Rickettsia prowazekii

Rickettsia prowazekii is a species of gram negative, Alpha Proteobacteria, obligate intracellular parasitic, aerobic bacteria that is the etiologic agent of epidemic typhus, transmitted in the feces of lice. In North America, the main reservoir for R. prowazekii is the flying squirrel. R. prowazekii is often surrounded by a protein microcapsular layer and slime layer; the natural life cycle of the bacterium generally involves a vertebrate and an invertebrate host, usually an arthropod, typically the human body louse. A form of R.. prowazekii that exists in the feces of arthropods remains stably infective for months. R. prowazekii also appears to be the closest free-living relative of mitochondria, based on genome sequencing.

History

Discovery

Henrique da Rocha Lima, a Brazilian doctor, discovered this bacterium in 1916. He named it after his colleague Stanislaus von Prowazek, who had died from typhus in 1915. Both Prowazek and Rocha Lima had been infected with typhus while studying its causative agent in a prison hospital in Hamburg, Germany.^[1]

Evolutionary History

The genome of R. prowazekii provides great insight, especially to the origin of modern-day mitochondria. The R. prowazekii is not only the closest known relative to mitochondria through genome sequencing, as it also has a highly derived genome like that of mitochondria. The genome for this organism is more than 1 million base pairs in size and contains 834 protein-encoding genes. Sequence data suggest that all extant mitochondria are derived from an ancestor of R. prowazekii as the result of a single endosymbiotic event. The evidence that modern mitochondria result from a single event comes from examination of the most bacteria-like mitochondrial genome, that of the protozoan Reclinomonas americana. Its genome contains 97 genes, of which 62 specify proteins. The genes encoding these proteins include all of the protein-coding genes found in all of the sequenced mitochondrial genomes. Yet, this genome encodes less than 2% of the protein-coding genes in the bacterium E. coli. It seems unlikely that mitochondrial genomes resulting from several endosymbiotic events could have been independently reduced to the same set of genes found in R. americana. Note that transient engulfment of prokaryotic cells by larger cells is not uncommon in the microbial world. In the case of mitochondria, such a transient relation became permanent as the bacterial cell lost DNA, making it incapable of independent living, and the host cell became dependent on the ATP generated by its tenant.^[2] Since the two are so similar, they likely have a very similar evolutionary history.^[3] It is commonly believed that mitochondria evolved through endosymbiosis, and R. prowazekii could have evolved in a similar way. It is even possible for a bacterium similar to R. prowazekii to have been the initiator of endosymbiosis.^[4][5]

Treatment

Vaccines against R. prowazekii were developed in the 1940s, and were highly effective in reducing typhus deaths among U.S. soldiers during World War II. Immunity following recovery from infection with, or by immunization against, R. prowazekii is lifelong in most cases. However, R. prowazekii can establish a latent infection, which can reactivate after years or decades (referred to as Brill-Zinsser disease). Treatment with tetracycline antibiotics is usually successful.

References

External links

• "Rickettsia prowazekii::Taxon Overview". PATRIC. Virginia Bioinformatics Institute. http://patricbrc.org/portal/portal/patric/Taxon?cType=taxon&cId=782. 
• NIAID Biodefense Research Agenda for Category B and C Priority Pathogens. National Institutes of Health, National Institute of Allergy and Infectious Diseases. January 2003. http://www.niaid.nih.gov/topics/BiodefenseRelated/Biodefense/Documents/categorybandc.pdf. 

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