan)

Oriental spotted fever

Typhus group

R. prowazekii (Worldwide)

Epidemic, recrudescent and sporadic typhus

R. typhi (Worldwide)

Murine typhus

Scrub typhus group''

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

(now a separate genus, Orientia)

Scrub typhus

Plant Pathogenesis

The following plant diseases have been assocaited with Rickettsia-like organisms.

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 genomes resemble that of mitochondria. }} The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding genes. }} 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. }} 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 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.

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.

References

Rickettsiales | Bacteriology

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