Microbes as "Live Wires"

via The Scientist
Mohamed Y. El-Naggar and Steven E. Finkel, May 1, 2013

Excerpt below.  Read the complete article here.

Scientists discovered the first metal-reducing bacteria, Shewanella and Geobacter, in the late 1980s. The dissimilatory metal-reducing bacteria (DMRB) metabolism, which couples biological electron transport chains to inorganic materials, gives us a unique opportunity to both study and harness such reduction-oxidation (redox) reactions at synthetic surfaces. In fact, if a synthetic electrode is poised at a favorable redox potential, it is possible to “trick” the metal-reducing bacteria into transferring their electrons to the electrode surface in the absence of any other electron acceptor. This not only provides a quantitative readout to study respiration in real time, it gives researchers precise control of the energetic redox conditions, thereby allowing them to direct the growth of the microbes, and even to culture some bacteria that may be difficult to grow in standard media.

These bacteria are being heavily investigated as practical biological catalysts in renewable energy technologies that now attract millions of dollars annually in government and industry funding. Microbial fuel cells and bacterial batteries, for example, are constructed with microbes that oxidize diverse organic fuels—including waste products such as raw sewage—then route the resulting electrons to fuel-cell anodes, where the flow is converted into electricity. Another emerging technology is microbial electrosynthesis, which essentially runs the process in reverse by supplying microbes with renewable (e.g., solar) electrical energy in order to drive reductive microbial metabolisms for the synthesis of biofuels and other high-value chemicals. Both these technologies—fuel-to-electricity and electricity-to-fuel—rely on the ability of microbes to donate and accept electrons at synthetic surfaces.

This notion of electron transport driving information flow and communication in microbial communities is new, and as yet untested, but it has potentially transformative physiological and technological implications. Compared to the relatively slow diffusion of entire molecules, electron flow is a rapid process, allowing cells to more quickly sense and respond to environmental change. Such an electronic signaling network, in addition to regulating cell-cell interactions on the population level, could even form the backbone of new synthetic microbial networks designed as sensors to detect specific environmental conditions, such as harmful or desirable chemicals, or variations in light or pH. Eventually, researchers may even learn to interface these networks with solid-state microelectronics, using the extracellular electron transport pathways of metal-reducers such as Shewanella to perform functions from bioremediation to energy production. This vision of integrated microbial circuits was unimaginable 10 years ago. But as we unravel the molecular and biophysical basis of long-distance electron transport, these bacteria may one day become essential components of everyday technologies. 

Mohamed Y. El-Naggar is an assistant professor of physics and Steven E. Finkel is an associate professor of biological sciences at the University of Southern California.