Bacteria are prokaryotic microorganisms and as such they lack intracellular membranous organelles, including mitochondria. In fact, bacteria not only do not have mitochondria, but the mitochondria themselves seem to originate from bacteria that were integrated into eukaryotic cells through a phenomenon of endosymbiosis.

There are eukaryotic unicellular organisms, descendants of primitive eukaryotes, that do not have mitochondria, but all multicellular eukaryotes have mitochondria. In the case of prokaryotes, none have, unless known, and therefore no bacteria.

Among the many functions of the mitochondria, one of the most prominent in obtaining chemical energy through the process known as cellular respiration. Cellular respiration occurs in two distinct stages: metabolite oxidation (Krebs cycle and beta-oxidation of fatty acids) and the ATP synthesis by oxidative phosphorylation.

Metabolite oxidation takes place in the mitochondrial matrix and oxidative phosphorylation, dependent on the electron transport chainis produced in the inner membrane of the mitochondria:

In bacteria, energy is produced in a schematically very similar process. The bacterial cell membranes would take the place of the mitochondrial membranes. Proton secretion and ATP synthesis take place at the cytoplasmic membrane or bacterial inner membrane:

This similarity in the scheme for obtaining ATP is just one sign of what has been shown with studies of genetic material: the endosymbiotic origin of mitochondria.

Endosymbiosis theory

The theory of the endosymbiotic origin of mitochondria maintains that about two billion years ago, when the concentration of atmospheric oxygen was continuously increasing, bacteria capable of using this oxygen to oxidize organic matter and produce energy appeared.

These bacteria established a endosymbiotic relationship with primitive eukaryotic cells and they lived inside them sharing with them the energy they produced. In return, the host cell got food for the bacteria and gave it protection.

Over time, this association proved to be very effective and grew ever closer until the two organizations became one. the bacterium remained integrated as an organelle of the host cell and thus the mitochondria appeared.

Mitochondria contain their own genetic material separate from nuclear DNA, the mitochondrial DNA. Studies of mitochondrial DNA have linked it to certain bacterial groups. At first they were related to the rickettsiaea group of intracellular parasitic proteobacteria to which the typhus-causing bacteria (Rickettsia typhi Y Rickettsia prowazekii).

Subsequently, a greater similarity with the DNA of Pelagibacter locatea kind of alpha proteobacteria very abundant, probably the most abundant on the entire planet. In a 2011 study, however, they found a closer relative of mitochondria. They would be a rare group of bacteria that represent less than 1% of the bacteria in surface ocean waters. This group has been given the name of Ocean Mitochondrial Affiliate Clade (OMAC).