Pili Expression in Geobacter sulfurreducens Lacking the Putative Gene for the PilB Pilus Assembly Motor

Multiple lines of evidence suggest that electrically conductive pili (e-pili) are an important conduit for long-range electron transport in Geobacter sulfurreducens, a common model microbe for the study of extracellular electron transport mechanisms. One strategy to study the function of e-pili has been to delete the gene for PilB, the pilus assembly motor protein, in order to prevent e-pili expression. However, we found that e-pili are still expressed after the gene for PilB is deleted. Conducting probe atomic force microscopy revealed filaments with the same diameter and similar current-voltage response as e-pili harvested from wild-type G. sulfurreducens or when e-pili are heterologously expressed from the G. sulfurreducens pilin gene in E. coli. Immunogold labeling demonstrated that a G. sulfurreducens strain expressing e-pili with a His-tag continued to express His-tag labelled e-pili when the PilB gene was deleted. Strains with the PilB gene deleted produced maximum current densities comparable to wild-type controls. These results demonstrate that deleting the gene for PilB is not an appropriate strategy for constructing strains of G. sulfurreducens without e-pili, necessitating a reinterpretation of the results of previous studies that have employed this approach.

Extracellular electron transfer increases fermentation in lactic acid bacteria via a hybrid metabolism

Energy conservation in microorganisms is classically categorized into respiration and fermentation, however recent work shows some species can use mixed or alternative bioenergetic strategies. We explored the utility of a flavin-based extracellular electron transport (FLEET) system for energy conservation within diverse lactic acid bacteria (LAB), microorganisms that mainly rely on fermentative metabolism and are important in food fermentations. The LAB Lactiplantibacillus plantarum uses extracellular electron transfer to increase its NAD+/NADH ratio, generate more ATP through substrate-level phosphorylation and accumulate biomass more rapidly. This novel, hybrid metabolism was dependent on a type-II NADH dehydrogenase (Ndh2) and conditionally required a flavin-binding extracellular lipoprotein (PplA) in the FLEET system to confer increased fermentation yield, metabolic flux, and environmental acidification in both laboratory media and food fermentation. The discovery of a single pathway that blends features of fermentation and respiration expands our knowledge of energy conservation metabolism and provides immediate biotechnology applications.

Screening Potential of Electrogenic Algae for Bio Power Production from Aquatic Habitats of Tamil Nadu

This paper illustrates the potential ofmicroalgae in electricity production.The present study focuses on searching for, indigenous microalgal strains from various aquatic sources exhibiting electrogenic activity. Sixteen microalgal strains were obtained from the diverse water bodies and assessed for electron transfer ability between cell and electrode. Six algal strains were screened out of eighteen for potential electrogenicity, based on cyclic voltammetry assay. In contrast to other algal strains, the cyclic voltammograms of Hindakia sp. isolated from paddy fields displayed an oxidation peak under anaerobic conditions in the potential range of +100 to +200 mV, while no distinct peaks were observed in other strains. This research broadened the exoelectrogen and identified model microalgae for investigating the extracellular electron transport process.

Cytochrome OmcS is not essential for long-range electron transport in Geobacter sulfurreducens strain KN400

The multi-heme c-type cytochrome OmcS, is one of the central components for extracellular electron transport in Geobacter sulfurreducens strain DL-1, but its role in other microbes, including other strains of G. sulfurreducens is currently a matter of debate. Therefore, we investigated the function of OmcS in G. sulfurreducens strain KN400, which is even more effective in extracellular electron transfer than strain DL-1. We found that deleting omcS from strain KN400 did not negatively impact the rate of Fe(III) oxide reduction and did not affect the strain’s ability to accept electrons via direct interspecies electron transfer. The OmcS-deficient strain also continued to produce conductive filaments, consistent with the concept that electrically conductive pili are the primary conduit for long-range electron transfer in G. sulfurreducens and closely related species. These findings, coupled with the lack of OmcS homologs in most other microbes capable of extracellular electron transfer, suggest that OmcS is not a common critical component for extracellular electron transfer.

Basicity of N5 in semiquinone enhances the rate of respiratory electron outflow inShewanella oneidensisMR-1

Extracellular electron transport (EET) occurs in environmental iron-reducing bacteria and is mediated by an outer membrane multi-heme cytochrome complex (Cyts). It has critical implications for global mineral cycling and electrochemical microbial catalysis. The rate of EET mediated by multiple heme redox centers significantly increases in the presence of flavins and quinones. Their electron free energy does not entirely account for the fact that differential effects on EET rate enhancement vary significantly by factors ≥100. Here, we report on whole-cell electrochemical analysis ofShewanella oneidensisMR-1 using six flavin analogs and four quinones. We demonstrated that protonation of the nitrogen atom at position 5 (N5) of the isoalloxazine ring is essential for electron outflow acceleration as a bound non-covalent cofactor of Cyts. EET mediated by Cyts was accelerated at a rate dependent on pKa(N5). The EET rate largely decreased in response to the addition of deuterated water (D2O), while low concentration of D2O (4 %) had little impact on electron free energy difference of the heme and non-covalent bound cofactors, strongly suggesting that the protonation of N5 limits the rate of EET. Our findings directly link EET kinetics to proton transport reaction via N5 and provide a basis for the development of novel strategies for controlling EET-associated biological reactions.Significance statementThe potential of various small molecules such as flavins and quinones to enhance the rate of extracellular electron transport (EET) has been exploited to develop environmental energy conversion systems. Flavins and quinones have similar molecular structures but their abilities to enhance EET vary by >100× inShewanella oneidensisMR-1. These large differences are inconsistent with conventional models, which rely on redox potentials or diffusion constant of shuttling electron mediators. In this study, we demonstrated that the basicity of the nitrogen atom of the isoalloxazine ring (N5) enhances the rate of electron outflow when a flavin or quinone is a non-covalent cofactor ofS. oneidensisMR-1 outer membranec-type cytochromes.