Rickettsia
Rickettsia (bacteria of the genus Rickettsia) are worldwide occurring organisms belonging to the bacteria family, which are found in many ticks, fleas, mites and lice and to which these serve as vectors (carriers).
In humans, they cause (endemic in Mediterranean countries, Eastern Europe, the tropics and North America) a whole series of diseases with different clinical pictures, which are medically combined in the group of rickettsial diseases. Examples are spotted fever (syn. typhus exanthematicus), Wolhynian fever (trench fever), rickettsial pox, Brill-Zinsser disease, Boutonneuse fever (Mediterranean tick-borne spotted fever) and Rocky Mountain spotted fever. Like viruses, rickettsiae thrive as intracellular parasites exclusively in living cells. In this way, they manage to evade the immune system of their hosts. The term "rickettsiae" is often used for all members of the order Rickettsiales.
These organisms were named rickettsiae in honour of the pathologist Howard Taylor Ricketts, who, among other things, researched Rocky Mountains spotted fever, the pathogen of which he was able to detect in the blood of infected humans and in the cattle tick species active as a vector. In 1909 he travelled to Mexico City with the aim of researching spotted fever. While there, he became infected with rickettsiae, fell ill and died in 1910.
Typical symptoms of rickettsialpox are fever, headache and exanthema. The diagnosis is usually made serologically.
The majority of all rickettsiae are sensitive to antibiotics of the tetracycline group; in an appropriate case, an infection can be treated with a two-week course of doxycycline. Alternatively, quinolones are also used. For central nervous system infections, chloramphenicol or (the tetracycline) doxycycline in combination with quinolones and/or rifampicin may be considered as antimicrobial drugs. In moist media, killing occurs at 50 °C in 15 minutes. The pathogens can also be effectively destroyed with conventional disinfectants.
Properties
Rickettsiae are Gram-negative, highly polymorphic (polymorphic, pleomorphic) organisms that do not form spores. They are often round (cocci) to oval bacteria with a diameter of 0.1 µm; they may also appear as rods (1-4 μm long) or filamentous (10 μm long). Occasionally they form chains, but usually they occur singly or in pairs. The survival of obligate intracellular rickettsiae depends entirely on their eukaryotic host cell (usually endothelial cells), into whose cytoplasm they must invade to be protected from the host defense system. Reproduction by transverse division also takes place inside the host cell. The bacteria are subsequently released by strangulation from the cell membrane (exocytosis) or by lysis, which destroys the host cell. At least R. conorii is capable of movement within the host cell.
Due to their dependence on the host cell, the bacteria cannot be kept in artificial culture media in the laboratory. They are therefore grown either in biological tissues or embryo cultures (typically chicken embryos are used). Because of their cell dependency and their reduced metabolism, rickettsiae were in the past often classified as microorganisms somewhere between viruses and the larger true bacteria, as an "intermediate species". For a long time, they were also referred to as "large viruses".
Mechanism of cell invasion
How rickettsiae manage to penetrate eukaryotic cells has been a mystery until now. At the end of 2005, scientists at the Institut Pasteur in Paris succeeded in identifying two key proteins involved in the penetration process using Rickettsia conorii. These are the bacterial protein rOmpB and the mammalian protein Ku70, which is normally found in the nucleus of mammalian cells. Apparently, however, it can also migrate to the cell membrane, where it is captured by Rickettsia's own rOmpB and used to enter the cell. The scientists also referred to Ku70 as the "molecular stooge" of the rickettsiae because of this "treacherous" property.
