Rickettsia may be considered the epitome of infectious, pathogenic bacteria High morbidity and mortality, environmental stability, low infectious dose, and persistence in infected hosts have contributed to Rickettsia prowazekii being weaponized as a biological warfare agent (Azad, 2007) Rickettsiae have also been implicated as the origin of the quintessential end product in the evolution of symbiosis and inheritance of bacteria, the mitochondria Rickettsia seemed to be characterized by two extreme phenotypic manifestations, with an absence of any representatives in between; rickettsiae as infectious pathogens and rickett-siae as inherited and obligate organelles of eukaryotic cells Only recently has the genomic sequencing of pathogenic Rickettsia allowed insight into the natural history of the pathogenic arm The discovery of Rickettsia that are obligatory symbionts of their insect hosts has demonstrated that Rickettsia do indeed span the whole spectrum of host-pathogen associations from symbionts without which a host cell cannot survive to pathogens that will kill their host cells readily The term symbiosis denotes here a long-term association that renders the symbiotic bacterium noninfectious for all practical purposes, but which in itself does not connote any beneficial or detrimental qualities

Rickettsia is a genus of obligate intracellular bacteria in the family Rickettsiaceae and the class a-Proteobacteria or, historically, the alpha group of the purple bacteria . They are Gramnegative and non-spore-forming. Infectious bacteria are often surrounded by a microcapsule composed of protein and a slime layer Following the release from phagosomes, rickettsiae grow freely mainly in the cytoplasm, dividing by binary fission Rickettsiae are not surrounded by a vacuolar membrane inside the cell Axenic cultures are not possible but many Rickettsia species can be propagated in chicken embryos, mammalian cell lines, and tissue cultures derived from ticks Only recently, insect cell lines have been available too Early on, the pleomorphic nature of Rickettsia had become a defining characteristic of the genus Morphological variations range from cocci and diplococci with a diameter of 0 1 ]m through rods to filament-like bacteria 10 ]m long. Many bacteria that did share pleomorphic features were originally lumped with the Rickettsia only to be separated later on (Kligler and Aschner, 1931; Philip, 1956; Weiss and Moulder, 1984; Dumler and Walker, 2005)

Rickettsiae do not have any flagella that might aid in mobility, but some Rickettsia species are far from nonmobile They cannot actively move outside a host cell, but within and between cells, Rickettsia, like other intracellular bacteria such as Listeria, Shigella, Mycobac-terium, and Burkholderia, can exploit the actin polymerization machinery of their hosts to induce actin-based motility (Carlsson and Brown, 2006; Stevens et al , 2006)

Together with its sister genus Orientia, Rickettsia constitute the family Rickettsiaceae Orientia tsutsugamushi causes scrub typhus in humans and is transmitted by an immature stage of blood-sucking mites with their saliva Rodents and mites are the natural reservoirs While being transmitted horizontally to mammalian hosts by the single larval stage (chig-gers), in adult mites O. tsutsugamushi manipulates the sex ratio of the next generation while being transmitted transovarially Formerly, Wolbachia, which are symbionts of insects, mites, isopods, spiders, scorpions, and nematodes, were included as a tribe within the Rickettsia-ceae Molecular data now place Wolbachia in the sister family Anaplasmaceae together with Aegyptianella, Anaplasma, Ehrlichia, Neoehrlichia, and Neorickettsia. The new genera Candidatus Xenohaliotis and Cand. Pelagibacter will end up in families of their own

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