Nanowires common in bacteria?

Microbes may use electrically conductive nanowires to help transport electrons

By | July 11, 2006

The practice of sprouting electrically conductive nanowires from the cell for electron transfer could be common across bacteria, not just those that reduce metal, scientists reported this week in Proceedings of the National Academy of Sciences (PNAS) . These findings, which appear to contradict a finding from an earlier study, could have broad implications for how microbes living in communities and biofilms distribute energy, affecting both ecology and human health, according to a study author. "It's not yet certain how far-reaching these structures are, but as a strategy to transfer electrons in a community structure we could investigate whether or not [nanowires] are found in all kinds of biofilms, from marine sediments to ones in Yellowstone to biofilms of pathogens, like in cystic fibrosis or tuberculosis," coauthor Yuri Gorby at Pacific Northwest National Laboratory in Richland, Wash., told The Scientist. As bacteria generate energy in the form of ATP, they must rid themselves of electrons. In 2005, Derek Lovley and his colleagues reported metal-reducing bacteria such as Geobacter sulfurreducens produce electrically conductive nanowires that apparently can help transfer electrons beyond the cell. Lovley's team found nanowires in one other metal-reducing bacterium (Shewanella oneidensis) and one non-metal-reducing bacterium (Pseudomonas aeruginosa), but they appeared nonconductive. There are instances when it makes sense for non-metal-reducing bacteria to produce conductive nanowires, Gordy and his team reasoned. For instance, cyanobacteria join carbon dioxide with electrons generated during photosynthesis to create organic compounds, and these bacteria might need to get rid of extra electrons when cultivated under limited carbon dioxide. Previous studies may have missed nanowires because bacteria in biofilms are surrounded by masses of matter that may have obscured the picture, Gorby said. Using scanning tunneling microscopy (STM) and tunneling spectroscopy, the researchers found the photosynthetic cyanobacterium Synechocystis, when grown under limited carbon dioxide, sprouted nanowires tens of microns long in bundles 50 to 150 nanometers in diameter that were highly electrically conductive. "It's very impressive work. The microscopy demonstrates very well the existence and function of these nanowires," James Tiedje at Michigan State University, not a coauthor, told The Scientist. The researchers also observed highly electrically conductive wires 10 to 20 nanometers across that were produced by the thermophilic fermentative bacterium Pelotomaculum thermopropionicum. The fact that two different types of bacteria generate nanowires "suggests nanowires may be broadly distributed across many groups," Gorby said. The ridged nanowire bundles Synechocystis produced bore a striking resemblance to electrically conductive appendages observed in the metal-reducing bacteria Shewanella oneidensis. Although Lovley and his team said the Shewanella wires were not electrically conductive, the wires are very fragile, Gorby noted, which could have muddied measurements of their conductivity. Lovley, however, suggested that each team might be talking about different nanowires from Shewanella. "Theirs are 30 to 50 times larger in diameter," he told The Scientist. Gorby and his colleagues also found that Shewanella mutants lacking genes for two electron transport proteins known as cytochromes displayed poorly conductive nanowires, as did mutants lacking a functional Type II secretion pathway, which helps transport cytochromes to their proper places in the cell. However, this mutant data does not present a convincing argument that cytochromes are involved in conductivity, Lovley noted. He said the authors should also conduct genetic studies to better identify the composition of these structures, as well as complementation studies that reintroduce wild-type variants of genes linked to nanowires into mutants lacking those genes, to see if function was restored. Daniel Bond at the University of Minnesota collaborates with both Lovley and Gorby, but did not participate in either study. He agreed that "there are a lot of strong feelings about this paper," and some researchers deem some of the evidence "circumstantial rather than confirmatory." Still, the STM imaging is convincing, he added, and the findings make sense. "Proteins are only capable of electron transfer over only short distances, and to get a bulky cell and bulky protein within nanometers or angstroms of a surface is hard, but to make a flexible appendage that can reach out solves a lot of problems," he said. Charles Q. Choi Links within this article Y.A. Gorby et al. "Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms." PNAS, published online July 10, 2006. N. Johnston. "Debaffling biofilms." The Scientist, August 2, 2004. Yuri Gorby G. Reguera et al. "Extracellular electron transfer via microbial nanowires," Nature, June 23, 2005. PM_ID: 15973408. D. Lovley. "Taming electricigens." The Scientist, July 1, 2006. J. Lucentini, "Living batteries," The Scientist, July 1, 2006. James Tiedje Daniel Bond


Avatar of: KJK Portland

KJK Portland

Posts: 1

August 10, 2006

What these articles are referring to as "nanowires" are in most cases type IV pili - which have already been recognized as important in biofilm formation, twitching motility, and some pathogenesis-related functions (in particular in Pseudomonas aeruginosa and some other pathogenic and non-pathogenic bacteria). Therefore, the discovery is not that these structures exist - but that they may have another function (ie. electrical conductivity) that has not been previously recognized.\n\nI think it would be better to call these structures by their established names - such as "Type IV Pili" - instead of renaming them with the trendy "nanowire". This would be especially appropriate with the structures that have not yet been shown to be electrically conductive. In addition to it being more accurate it would prevent confusion in the scientific community.\n\n

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