Researchers at the University of Antwerp, Belgium, made a breakthrough in understanding the inner design of the so-called cable bacteria – unique microorganisms capable of conducting electricity across distances up to several centimeters. The team’s work reveals the molecular structure of conductive filaments that turn those bacteria into “living wires”. This feature involves a mechanism totally different from that of other biologic conductors known so far: not proteins with iron cofactors as found in other electroactive microbes but nickel organic compounds that form an ordered metal-organic framework. The discovery not only explains how the bacteria conduct electric current with efficiency which is unusually high for biomaterials, but also reveals new prospects for developing biodegradable and flexible electronics of the future.
The researchers studied Candidatus Electrothrix gigas, a lab-cultivated bacterium shaped as long filaments of serially connected cells. The outer membrane of each such filament contains dozens of parallel fibers responsible for electron transfer over distances which exceed those typical of microorganisms. To uncover their inner morphology the team employed electronic and X-ray microscopy, various spectroscopy methods and computer modelling.
The study demonstrated that each of the electron-conducting fibers with a diameter of around 49 nm contains an average of intertwined nanoribbons some 1.4 nm thick. Inside them, nickel atoms are connected to sulphurous organic molecules. Those nickel organic units occur in a sequence along the fiber to form an extended conductive structure. Its composition is similar to that of synthetic nickel bis(dithiolene) polymers known to have high electrical conductivity, however such conductive structures have never been found in biologic systems.
According to the authors, it is precisely the design that explains the unusual conductivity of cable bacteria. Most of the known biologic systems use iron-containing nodes with electrons “floating” between them in sequence. In cable bacteria filaments, no significant accumulations of iron were found. Instead, electrons there can travel along the extended nickel organic structures making up the nanoribbons. In turn, several nanoribbons are packed into a single filament similar to a multiconductor cable. This type of structure ensures high conductivity while remaining flexible.
Electrical conductivity of individual cable bacteria filaments was previously estimated at 5 to 500 S/cm, with significant variation between filaments. In comparison, copper, a material commonly used to produce electric cables, features around 596 thousand S/cm. However, the previously cited figures for bacteria were estimated for the entire surface area of the filament while, according to the new model, electric current runs mostly through the much thinner nanoribbons inside it. When taking into account the nanoribbon surface area only, their estimated conductivity ranges between 150 and 15 thousand S/cm. The figure at the top of the range is about 40 times lower than that of copper, but such conductivity is exceptionally high for a biologic system. The research team places that measurement among the highest among all the known metal-organic structures.
The authors claim it to be the first known case of a metal-organic framework built by a living organism. The discovery may prompt some innovative approaches to creating bioelectronics materials: the bacteria effectively work to construct flexible structures capable of self-assembly which are both conductive and biodegradable. Understanding their molecular design may eventually help to artificially synthesize similar conductors and apply their natural principles in developing new electronic materials.
