Some bacteria utilize electrically conductive protrusions to transfer electrons for various purposes. These are known as bacterial nanowires.
Purpose of electron transfer
Respiration is the production of energy, which bacteria have found numerous and diverse ways of doing. Many microorganisms take part in extra-cellular respiration, which involves electron transfer to or from a substance outside of the cell.
This is common in bacteria that have evolved to take energy from metals such as iron or manganese, and do so by donation of electrons in order to reduce a metal compound and absorb it in a more soluble, less toxic, or more easily metabolised state.
Some bacteria that are able to photosynthesise (cyanobacteria) also possess nanowires. They are used to transfer electrons from the cell under intense light conditions, changing the electrical potential of the bacteria and protecting them from taking in too much energy at once and becoming damaged.
Adjacent bacteria are also able to pass electrons between one another, even between differing species, as occurs between Geobacter metallireducens and Geobacter sulfurreducens.
In some circumstances, this may constitute a symbiotic relationship, where cyanobacteria provide excess electrons to other species to maintain the microbial mat in which they survive.
Electron transfer is also sometimes used as a signalling mechanism and can contribute to the construction of a biofilm shared by neighbouring bacteria.
Types of Bacterial Nanowire
Bacteria have developed a diverse range of nanowires in different forms, of which there are three main types.
Pili are hair-like structures that protrude from the surface of a bacterium that are used commonly for the functions of adhesion, movement and biofilm formation.
Some species of bacteria, including Geobacter sulfurreducens and Acidithiobacillus ferroxidans, both of which take part in extra-cellular respiration, and Synechocystis sp., which is a freshwater cyanobacteria, utilize their pili for electron transfer.
Flagella are long, thin, whip like appendages that extend from the interior to the exterior of a cell body. They are used primarily for locomotion and as a sensory organ. Similarly to pili, several species of bacteria have evolved to use their flagellum as nanowires.
Some bacteria use extended membrane protrusions coated with proteins to engage in electron transfer, such as Shewanella oneidensis, which uses the appendage to reduce metal ions.
Many such bacteria also possess pili or flagella that may be involved or separately capable of taking part in electron transfer, and the reasons behind the evolution of a separate membrane protrusion are yet to be completely elucidated.
Mechanisms of electron transfer
Several mechanisms of electron transfer through pili, flagella or membrane protrusions have been hypothesized, but the specific method in most species of bacteria has not yet been identified.
One such mechanism is a metal-like conductivity, where overlapping π orbitals between atoms allow electrons to be shared freely and flow between orbitals.
Geobacter that take part in electron transfer through their pili possess a densely packed chain of aromatic compounds through the center of the pili, allowing for the movement of electrons through it.
Electron hopping is a second method of electron transfer that often utilizes iron-containing heme proteins.
Hopping occurs when electrons in an excited state move to a nearby electron-hole in an adjacent atom, leaving an electron-hole in the orbital which they left. This can occur repeatedly, allowing for the movement of electrons down a chain of molecules.
Applications of Bacterial Nanowires
Circulating tumor cells (CTC) break away from primary tumous and help discern the type of cancer a patient has. Recent developments in nanowire technology have demonstrated the ability of nanowires attached to substrates by standard photolithography and chemical wet etching to capture and release CTCs in order to analyse them.
Many industrial and commercial applications exist for bacteria possessing nanowires. The concept of microbial fuel cells, which would allow electrons to transfer through the biofilm has been proposed, similarly to the creation of bioelectronics that would use nanowire possessing bacteria to transfer current, even when underwater.
Additional applications in the remediation of waste products such as arsenic, chromium and even uranium exist thanks to the bacteria’s ability to break down these metals through electron transfer.