Rickettsia

Overview
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). 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, African Tick Bite Fever, Rocky Mountain spotted fever, Australian Tick Typhus, Flinders Island Spotted Fever and Queensland Tick 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 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.

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


 * R. africae (South Africa)
 * African Tick Bite 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.


 * 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.

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 D deficiency disease, not an infectious disease.