Rickettsia
Rickettsia is a genus of nonmotile, gram-negative, nonspore-forming, highly pleomorphic bacteria that may occur in the forms of cocci (0.1 μm in diameter), bacilli (1–4 μm long), or threads (up to about 10 μm long). The term "rickettsia" has nothing to do with rickets (which is a deficiency disease resulting from lack of vitamin D); the bacterial genus "Rickettsia" instead was named after Howard Taylor Ricketts, in honor of his pioneering work on tick-borne spotted fever. Properly, "Rickettsia" is the name of a single genus, but the informal term "rickettsia", plural "rickettsias", usually not capitalised, commonly  applies to any members of the order Rickettsiales. Being obligate intracellular parasites, rickettsias depend on entry, growth, and replication within the cytoplasm of living eukaryotic host cells (typically endothelial cells). Accordingly, "Rickettsia" species cannot grow in artificial nutrient culture; they must be grown either in tissue or embryo cultures; typically, chicken embryos are used, following a method developed by Ernest William Goodpasture and his colleagues at Vanderbilt University in the early 1930s. "Rickettsia" species are transmitted by numerous types of arthropod, including chigger, ticks, fleas, and lice, and are associated with both human and plant diseases. Most notably, "Rickettsia  species are the pathogens responsible for typhus, rickettsialpox, boutonneuse fever, African tick-bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever, and Queensland tick typhus (Australian tick typhus). The majority of Rickettsia" bacteria are susceptible to antibiotics of the tetracycline group. Classification. The classification of "Rickettsia" into three groups (spotted fever, typhus, and scrub typhus) was initially based on serology. This grouping has since been confirmed by DNA sequencing. All three of these groups include human pathogens. The scrub typhus group has been reclassified as a related new genus, "Orientia", but they still are in the order Rickettsiales and  accordingly still are grouped with the rest of the rickettsial diseases. Rickettsias are more widespread than previously believed and are known to be associated with arthropods, leeches, and protists. Divisions have also been identified in the spotted fever group and this group likely should be divided into two clades. Arthropod-inhabiting rickettsiae are generally associated with reproductive manipulation (such as parthenogenesis) to persist in host lineage In March 2010, Swedish researchers reported a case of bacterial meningitis in a woman caused by "Rickettsia helvetica" previously thought to be harmless. Flora and fauna pathogenesis. Plant diseases have been associated with these  Rickettsia-like organisms (RLOs): Infection occurs in nonhuman mammals; for example, species of "Rickettsia" have been found to afflict the South American guanaco, "Lama guanacoe". Pathophysiology. Rickettsial organisms are obligate intracellular parasites and invade vascular endothelial cells in target organs, damaging them and producing increased vascular permeability with consequent oedema, hypotension, and hypoalbuminaemia. Genomics. Certain segments of rickettsial genomes resemble those of mitochondria. The deciphered genome of "R. prowazekii" is 1,111,523 bp long and contains 834 genes. Unlike free-living bacteria, it contains no genes for anaerobic glycolysis or genes involved in the biosynthesis and regulation of amino acids and nucleosides . In this regard, it is similar to mitochondrial genomes; in both cases, nuclear (host) resources are used. ATP production in "Rickettsia" is the same as that in mitochondria. In fact, of all the microbes known, the "Rickettsia" is probably the closest relative (in a phylogenetic sense) to the mitochondria. Unlike the latter, the genome of "R. prowazekii", however, contains a complete set of genes encoding for the tricarboxylic acid cycle and the respiratory chain complex. Still, the genomes of the "Rickettsia", as well as the mitochondria, are frequently said to be "small, highly derived products of several types of reductive  evolution". The recent discovery of another parallel between "Rickettsia" and viruses may become a basis for fighting HIV infection. Human immune response to the scrub typhus pathogen, "Orientia tsutsugamushi", appears to provide a beneficial effect against HIV infection progress, negatively influencing the virus replication process. A probable reason for this actively studied phenomenon is a certain degree of homology between the rickettsiae and the virus, namely, common epitope(s) due to common genome fragment(s) in both pathogens. Surprisingly, the other infection reported to be likely to provide the same effect (decrease in viral load) is the virus-caused illness dengue fever . Comparative analysis of genomic sequences have also identified five conserved signature indels in important proteins, which are uniquely found in members of the genus "Rickettsia". These indels consist of a four-amino-acid insertion in transcription repair coupling factor Mfd, a 10-amino-acid insertion in ribosomal protein L19, a one-amino-acid insertion in FtsZ, a one-amino-acid insertion in major sigma factor 70, and a one-amino-acid deletion in exonuclease VII. These indels are all characteristic of the genus and serve as molecular markers for "Rickettsia". Bacterial small RNAs play critical roles in virulence and stress /adaptation responses. Although their specific functions have not been discovered in "Rickettsia", few studies showed the expression of novel sRNA in human microvascular endothelial cells (HMEC) infected with "Rickettsia". Genomes of intracellular or parasitic bacteria undergo massive reduction compared to their free-living relatives. Examples include Rickettsia for alpha proteobacteria, T. whipplei for Actinobacteria, Mycoplasma for Firmicutes (the low G+C content Gram-positive), and Wigglesworthia and Buchnera for gamma proteobacteria. Naming. The genus "Rickettsia" is named after Howard Taylor Ricketts (1871–1910), who studied Rocky Mountain spotted fever in the Bitterroot Valley of Montana, and eventually died of typhus after studying  that disease in Mexico City.