Microbial nanowires – a biological, electrically conductive fibre produced by many species of bacteria¹ – are commonly used by electrogens to deliver electrons to each other and to inorganic surfaces, but the mechanism by which they conduct electricity is poorly understood. This is partily due to the nature of the nanowires themselves, whose structure is notoriously difficult to model. What is known that the wires, long assumed to be modified type IV pili, can grow up to 20 microns long and have c-cytochromes spaced along their length, which are essential for their operation. There are two major competing models for the transfer mechanism, known as the metal-like conductivity (MLC) hypothesis and the superexchange conductivity (SEC) hypothesis.

Metal-like conductivity

The MLC hypothesis holds that the c-cytochromes on the surface of the pilus are not fundamental to the conductivity of the nanowire, but are merely used to transfer electrons from the nanowire to surfaces and to other bacteria. Instead, this model claims that thepilusis conductive, and conductive in a metal-like fashion, due to overlapping pi orbitals in aromatic residues stacked within the wire. Nanowires are conductive even when dried and separated from their bacteria, which supports the notion of this pi-pi stacking over a model in which the c-cytochromes need to be bilogically functional. Perhaps the most convincing evidence of this metal-like conductivity was found in April 2011, when Malvankar et. al. demonstrated that nanowire conductivity is tunable, much like metal wires.²  Specifically, Malvankar et. al. noted that the conductivity of nanowires can be controlled not only by regulating gene expression, as would be expected in a biological material, but also by varying voltage and, perhaps most important of all, by varying temperature, with the nanowire response to both acting very similar to that of metal wires. It is, however, worth noting that these experiments were performed on whole conductive biofilms, not isolated nanowires, and as yet there has been no attempt to determine if other components within the biofilm might affect conductivity.

Fig 1: Malvankar et. al.’s setup for testing the tunable conductivity of nanowire-containing biofilms.²

 

Superexchange conductivity

The SEC hypothesis maintains that the c-cytochromes on the surface of the type IV pili acting as nanowires behave much how c-cytochromes are expected to behave; that they accept electrons from cytochromes downwire and pass them on. Thus, electrons ‘hop’ up or down the wire, transferred between cytochromes on its surface. This hypothesis has the advantage of being similar to other things often observed in nature, unlike the MLC hypothesis. With the exception of tunable conductivity experiments (which were done on complete biofilms), much of the work supporting the MLC model is with wires that have been removed from their natural environment, and proponents of the SEC model note that there is no reason to assume that a nanowire capable of deomstrating behaviour when purified, dried and mounted on a slide  necessarily exhibits that behaviour when attached to a living cell. For years, the major point against the SEC hypothesis was that in models of the type IV pilus used as a nanowire, the c-cytochromes on the surface are spaced too far apart to allow for electron ‘hopping’. This was solved in October 2013 by Reardon and Meuller, who recognised that scientists had been making the mistake of working with single nanowires.³ When several nanowires were modelled together as a bundle, it became clear that the cytochromes on their surfaces lined up in a spiral around the bundle, and were easily close enough together to allow electron hopping down the length of the bundle.

Fig 2: Reardon and Meuller’s nanowire model, in which c-cytochromes of neighbouring nanowires spiral around the nanowire bundle. ³

A fundamental revolution

Superexchange or metal-like conductivity? The debate rages on. However, a new paper by Pirbadian et. al. released recently in PNAS may help put the matter to rest. By taking the first in vivo video of nanowires actually forming in the commonly studied electrogen Shewenella oneidensis, Pirbadian et. al. have demonstrated that a fundamental assumption underlying the entire field of study of electron transfer in nanowires may be false – nanowires may not be type IV pili at all.⁴ Instead, they appear to be extrusions of the outer membrane of the cells, with cytochromes heavily localised at the points of extrusion. This paper is very strong support for the SEC model, but if nanowires are indeed outer membrane extensions instead of modified type IV pili, this so fundamentally changes the existing models of nanowire structure that all evidence to date on the topic of nanowire conductivity will need to be reexamined in the light of this new information.

Fig 3: in-vitro observation of nanowire formation in Shewanella oneidensis under different flourescent stains.⁴ The supplementary materials for this article also contain several time-lapse videos of this process.

 

Fig 4: Proposed periplasmic extension nanowire model.⁴

 

Article written by Jacqueline Rochow, Supervisor Professor Jim Mitchell.

References

1. Reguera, G., McCarthy, K.D., Mehta, T., Nicoll, J.S., Tuominen, M.T., and Lovley, D.R., ‘Extracellular electron transfer via microbial nanowires’, Nature, 435, 1098-1101 (2005)

2. Malvankar, N.S., Vargas, M., Nevin, K.P., Franks, A.E., Leang, C., Kim, B., Inoue, K., Mester, T., Covalla, S.F., Johnson, J.P., Rotello, V.M., Tuominen, M.T., and Lovley, D.R., ‘Tunable metal-like conductivity in microbial nanowire networks’, Nature Nanotechnology, 6, 573-579 (2011)

3. Reardon, P.N., and Mueller, K.T., ‘Structure of the type IVa major pilin from the electrically conductive bacterial nanowires of Geobacter sulfurreducens‘, Journal of Biological Chemistry, 288(41), 29260-29266 (2013)

4. Pirbadian, S., Barchinger, S.E., Leung, K.M., Byun, H.S., Jangir, Y., Bouhenni, R.A., Reed, S.B., Romine, M.F., Saffarini, D.A., Shi, L., Gorby, Y.A., Golback, J.H. and El-Naggar, M.Y., ‘Shewanella oneidensis MR-1 nanowires are outer protein and periplasmic extensions of extracellular electron transport components’, Proceedings of the National Academy of Sciences, 111(35), 12883-12888 (2014)