The discovery of microRNAs encapsulated in milk-derived exosomes has revealed stability under extreme conditions reflecting the protection of membranes. We attempted to determine the variations in nanoparticles derived from milk after fermentation, and provide evidence to determine the effects of these exosomes on cells with potential bioactivity. Using scanning electron microscopy and dynamic light scattering, we compared the morphology and particle size distribution of exosomes from yogurt fermented with three different combinations of strains with those from raw milk. The protein content of the exosome was significantly reduced in fermented milk. The cycle threshold showed that the expression of miR-29b and miR-21 was relatively high in raw milk, indicating a loss of microRNA after fermentation. Milk-derived exosomes could promote cell growth and activate the mitogen-activated protein kinase pathway. These findings demonstrated biological functions in milk exosomes and provided new insight into the nutrient composition of dairy products.

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Efficient extracellular electron transfer (EET) of exoelectrogens is essentially for practical applications of versatile bioelectrochemical systems. Intracellular electrons flow from NADH to extracellular electron acceptors via EET pathways. However, it was yet established how the manipulation of intracellular NADH impacted the EET efficiency. Strengthening NADH regeneration from NAD, as a feasible approach for cofactor engineering, has been used in regulating the intracellular NADH pool and the redox state (NADH/NAD ratio) of cells. Herein, we first adopted a modular metabolic engineering strategy to engineer and drive the metabolic flux toward the enhancement of intracellular NADH regeneration. We systematically studied 16 genes related to the NAD-dependent oxidation reactions for strengthening NADH regeneration in the four metabolic modules of S. oneidensis MR-1, i.e., glycolysis, C1 metabolism, pyruvate fermentation, and tricarboxylic acid cycle. Among them, three endogenous genes mostly responsible for increasing NADH regeneration were identified, namely gapA2 encoding a NAD-dependent glyceraldehyde-3-phosphate dehydrogenase in the glycolysis module, mdh encoding a NAD-dependent malate dehydrogenase in the TCA cycle, and pflB encoding a pyruvate-formate lyase that converted pyruvate to formate in the pyruvate fermentation module. An exogenous gene fdh* from Candida boidinii encoding a NAD-dependent formate dehydrogenase to increase NADH regeneration in the pyruvate fermentation module was further identified. Upon assembling these four genes in S. oneidensis MR-1, ∼4.3-fold increase in NADH/NAD ratio, and ∼1.2-fold increase in intracellular NADH pool were obtained under anaerobic conditions without discharge, which elicited ∼3.0-fold increase in the maximum power output in microbial fuel cells, from 26.2 ± 2.8 (wild-type) to 105.8 ± 4.1 mW/m (recombinant S. oneidensis), suggesting a boost in the EET efficiency. This modular engineering method in controlling the intracellular reducing equivalents would be a general approach in tuning the EET efficiency of exoelectrogens.

BackgroundThe microbial fuel cell (MFC) is a green and sustainable technology for electricity energy harvest from biomass, in which exoelectrogens use metabolism and extracellular electron transfer pathways for the conversion of chemical energy into electricity. However, Shewanella oneidensis MR-1, one of the most well-known exoelectrogens, could not use xylose (a key pentose derived from hydrolysis of lignocellulosic biomass) for cell growth and power generation, which limited greatly its practical applications.ResultsHerein, to enable S. oneidensis to directly utilize xylose as the sole carbon source for bioelectricity production in MFCs, we used synthetic biology strategies to successfully construct four genetically engineered S. oneidensis (namely XE, GE, XS, and GS) by assembling one of the xylose transporters (from Candida intermedia and Clostridium acetobutylicum) with one of intracellular xylose metabolic pathways (the isomerase pathway from Escherichia coli and the oxidoreductase pathway from Scheffersomyces stipites), respectively. We found that among these engineered S. oneidensis strains, the strain GS (i.e. harbouring Gxf1 gene encoding the xylose facilitator from C. intermedi, and XYL1, XYL2, and XKS1 genes encoding the xylose oxidoreductase pathway from S. stipites) was able to generate the highest power density, enabling a maximum electricity power density of 2.1 ± 0.1 mW/m2.ConclusionTo the best of our knowledge, this was the first report on the rationally designed Shewanella that could use xylose as the sole carbon source and electron donor to produce electricity. The synthetic biology strategies developed in this study could be further extended to rationally engineer other exoelectrogens for lignocellulosic biomass utilization to generate electricity power.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-017-0881-2) contains supplementary material, which is available to authorized users.

1,4-pentanediol or monohydric alcohol were achieved with high selectivity from levulinic acid under relatively mild conditions using nanoporous Ru.