Rickettsia
Rickettsia rickettsii in the host cell
Bacteria of the genus Rickettsia are parasitic organisms that are found in many ticks, fleas, mites and lice as vectors ( carriers ).
In humans they cause a number of diseases with different clinical syndromes, which are summarized medically to the group of rickettsial diseases. Be mentioned typhus ( syn. typhoid Exanthematicus ) Rickettsialpox, Brill - Zinsser disease, Boutonneuse fever ( Mediterranean tick typhus ) and the Rocky Mountain spotted fever as examples. Rickettsias such as viruses grow as intracellular parasites only in living cells. In this way, they manage to escape the immune system of their hosts. The term " Rickettsia " is often used for all members of the order Rickettsiales the.
The name derives from the pathologist Howard Taylor Ricketts, who explored including the Rocky Mountain spotted fever, which he could pathogen detected in the blood of infected people and the active and Vector Viehzeckenart. In 1909 he traveled to Mexico City to study with the aim of typhus. He was infected with Rickettsia, so that he died in 1910 from the disease.
The majority of rickettsiae is sensitive to antibiotics of the tetracycline group. In humid media a kill takes place at 50 ° C in 15 minutes. Even with conventional disinfectants can effectively destroy the pathogens.
Properties
Rickettsiae are gram-negative, highly polymorphic ( = multiform, pleomorphic ) organisms that do not form spores. Often it is round ( = cocci ) to oval bacteria with a diameter of 0.1 microns; they can also be as rods (1-4 microns long) or thread -like (10 microns long) occur. Occasionally, they form chains, usually they come but before individually or in pairs. The survival of the obligate intracellular rickettsia depends entirely on its eukaryotic host cell ( mostly endothelial cells) from, in their cytoplasm, they must penetrate in order to be protected from the immune system of the host. The propagation by transverse division takes place inside the host cell. The release of the bacteria is followed by constriction of the cell membrane ( exocytosis ) or by lysis, whereby the host cell is destroyed. R. conorii at least is mounted for movement within the host cell capable.
Because of their dependence on the host cell, the parasite can not be maintained in artificial growth media in the laboratory. They therefore breeds either in biological tissues or embryo cultures (typically, chicken embryos are used). Because of their dependence and reduced cell metabolism, rickettsiae were often classified in the past as microorganisms somewhere between the viruses and the bacteria real, as if an "intermediate species ". They were designated long as so-called " Great viruses ".
Mechanism of cell invasion
How can rickettsiae to invade eukaryotic cells, that has been a mystery. Scientists at the Pasteur Institute in Paris has succeeded in late 2005 to identify two key proteins involved in the penetration process of Rickettsia conorii based. Is the bacterial protein rOmpB and to the mammalian Ku70 protein, which normally takes place in the nucleus of mammalian cells. Obviously, it can also migrate to the cell membrane where it is held by the rickettsial own rOmpB and used to penetrate the cell. The scientists identified Ku70 due to this " treacherous " property as " molecular stooge " of the rickettsiae.
Endosymbiontentheorie
Rickettsia prowazekii the type is of particular interest in relation to the endosymbiotic the mitochondria. In the unicellular Reclinomonas americana mitochondria were the largest known genome found. It therefore assumes that in this mitochondrion genetically the closest match to the original symbiote is still left. When comparing bacteria with this protozoa in turn showed the completely sequenced genome of Rickettsia prowazekii is the best match. So one can assume that Rickettsia has a close relationship with the ancestors of mitochondria.
What is certain, however, this assumption is not considered. The genome of Rickettsia prowazekii as also to be found in the mitochondria genome is very small. Both are probably shrunk in the billions of years of evolution, which is why a strong similarity of gene structure does not allow for a clear statement. That is why, for example, Rickettsia and the original bacterium, which formed the symbionts, are descended from a common ancestor.
Grouping
With regard to their property are usually divided into the following three groups as Rickettsia pathogenic for humans:
- Tick bite fever ( spotted fever )
- Spotted fever ( typhus )
- Tsutsugamushi ( scrub typhus ) (Note: Name unclear! )
System
The following types are summarized in the genus Rickettsia:
- Rickettsia aeschlimannii Beati et al. 1997
- Rickettsia africae Kelly et al. 1996
- Rickettsia asiatica Fujita et al. 2006
- Rickettsia australis Philip 1950
- Rickettsia bellii Philip et al. 1983
- Rickettsia canadensis Corrig. McKiel et al. 1967
- Rickettsia conorii Brumpt 1932
- Rickettsia felis Bouyer et al. 2001, emend. La Scola et al. 2002
- Rickettsia heilongjiangensis Fournier et al. 2006
- Rickettsia helvetica Beati et al. 1993
- Rickettsia honei Stenos et al. 1998
- Rickettsia japonica Uchida et al. 1992
- Rickettsia massiliae Beati and Raoult 1993
- Rickettsia parkeri Lackman et al. 1965
- Rickettsia peacockii Niebylski et al. 1997
- Rickettsia prowazekii da Rocha -Lima in 1916 ( type species )
- Rickettsia raoultii Mediannikov et al. 2008
- Rickettsia rickettsii ( Wolbach 1919) Brumpt 1922
- Rickettsia sibirica Zdrodovskii 1948
- Rickettsia slovaca Sekeyová et al. 1998
- Rickettsia tamurae Fournier et al. 2006
- Rickettsia tsutsugamushi ( Hayashi 1920) Ogata 1931
- Rickettsia typhi ( Wolbach & Todd 1920) Philip 1943