Microbial Mitopathogens
Mitochondria are structures found in eukaryotic cells which generate most of the cell’s supply of adenosine triphosphate (ATP), used as a source of chemical energy. They have been described as “cellular power plants”. One of the key differences between eukaryotes and prokaryotes is the numerous subcellular compartments present in eukaryotic cells. In addition to the nucleus, eukaryotes possess other sub-cellular organelles such as mitochondria, and chloroplasts in plants. These organelles look very much like prokaryotes, and it is generally believed that they were originally derived from endosymbiotic prokaryotes more than a billion years ago. Mitochondria possess their own DNA (mtRNA), ribosomes, structural and functional proteins and energy generation system. mtDNA is circular, just like bacterial DNA, and mitochondrial ribosomes more closely resemble bacterial 70S ribosomes than the eukaryotic 80S ribosomes found in the cytoplasm. The mitochondrial genetic code is more similar to the bacterial genetic code than the eukaryotic chromosomal code.
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A recent paper considers the possible existence of various kinds of mitochondrion-specific microbial pathogens, i.e., pathogenic micro-organisms that may damage or destroy the mitochondria from within (Microbial mito-pathogens: Fact or fiction? Medical Hypotheses 2008 70: 1051 1053). This is highly topical since it follows the recent description of a virus pathogen of Mimivirus, a “virophage” (to parallel “bacteriophage”, the name for a virus that infects bacteria) called Sputnik (The virophage as a unique parasite of the giant mimivirus. Nature 2008 Aug 6). Sputnik does not replicate by itself and relies on a virus host in order to reproduce. Sputnik-infected virus particles are abnormal, with thicker capsules, and they produce fewer progeny. Sputnik has a circular chromosome of 18,000 bases containing 21 protein-coding genes. Among these are three that appear to have been kidnapped from the host virus, as well as others that seem to have been transferred from viruses that infect bacteria or archaea. Viruses were known to pick up genes from host organisms, but the discovery of Sputnik is the first time that horizontal transfer between viruses has been observed.
If mitochondria are evolutionarily related to bacteria, it is plausible that they could be harmed or destroyed by bacteriophages, and the consequences of that could be very bad.
The first mitochondrial-specific viruses (mitoviruses) reported occur in mitochondria of the fungi that cause chestnut blight and Dutch elm disease. These mitoviruses are naked dsRNA-viruses that belong to the family Narnaviridae. Since all the conditions for virus replication seem to be present in mitochondria, it is quite possible that the occurrence of mitoviruses is a phenomenon with a broader range than just fungi.
Viroids are structurally and functionally different from viruses, small circular RNAs able to replicate autonomously (without a helper virus) in the nuclei or chloroplasts of plants. To date, viroids are only known in plants, but it is conceivable that mitoviroids could occur in mitochondria.
Bdellovibrio bacteriovorans is an example of a parasitic intra-bacterial bacterium, i.e., the bacterial counterpart of a bacteriophage, that infects Gram-negative bacteria such as Escherichia coli. Since mitochondria have a double membrane system similar to that of Gram-negative bacteria, it is again possible that there could be mitobacteria similar to Bdellovibrio or chlamydiae.
Although mito-pathogens have not yet been demonstrated in humans, their theoretical degenerative effect regarding energy production might explain diseases that have not yet been understood, including prion diseases and post-traumatic muscle dystrophy. Therefore, these kinds of micro-organisms should be kept in mind for the future.
Tags: Bacteria, Biology, Fungi, Microbiology, Podcast, Science, Virology
I wish that the “Medical Hypothesis” reference was open access. I cannot obtain a copy through my institution!