Electromicrobiology: Emerging discipline
I started this article three years ago. Now the field has matured enough to complete the story. Yes, electricity from microorganisms! So it is called electro-microbiology, and I think it is destined to proliferate.
Electricity is the flow of electrons from high abundance to low abundance between two spots connected by a wire. The difference in electron pressure between these spots is called volts. In biology, most of the work such as heart beat, walk, talk or thought derives energy from the flow of electrons generated from oxidation of foodstuffs. These electrons sequentially jump or travel from one biological acceptor to another. Metal containing cytochromes, and several vitamin forms, differing by only few millivolts, act as biological electron acceptors. For humans and other oxygen-respiring organisms, the ultimate electron acceptor is oxygen to produce water and the biological form of energy called ATP.
Now scientists have identified wires produced by microorganisms through which electrons flow from one place to another.
Anaerobic bacteria strived on the earth, when there was no oxygen in the atmosphere, by transferring electrons to metals. Later, scientists found a clue to the role of bacteria in the corrosion and transformation of iron and steel. This led to the discovery of metal transforming Shewanella and Geobacter bacteria.
Recent nano-scale experiments have demonstrated that one of these bacterial cells can transmit electrons to another cell separated by many cell lengths! Scientists found that there are wires, multiples of them crisscrossing many cells. In the Shewanella bacteria long appendages called pili or nanowires are embedded lengthwise with flavins, a type of B2 vitamin, which are biological electron transporters. Its cell membranes are studded with iron containing cytochromes that can accept and donate electrons. So the electrons could travel long distance through the wires from one cell to another. Scientists also found that bacteria could deposit electrons to artificial electrodes!
Bacterial cells can transfer electrons in different ways too. Last year, an international effort (Nature 491:218 (2012)) demonstrated the existence of centimeter-long bacterial strings in marine sediments. Thousands of single bacteria were arranged end to end in a shared periplasmic sheath. The cell at the bottom could derive electrons from hydrogen sulfide from submerged thermal vents. The electrons could travel through the periplasmic space to the cell at the top that has access to oxygen. Oxidation of the electrons generates energy for survival of all the bacterial cells in the string.
It is the discovery that Shewanella and Geobacter and other bacteria can (1) grow in the absence of oxygen, (2) oxidise organic molecules to generate electrons, (3) reversibly deposit electrons to metals and electrodes, and (4) transport the generated electrons thousands of cell distances away allowed scientists to conceive the idea of making microbial circuits or living bioelectronic devices. Recent reviews (The Scientist 27(5): article #35299 (2013); Ann Rev Microbiol 66:391 (2012) by pioneers in the field discussed the potential use of microbes in making bio transistors, capacitors, environmental sensors, and wires for electronic devices.
While an in depth analysis of the electric properties of bacteria are being pursued by academic scientists (PNAS 109:10042), others are at work to harness a more economic and environmental benefit. Disposing of agricultural, domestic, human and animal waste has been an everlasting problem throughout human civilization. Discovery of bacterial reductive properties discussed above led scientists at the famous Craig Venter Institute and other universities to device microbial fuel cells that should convert sewer waste into electricity instead of generating wasteful foul gas and sludge in the conventional and expensive oxidation treatment plants.
The writer, a former Dhaka University teacher, is a biomedical scientist working in the USA.
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