MR-1 are described as a form of conductive pilus associated with the membrane. Also, the

lipids colocalized on these extracellular structures aid in performing a multistep redox

hopping mechanism that contributes to electron transfer (Figure 11.4).

Recently, Subramanian et al. [32], used electron cryotomography microscopy to de­

termine the ultrastructure of the nanowire produced by the S. oneidensis strain. In the outer

membrane and periplasm of the strain, EET was observed on the microbial nanowires. In

this study, the authors used light and electrons which revealed that these structures on

S. oneidensis were curved and an extension tabulation form was observed. Moreover, less

c-cytochrome in mutant strain compared to the wild type strain was observed. The peri­

plasm and outer membrane proteins were consistent with cytochromes [32]. The result

obtained revealed that S. oneidensis MR-1 nanowires were outer membrane vesicles with

variable lengths [33].

11.5 Geobacter and Shewanella EET Mechanism

11.5.1 The Hopping Mechanism by S. Oneidensis Strain MR-1 Nanowires

Several mechanisms are used by the bacteria in electron transfer including DET, mediated

electron transfer, and through nanowires, as represented in Figure 11.3. For the S. onei­

densis strain MR-1 nanowires, the electron transfer by extracellular nanowires occurred

through the hopping mechanism. The hopping mechanism is a transferred electron that

can be determined between two sites in a solid specimen from one molecule to another

with the acquisition of energy (Figure 11.5) [23].

This concept has been described in connection with ionic conduction in amorphous

non-metallic solids and after it has been extended to electrons. The hopping transition can

be determined by both the distance between the two sites and the potential. If the po­

tential barrier width is larger than 10 Å, it causes the electrons to hop rather than tunnel

from one molecule to the neighboring one. In fact, the hopping process is similar to the

atomic diffusion process. However, in hopping, there is no electron transfer until the

thermal motion of nuclei permits electron motion over the barrier by rearrangement of

the molecule. In the electron hopping mechanism, it was suggested that the outer surface

cytochromes were aligned along the filament, which enabled sufficient electronic cou­

pling. To form the microbial nanowires, the S. oneidensis MR-1 strain requires cyto­

chromes called MtrC and OmcA, which can be involved in electron transfer [4,34]. The

composition analysis of the bacterial nanowires by electron microscopy imaging of

Shewanella S. oneidensis MR-1 nanowires showed that the outer membrane extensions

contained components rather than pilin-based structures, which include cytochromes that

improve the electron hopping pattern. These multiheme cytochromes of the MtrC and

OmcA are localized on the outer membranes. They can be associated with the nanowires

of Shewanella and are mediators for electron transfer.

11.5.2 Tunneling Mechanism

Tunneling is a mechanical phenomenon in which the excited state electron can tunnel to

the neighboring molecule in one of the multiple consecutive steps by exchanging energy

through the tunneling process. In the tunneling effect, an electron moves through the

Microbial Nanowires

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