Microorganisms continue to surprise researchers by exhibiting new, and oftentimes peculiar physiologies. The recent discovery of bacterial ‘nanowires,’ or conductive extracellular appendages, in metal-reducing bacteria such as Shewanella and Geobacter spp. is further evidence of this concept. They really are quite incredible:
When discovered a couple years ago, nanowires were originally proposed to be an important part of how bacteria respire on solid-phase electron acceptors such as Fe- and Mn-oxide minerals (these bacteria have evolved to ‘breathe’ these metals in environments where there is no oxygen present). It is thought that by making nanowires, the bacteria can access minerals from incredibly far distances, increasing their chances for survival. Although not confirmed, it is proposed that nanowires are rich in conductive proteins called cytochromes that shuttle electrons down the wire in an energy cascade. Most of the work on nanowires has been done on single species in isolated cultures, so researchers still have a very limited understanding of the role nanowires might play in large anoxic bacterial communities.
A recent study showed that Shewanella oneidensis produces nanowires to access oxygen from deep within sediment if no other electron acceptors are within reach. Therefore, the subsurface may be ‘hardwired,’ i.e. nanowires could connect entire microbial communities—even across oxic-anoxic boundaries. What remains to be seen on this scale, however, is how nanowires are involved in the reduction of minerals or other metals. In fact, one study even asserts that nanowires are not unique to metal-reducing bacteria, but are widespread across the bacterial world including cyanobacteria and thermophilic bacteria, putting forth the notion that nanowires facilitate interspecies communication and electron transfer.
Interestingly, nearly every time a new paper comes out on bacterial nanowires, controversy between competing groups usually follows. The disagreements between camps (particularly those that study either Shewanella and Geobacter species) even go public (see this recent article in The Scientist for an example, and an older article in Science highlighting, among other things, the race to get their respective genomes sequenced). But regardless of these differences, I think that both sides can agree that the discovery of nanowires is exciting for microbiology. Not only are nanowires likely important in nature, but they hold the potential for applications across a wide range of technologies including microbial fuel cells and bioremediation.
Related papers:
D. Ntarlagiannis et al., 2007, Microbial nanowires: Is the subsurface “hardwired?” Geophys. Res. Lett. 34, L17305. DOI: 10.1029/2007GL030426
Y. A. Gorby et al., 2006, Electrically conductive bacterial nanowires produced by Shewanella oneidensis strain MR-1 and other microorganisms. Proceed. Natl. Acad. Sci. 103, 11358-11363. DOI: 10.1073/pnas.0604517103
G. Reguera et al., 2005, Extracellular electron transfer via microbial nanowires. Nature, 435, 1098-1101. DOI: 10.1038/nature03661
See also:
Bacteria may be wiring up the soil
