Rickettsia felis
Rickettsia prowazekii
Rickettsia rickettsii
Rickettsia typhi
Rickettsia conorii
Rickettsia africae
etc.
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, Brill-Zinsser disease, Boutonneuse fever, Rocky Mountain spotted fever, and endemic 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.