Geobacter sulfurreducens bacteria grow on biofilms and have the particular ability of using polarized electrodes as the final electron acceptor of their respiratory chain. In these biofilms, electrons are transported through distances of more than 50 μm before reaching the electrode. The way in which electrons are transported across the biofilm matrix through such large distances remains under intense discussion. None of the two mechanisms proposed for explaining the process, electron hopping through outer membrane cytochromes and metallic like conduction through conductive PilA filaments, can account for all the experimental evidence collected so far. Aiming at providing new elements for understanding the basis for electron transport, in this perspective article we present a modelled structure of Geobacter pilus. Its analysis in combination with already existing experimental evidence gives support to the proposal of the "stepping stone" mechanism, in which the combined action of pili and cytochromes allows long range electron transport through the biofilm.

Summary During chemotactic signaling by Escherichia coli, the small cytoplasmic CheW protein couples the histidine kinase CheA to chemoreceptor control. Although essential for assembly and operation of receptor signaling complexes, CheW in stoichiometric excess disrupts chemotactic behavior. To explore the mechanism of the CheW excess effect, we measured the physiological consequences of high cellular levels of wild-type CheW and of several CheW variants with reduced or enhanced binding affinities for receptor molecules. We found that high levels of CheW interfered with trimer assembly, prevented CheA activation, blocked cluster formation, disrupted chemotactic ability, and elevated receptor methylation levels. The severity of these effects paralleled the receptor binding affinities of the CheW variants. Because trimer formation may be an obligate step in the assembly of ternary signaling complexes and higher-order receptor arrays, we suggest that all CheW excess effects stem from disruption of trimer assembly. We propose that the CheW-binding sites in receptor dimers overlap their trimer contact sites and that high levels of CheW saturate the receptor binding sites, preventing trimer assembly. The CheW-trapped receptor dimers seem to be improved substrates for methyltransferase reactions, but cannot activate CheA or assemble into clusters, processes that are essential for chemotactic signaling.

5Whereas most of the studies conducted nowadays to boost electrode performance in bioelectrochemical systems (BES) are focused on carbonaceous scaffolds, in this study we demonstrate that ice-templated titanium-based ceramics (ITTC) can provide a new alternative for this purpose. We combined the chemistry of titanium suboxides (Ti 4 O 7 ) with an ice-templating technique (ISISA) to produce electrically conducting and highly porous (88% porosity) 3D architectures. The ITTC platforms were characterized by strongly aligned macrochannels that provided a direct path for substrate supply under a flow-through configuration, while supporting the growth of 10 electroactive Geobacter sulfurreducens biofilms. This new electrode material is demonstrated to outperform graphite when used as an anode in bioelectrochemical reactors, providing volumetric current densities of 9500 A.m -3 , equating to projected current densities of 128.7 A.m -2 and maximum power densities of 1.9 kW.m -3 . The performance of the ITTC scaffolds levels that of any of the available materials on the current state of research. The presented alternative may lead to the start of a branch into the exploration of conducting ITTC materials in the growing area of bioelectrochemical technologies. 15 65