Derek Lovley and colleagues of the University of Massachusetts at Amherst made their discovery in networks of “bacterial filaments”. These are also known as “microbial nanowires” because they conduct electrons along their length. These are produced naturally by some bacteria and are about 3-5 nm wide and up to tens of micrometres long. The filaments bind bacteria together into clumps called microbial biofilms.

Lovley's team looked at nanowires produced by the bacterium Geobacter sulfurreducens. They measured electrical conductivities in the wires of around 5 mS/cm, which is comparable to those of synthetic organic metallic nanostructures that are commonly used in the electronics industry. The wires were also seen to conduct over distances of centimetres, which is thousands of times the length of a bacterium itself.

Metallic first for biofilms

The researchers claim that this is the first time that metallic-like conductivity has been found in a biological material. Indeed, microbial biofilms are generally thought of as being electronic insulators.

Geobacter are anaerobic organisms that live in aquatic sediments and soils worldwide. They “breathe” by transferring electrons to iron oxides found in soil. This means that they could also be used to clean up groundwater contaminated with pollutants such as toxic and radioactive metals. “The bacteria use iron oxide in the same way that animals use oxygen,” explains team member Nikhil Malvankar. He says that what Geobacter does with its conducting nanowires is akin to a human breathing through a 10 km-long snorkel.

In the laboratory, Geobacter can grow on electrodes instead of on iron oxides and produce thick, electrically conductive biofilms. Lovely and colleagues took advantage of this fact for their experiments, in which they observed networks of nanowires spreading throughout the biofilm that was grown in a microbial fuel cell with acetate as the electron donor. This electron donor was modified so that the anode of the fuel cell – which acts as an electron acceptor to help the biofilm grow – was made up of two gold electrodes separated by a non-conducting gap of 50 μm.

Biological transistor

When a third electrode is then added to the system, the team discovered that the biofilm can act as a biological transistor that can be switched on and off by applying a voltage. “What's more, the conductivity of the biofilm can be tuned by simply changing the temperature – just like what happens in any metallic material,” says team member Mark Tuominen.

Using an atomic-force microscope with a conductive tip, the researchers observed that the current between the anode and the cathode increased as the biofilm grew on the electrodes. Confocal laser-scanning microscopy also showed that the cells formed a film that spread across the non-conducting gap. This bridge allowed the team to measure the conductivity of the biofilm.

“Long-range, metallic-like conductance along such protein filaments is a paradigm shift in biology that changes the way we think about how micro-organisms interact with their environment and each other,” says Lovley. “The structures can also interface with electronics, as we have shown.”

New energy-capture strategies

The findings could influence the design of energy-capture strategies, such as conversion of biomass and wastes to methane or electricity, he adds. Looking further into the future, the discovery could lead to the development of new electronic materials – either produced by the micro-organisms themselves or engineered based on insights gleaned from the biological materials.

“I believe that research dedicated to improving our fundamental understanding of the components and mechanisms involved in charge transfer along bacterial nanowires is important and could have broad-reaching academic and practical implications,” comments Yuri Gorby of the University of Southern California, who was not involved in the work. “However, it is essential that those of us involved in this research maintain the highest standards for generating quality data that will withstand the test of time.”

The Massachusetts team is now looking into the mechanisms behind the metallic-like conductivity. “One of our future strategies is to genetically modify the amino-acid composition of the filaments and determine how this subsequently affects the conductivity of the bacterial nanowires,” reveals Lovley.

The work was reported in Nature Nanotechnology doi:10.1038/nnano.2011.119