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

Midichloria is a genus of Gram-negative, non spore-forming bacteria, with 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 the hard tick Ixodes ricinus. 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 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 seems to suggest a mutualistic association.


Only one species, Midichloria mitochondrii, is described in this genus.[1] 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.[2] Some poorly studied candidate species belonging to this family may include Nicolleia massiliensis and the unclassified Montezuma strain.[3]


The name of this bacterial genus, Midichloria (mi.di.chlo’ria. N.L. fem. n.), is derived from the midi-chlorians, a symbiotic, microscopic life form described in the fictional Star Wars universe.[1]


The genome of Midichloria mitochondrii has been sequenced by an international scientific consortium formed by researchers belonging to the University of Milan, the University of Sydney, the University of Valencia, the University of Pavia, and the University of Milan Bicocca.[4]

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 Midichloria mitochondrii contains the gene sets for the synthesis of the flagellum and of a cytochrome oxidase of the cbb3 kind.

Midichloria and the origin of mitochondria[edit]

The Rickettsiales are widely regarded as being the closest relatives to mitochondria. Based on the fact that the Midichloria genes for the flagellum and for the cbb3 cytochrome oxidase were proven to be ancestral, it was inferred that they were present in the bacterium that established the symbiosis with the ancestor of the eukaryotic cell to become the mitochondrion. The sequencing of the genome of Midichloria mitochondrii thus allowed an updated reconstruction of the free-living mitochondrial 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.

Phylogeny of Rickettsiales

Magnetococcus marinus


Rhodospirillales, Sphingomonadales, Rhodobacteraceae, Rhizobiales, etc.




subgroups Ib, II, IIIa, IIIb, IV and V











Robust 16S + 23S phylogeny of Rickettsidae from Ferla et al. (2013)[5]


  1. ^ a b 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. PMID 17082386. doi:10.1099/ijs.0.64386-0. 
  2. ^ 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. PMC 3685259Freely accessible. PMID 23503305. doi:10.1128/AEM.03971-12. 
  3. ^ 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. PMC 3859672Freely accessible. PMID 24349502. doi:10.1371/journal.pone.0083383. 
  4. ^ 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. PMID 21690562. doi:10.1093/molbev/msr159. 
  5. ^ 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. PMC 3859672Freely accessible. PMID 24349502. doi:10.1371/journal.pone.0083383. 
  • 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: 2596–2602. doi:10.1128/aem.70.5.2596-2602.2004. 
  • 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: 43–53. doi:10.1016/j.tice.2003.08.004. 
  • 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: 485–494. doi:10.1017/s0031182007004052.