Further to our recent post regarding Reckettsia altering evolution of whitefly, we have received this excellent article on Rickettsia edited by Deskarati regular  Alan Mason –

Rickettsia is a genus of non-motile, Gram-negative, non-sporeforming, highly pleomorphic bacteria that can present as cocci (little berries, 0.1 μm in diameter), rods (1–4 μm long) or thread-like (10 μm long).

Specialised parasitic lifestyle – Obligate intracellular parasites, the Rickettsia survival depends 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.

A Puzzle to Classify – In the past they were positioned somewhere between viruses and true bacteria. The majority of Rickettsia bacteria are susceptible to antibiotics of the tetracycline group.   They are thought to be the closest living relatives to bacteria that were the origin of the mitochondria organelle that exists inside most eukaryotic cells.

Mitochondria provide the power for cells and today they appear to be a normal part of all cells. However, in the past they are believed to have arisen as originally independent bacteria that became incorporated into early cells that lacked these energy-creating mechanisms.  This is a particularly intimate form of symbiosis, where two originally separate organisms come together to each others’  benefit.  It is analysed in more detail under “Genomics”.

Agents for a Wide Range of Diseases with Exotic Names – The study of Rickettsia became important because some species cause quite serious and life-threatening human diseases. Rickettsia species are carried by many ticks, fleas, and lice, and cause diseases in humans such as typhus, rickettsial pox, Boutonneuse fever, African tick bite fever, Rocky Mountain spotted fever, Flinders Island spotted fever and Queensland tick typhus (Australian Tick Typhus).

They have also been associated with a range of plant diseases, notably, Beet latent Rosette, Citrus Greening bacterium, Clover leaf, Grapevine infectious necrosis, Grapevine Pierce’s, Grapevine yellos, Larch witch’s broom disease and Peach phony.

Classification – The classification of Rickettsia into three groups (spotted fever, typhus and scrub typhus) was based originally on serology, (the study of tissue fluids and agglutination or clumping of different sera when mixed together). This grouping has since been confirmed by DNA sequencing. All three of these contain human pathogens. The scrub typhus group has been reclassified as a new genus – Orientia – but many medical textbooks still list this group under the rickettsial diseases.

However more recently it has become apparent that rickettsia 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 it has been suggested that this 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 woman caused by Rickettsia helvetica previously thought to be harmless. Infection occurs in non-human mammals; for example, species of Rickettsia have been found to afflict the South American Guanaco, Lama guanacoe.

Genomics – Certain segments of Rickettsial genomes resemble that of mitochondria. The deciphered genome of R. prowazekii is 1,111,523 bp long and contains 834 protein-coding 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 rickettsia, 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 rickettsia 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.

NamingThe 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. Despite the similar name, Rickettsia bacteria do not cause rickets, which is a result of vitamin D deficiency.

(The original article from Wikipedia has been shortened, more paragraphs and headings have been used, and a few words of explanation added.  Interested readers should go to the original for greter detail and for  references.)



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