Midichloria

Midichloria is a genus of Gram-negative, nonspore-forming bacteria, with a bacillus shape around 0.45 µm in diameter and 1.2 µm in length. First described in 2004 with the temporary name IricES1, Midichloria species are symbionts of several species of hard ticks (e.g., Ixodes ricinus and Ixodes uriae of the Ixodidae family).[1] They live in the cells of the ovary of the females of this tick species. These bacteria have been observed in the mitochondria of the host cells, a trait that has never been described in any other symbiont of animals.

Midichloria
Scientific classification
Domain:	Bacteria
Phylum:	Proteobacteria
Class:	Alphaproteobacteria
Subclass:	Rickettsidae
Order:	Rickettsiales
Family:	Midichloriaceae
Genus:	Midichloria

Midichloria bacteria seem to consume the mitochondria they parasitize, possibly using them as a source of energy and/or molecules to multiply. The interaction of these symbionts with their host is currently unknown, though the 100% prevalence in the females of the host tick suggests a mutualistic association.[2] Although there is no evidence that even up to 20 bacteria residing within the host cell mitochondria produce deleterious effects in Rhipicephalus bursi, the prevalence in R.bursi was estimated at 33% in females and 14% in males, suggesting that the relationship may be more complicated than previously thought.[1] In I.ricinus, between 10-20 bacteria are known to produce deleterious effects, leading to the conclusion that Midichloria consume mitochondria.[3]

Species

Only one species, Midichloria mitochondrii, is described in this genus.[4] Molecular screenings, however, have detected the presence of related bacteria in other tick species, as well as in other blood-sucking arthropods, suggesting the possibility of horizontal transmission of these bacteria.

It was given its own family, the Midichloriaceae, in the Rickettsiales.[5] Some poorly studied candidate species belonging to this family may include Nicolleia massiliensis and the unclassified Montezuma strain.[6]

Naming

The name of this bacterial genus, Midichloria, is derived from the fictional midichlorians, a symbiotic, microscopic life form described in the Star Wars stories.[4]

Genome

The genome of M. mitochondrii has been sequenced by an international scientific consortium formed by researchers at the University of Milan, the University of Sydney, the University of Valencia, the University of Pavia, and the University of Milan Bicocca.[7]

The genome is 1.2 Mb, and it is, for most characteristics, very similar to the genomes of the other Rickettsiales, with two notable exceptions; the genome of M. mitochondrii contains the gene sets for the synthesis of the flagellum and of a cytochrome cbb3 oxidase.

Midichloria and the origin of mitochondria

Schematic ribosomal RNA phylogeny of Alphaproteobacteria

Magnetococcidae     Magnetococcus marinus       Caulobacteridae       Rhodospirillales, Sphingomonadales,   Rhodobacteraceae, Rhizobiales, etc.       Holosporales       Rickettsidae     Pelagibacterales     Pelagibacteraceae       Pelagibacter         Subgroups Ib, II, IIIa, IIIb, IV and V       Rickettsiales       Proto-mitochondria       Anaplasmataceae           Ehrlichia       Anaplasma         Wolbachia         Neorickettsia         Midichloriaceae       Midichloria       Rickettsiaceae       Rickettsia       Orientia

The cladogram of Rickettsidae has been inferred by Ferla et al. [8] from the comparison of 16S + 23S ribosomal RNA sequences.

The Rickettsiales are widely believed to be the closest relatives to mitochondria. Based on the fact that the Midichlorian genes for the flagellum and for the cbb3 cytochrome oxidase were proven to be ancestral, the genes were inferred to have been present in the bacterium that established the symbiosis with the ancestor of the eukaryotic cell, that became the mitochondrion.

Hence, sequencing the genome of M. mitochondrii allowed an improved reconstruction of the mitochondrias' hypothetical free-living ancestor: It was a motile bacterium able to survive in microaerophilic conditions. Both these characteristics may have played an important role in the beginning of the symbiosis between the eukaryotic cell and the mitochondrion.

References

1. Epis, S.; Sassera, D.; Beninati, T.; Lo, N.; Beati, L.; Piesman, J.; Rinaldi, L.; McCOY, K. D.; Torina, A.; Sacchi, L.; Clementi, E. (April 2008). "Midichloria mitochondrii is widespread in hard ticks (Ixodidae) and resides in the mitochondria of phylogenetically diverse species". Parasitology. 135 (4): 485–494. doi:10.1017/S0031182007004052. ISSN 1469-8161.
2. Lo, N.; Beninati, T.; Sassera, D.; Bouman, E. a. P.; Santagati, S.; Gern, L.; Sambri, V.; Masuzawa, T.; Gray, J. S.; Jaenson, T. G. T.; Bouattour, A. (2006). "Widespread distribution and high prevalence of an alpha-proteobacterial symbiont in the tick Ixodes ricinus". Environmental Microbiology. 8 (7): 1280–1287. doi:10.1111/j.1462-2920.2006.01024.x. ISSN 1462-2920.
3. Sacchi, L; Bigliardi, E; Corona, S; Beninati, T; Lo, N; Franceschi, A (2004-02-01). "A symbiont of the tick Ixodes ricinus invades and consumes mitochondria in a mode similar to that of the parasitic bacterium Bdellovibrio bacteriovorus". Tissue and Cell. 36 (1): 43–53. doi:10.1016/j.tice.2003.08.004. ISSN 0040-8166.
4. Sassera, D; Beninati, T; Bandi, C; Bouman, EA; Sacchi, L; Fabbi, M; Lo, N (November 2006). "'Candidatus Midichloria mitochondrii', an endosymbiont of the tick Ixodes ricinus with a unique intramitochondrial lifestyle". International Journal of Systematic and Evolutionary Microbiology. 56 (Pt 11): 2535–40. doi:10.1099/ijs.0.64386-0. PMID 17082386.
5. Montagna, M; Sassera, D; Epis, S; Bazzocchi, C; Vannini, C; Lo, N; Sacchi, L; Fukatsu, T; Petroni, G; Bandi, C (2013). ""Candidatus Midichloriaceae" fam. Nov. (Rickettsiales), an ecologically widespread clade of intracellular alphaproteobacteria". Applied and Environmental Microbiology. 79 (10): 3241–8. doi:10.1128/AEM.03971-12. PMC 3685259. PMID 23503305.
6. Ferla, M. P.; Thrash, J. C.; Giovannoni, S. J.; Patrick, W. M. (2013). "New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability". PLoS ONE. 8 (12): e83383. doi:10.1371/journal.pone.0083383. PMC 3859672. PMID 24349502.
7. Sassera, D.; Lo, N.; Epis, S.; D'Auria, G.; Montagna, M.; Comandatore, F.; Horner, D.; Pereto, J.; Luciano, A. M.; Franciosi, F.; Ferri, E.; Crotti, E.; Bazzocchi, C.; Daffonchio, D.; Sacchi, L.; Moya, A.; Latorre, A.; Bandi, C. (20 June 2011). "Phylogenomic Evidence for the Presence of a Flagellum and cbb3 Oxidase in the Free-Living Mitochondrial Ancestor". Molecular Biology and Evolution. 28 (12): 3285–3296. doi:10.1093/molbev/msr159. PMID 21690562.
8. Ferla MP, Thrash JC, Giovannoni SJ, Patrick WM (2013). "New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability". PLOS One. 8 (12): e83383. doi:10.1371/journal.pone.0083383. PMC 3859672. PMID 24349502.

• Beninati, T.; Lo, N.; Sacchi, L.; Genchi, C.; Noda, H.; Bandi, C. (2004). "A novel alpha-proteobacterium resides in the mitochondria of ovarian cells of the tick Ixodes ricinus". Appl. Environ. Microbiol. 70 (5): 2596–2602. doi:10.1128/aem.70.5.2596-2602.2004. PMC 404433. PMID 15128508.
• Sacchi, L.; Bigliardi, E.; Corona, S.; Beninati, T.; Lo, N.; Franceschi, A. (2004). "A symbiont of the tick Ixodes ricinus invades and consumes mitochondria in a mode similar to that of the parasitic bacterium Bdellovibrio bacteriovorus". Tissue Cell. 36 (1): 43–53. doi:10.1016/j.tice.2003.08.004. PMID 14729452.
• Lo N, Beninati T, Sacchi L, Bandi C (2006b). An alpha-proteobacterium invades the mitochondria of the tick Ixodes ricinus. In Insect Symbiosis II, edited by K. Bourtzis and T. Miller, CRC Press Boca Raton.
• Epis, S; Sassera, D; Beninati, T; Lo, N; Beati, L; Piesman, J; Rinaldi, L; McCoy, KD; Torina, A; Sacchi, L; Clementi, E; Genchi, M; Magnino, S; Bandi, C (2008). "Midichloria mitochondrii is widespread in hard ticks (Ixodidae) and resides in the mitochondria of phylogenetically diverse species". Parasitology. 135 (4): 485–494. doi:10.1017/s0031182007004052.