da Rocha-Lima, 1916
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.
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. This bacteria lacks a flagella and is aerobic. It is stained gram-negative.
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. Since the two are so similar, they likely have a very similar evolutionary history. 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.
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.
- Henrique da Rocha Lima at Who Named It?
- Berg, Jeremy; Tymoczko, John; Stryer, Lubert (2007). Biochemistry, 6th ed. New York: W. H. Freeman and Company. p. 505. ISBN 978-0-7167-8724-2.
- Voet, Donald; Voet, Judith (2004). "21: Citric Acid Cycle". Biochemistry 3. Hoboken, New Jersey: John Wiley & Sons. p. 770. ISBN 978-0-471-19350-0. "The observation that the PDCs [pyruvate dehydrogenase complexes] of both eukaryotes and gram-positive bacteria have the dodecahedral form suggests that mitochondria are descended from gram-positive bacteria. Yet mitochondria are enclosed by two membranes as are gram-negative bacteria, whereas gram-positive bacteria have only one membrane. However, it has recently been demonstrated by genome sequence comparisons that mitochondria are, in fact, closely related to the obligate intracellular parasite Rickettsia prowazekii, the causative agent of the disease typhus. These bacteria are gram-negative but have the dodecahedral form of PDC."
- Cavalier-Smith, T. (1975). "The origin of nuclei and of eukaryotic cells". Nature 256 (5517): 463–8. doi:10.1038/256463a0. PMID 808732.
- Kurland, Charles G.; Andersson, Siv G. E.; Zomorodipour, Alireza; Andersson, Jan O.; Sicheritz-Pontén, Thomas; Alsmark, U. Cecilia M.; Podowski, Raf M.; Näslund, A. Kristina et al. (1998). "The genome sequence of Rickettsia prowazekii and the origin of mitochondria". Nature 396 (6707): 133–40. doi:10.1038/24094. PMID 9823893.
- "Rickettsia prowazekii::Taxon Overview". PATRIC. Virginia Bioinformatics Institute.
- NIAID Biodefense Research Agenda for Category B and C Priority Pathogens. National Institutes of Health, National Institute of Allergy and Infectious Diseases. January 2003.