Rickettsia is a unique genus classified between a virus and a bacteria. The organism is unique in that it has a cell wall and exists only in living cells like a virus, but is susceptible to antibiotics like bacteria.
The method of growing Rickettsia in chicken embryos was invented by Ernest William Goodpasture and his colleagues at Vanderbilt University in the early 1930s.
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".
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.
