Nanowires common in bacteria?
Microbes may use electrically conductive nanowires to help transport electrons
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.
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.
G. Reguera et al. "Extracellular electron transfer via microbial nanowires," Nature
, June 23, 2005.
D. Lovley. "Taming electricigens." The Scientist
, July 1, 2006.
J. Lucentini, "Living batteries," The Scientist
, July 1, 2006.