Microbial electrochemical techniques describe a variety of emerging technologies that use electrode–bacteria interactions for biotechnology applications including the production of electricity, waste and wastewater treatment, bioremediation and the production of valuable products. Central in each application is the ability of the microbial catalyst to interact with external electron acceptors and/or donors and its metabolic properties that enable the combination of electron transport and carbon metabolism. And here also lies the key challenge. A wide range of microbes has been discovered to be able to exchange electrons with solid surfaces or mediators but only a few have been studied in depth. Especially electron transfer mechanisms from cathodes towards the microbial organism are poorly understood but are essential for many applications such as microbial electrosynthesis. We analyze the different electron transport chains that nature offers for organisms such as metal respiring bacteria and acetogens, but also standard biotechnological organisms currently used in bio-production. Special focus lies on the essential connection of redox and energy metabolism, which is often ignored when studying bioelectrochemical systems. The possibility of extracellular electron exchange at different points in each organism is discussed regarding required redox potentials and effect on cellular redox and energy levels. Key compounds such as electron carriers (e.g., cytochromes, ferredoxin, quinones, flavins) are identified and analyzed regarding their possible role in electrode–microbe interactions. This work summarizes our current knowledge on electron transport processes and uses a theoretical approach to predict the impact of different modes of transfer on the energy metabolism. As such it adds an important piece of fundamental understanding of microbial electron transport possibilities to the research community and will help to optimize and advance bioelectrochemical techniques.
CO 2 reduction by combined electro-and bio-catalytic reactions is a promising technology platform for sustainable production of chemicals from CO 2 and electricity. While heterogeneous electrocatalysts can reduce CO 2 to a variety of organic compounds at relatively high reaction rates, these catalysts have limitations achieving high selectivity for any single product beyond CO. Conversely, microbial CO 2 reduction pathways proceed at high selectivity; however, the rates at bio-cathodes using direct electron supply via electricity are commonly limiting. Here we demonstrate the use of non-precious metal cathodes that produce hydrogen in situ to support microbial CO 2 reduction to C 1 and C 2 compounds. CoP, MoS 2 and NiMo cathodes perform durable hydrogen evolution under biologically relevant conditions, and the integrated system achieves coulombic efficiencies close to 100% without accumulating hydrogen. Moreover, the one-reactor hybrid platform is successfully used for efficient acetate production from electricity and CO 2 by microbes previously reported to be inactive in bioelectrochemical systems.
Amphiphilic polycations are an alternative to biocides but also toxic to mammalian cells. Antimicrobially active hydrophilic polycations based on 1,4-dibromo-2-butene and tetramethyl-1,3-propanediamine named PBI are not hemotoxic for porcine red blood cells with a hemocytotoxicity (HC50) of more than 40,000 μg · mL(-1). They are quickly killing bacterial cells at their MIC (minimal inhibitory concentration). The highest found selectivity HC50 /MIC is more than 20,000 for S. epidermidis. Investigations on sequentially prepared PBIs with defined molecular weight Mn and tailored end groups revealed that there is a dependence of antimicrobial activity and selectivity on Mn and nature of the end groups.
BackgroundMicrobial electrosynthesis is a novel approach that aims at shifting the cellular metabolism towards electron-dense target products by extracellular electron supply. Many organisms including several acetogenic bacteria have been shown to be able to consume electrical current. However, suitable hosts for relevant industrial processes are yet to be discovered, and major knowledge gaps about the underlying fundamental processes still remain.ResultsIn this paper, we present the first report of electron uptake by the Gram-positive, ethanol-producing acetogen, Clostridium autoethanogenum. Under heterotrophic conditions, extracellular electron supply induced a significant metabolic shift away from acetate. In electrically enhanced fermentations on fructose, acetate production was cut by more than half, while production of lactate and 2,3-butanediol increased by 35-fold and threefold, respectively. The use of mediators with different redox potential revealed a direct dependency of the metabolic effect on the redox potential at which electrons are supplied. Only electrons delivered at a redox potential low enough to reduce ferredoxin caused the reported effect.ConclusionsProduction in acetogenic organisms is usually challenged by cellular energy limitations if the target product does not lead to a net energy gain as in the case of acetate. The presented results demonstrate a significant shift of carbon fluxes away from acetate towards the products, lactate and 2,3-butanediol, induced by small electricity input (~0.09 mol of electrons per mol of substrate). This presents a simple and attractive method to optimize acetogenic fermentations for production of chemicals and fuels using electrochemical techniques. The relationship between metabolic shift and redox potential of electron feed gives an indication of possible electron-transfer mechanisms and helps to prioritize further research efforts.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-016-0663-2) contains supplementary material, which is available to authorized users.
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