Bacterial hair-like extensions appear to be capable of conducting electricity down their length, possibly playing a key role in respiration by allowing the cells to dump electrons at distances far outside the cell.
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Gross L, PLoS Biology Vol. 4/8/2006, e282
The results, reported online today (11th October) in Proceedings of the National Academy of Sciences, add to a controversial body of literature about the function of these conductive pili, or "nanowires." "It is the first time in which [researchers] actually measure electron transport along the wires at micrometer distances, [which] make it a biologically relevant process," said microbiologist Gemma Reguera of Michigan State University, who was not involved in the research. "This suggests they could be relevant mode of respiration for bacteria."
"It's an incredibly important finding," agreed microbiologist Derek Lovley of the University of Massachusetts, who also did not participate in the study. "It's fascinating that these microorganisms can make electricity and can get electrons outside the cell."
Shewanella oneidensis MR-1 are among a class of bacteria that can generate energy using solids, such as metal oxides, as electron acceptors. Unlike oxygen, for example, which diffuses into cells to accept the electrons produced during respiration, these solids are found outside cells. These bacteria must thus find a way to transport their electrons to solid surfaces across the cell membrane. A number of strategies have been proposed for how bacteria can accomplish this. If the cells are in direct contact with the solids, electron transfer proteins on the cell membrane can transfer the electrons. Alternatively, small soluble molecules may act as chauffeurs, shuttling the electrons to their final destination.
Recently, a third mechanism of electron dumping has been proposed: Bacteria use nanowires to conduct the electrons to areas where the metal electron acceptors may be more abundant. Evidence that nanowires actually conduct electrons, or electricity, down their length has been lacking, however. To resolve this lingering question, biophysicist Moh El-Naggar of the University of Southern California and his colleagues grew S. oneidensis under conditions that promote the production of lots of nanowires, namely by limiting the number of available electron acceptors. They then rested platinum rods at each end of a nanowire and applied a voltage. Sure enough, the nanowire conducted the current. When the nanowires were snipped, the current stopped.
"It's the first demonstration that these bacterial nanowires are actually conductive," El-Naggar said. "The question is now, what are the implications for these bacterial nanowires in entire microbial communities?"
Until in vivo measurements can be made, it is impossible to know if the bacteria are using the nanowires as a mechanism for transporting electrons for respiration, El-Naggar cautioned. Unfortunately, the techniques available today are adopted from research on inorganic wires, which may impact any findings, he said. But when the group repeated the experiment using a different technique, they got the same results. "Our research indicates that bacteria produce nanowires that are capable of mediating electron transport over long distances."
The team also repeated the experiment using mutant bacteria that lacked two electron transfer proteins known as cytochromes, suspected to be important for conducting electricity. These mutants did not conduct a current. If it turns out these bacteria are indeed linking up into complex biological circuits, "the implications are huge," El-Naggar said. "If [the nanowires are] central to the functioning and to the survival of the community, it enables us to either try to optimize it or even disrupt it."
In the case of microbial fuel cells, for example, which produce electricity by oxidizing biofuels, understanding how these nanowires work could allow researchers to increase the efficiency of the process. Conversely, in the case of pathogenic biofilms, it could provide a target to try to disrupt bacterial function.
Another potential application of these nanowires is in bioremediation of toxic heavy metals, said chemical engineer Plamen Atanassov of the University of New Mexico, who was not involved in the research. "The hope is that bacteria like Shewanella with its ability to reduce metal oxides will be successfully deployed as a bioremediation agent."
But more research is needed before these applications are realized, said Reguera. "We first have to do these baby steps of characterizing the physical and biology properties of the wires themselves," she said, such as what they are made of. "And perhaps, once they know that, they may be able to mass produce them and explore applications in nanotechnology."
M.Y. El-Naggara, et al., "Electrical transport along bacterial nanowires from Shewanella oneidensis MR-1," PNAS, www.pnas.org/cgi/doi/10.1073/pnas.1004880107, 2010.