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'''Rickettsia''' is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria 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). Rickettsiae. ''In:'' Barron's Medical Microbiology ''(Barron S ''et al'', eds.), Walker DH, , , Univ of Texas Medical Branch, 1996, (via NCBI Bookshelf) ISBN 0-9631172-1-1 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.
''Rickettsia'' species are carried as parasites by many ticks, fleas, and lice, and cause diseases such as typhus, rickettsialpox, Boutonneuse fever and Rocky Mountain spotted fever 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 relatives to bacteria that were the origin of the mitochondria organelle that exist in most eukaryotic cells.
The method of growing Rickettsia in chicken embryos was invented by Ernest William Goodpasture and colleagues at Vanderbilt University in the early 1930s.

Contents

Human pathogenesis

Spotted fever group

Typhus group

Scrub typhus group

Plant pathogenesis

Genomics

Naming

References

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 (endemic) typhus

Scrub typhus group

:
★ The causative agent of scrub typhus formerly known as ''R. tsutsugamushi'' has been reclassified into the genus ''Orientia''.

Plant pathogenesis

The following plant diseases have been assocaited with Rickettsia-like organisms European Handbook of Plant Diseases., Smith IM, Dunez J, Lelliot RA, Phillips DH, Archer SA, , , Blackwell Scientific Publications, 1988, ISBN 0-632-01222-6 .
:
★ 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.^[1] 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.

Naming

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, ''hrake'', and is a vitamin deficiency disease, not an infectious disease.

References

1.