Respiratory glycerol metabolism of Actinobacillus succinogenes 130Z for succinate production

Metabolic engineering of Mannheimia succiniciproducens for malic acid production using dimethylsulfoxide as an electron acceptor

Microbial production of various TCA intermediates and related chemicals through the reductive TCA cycle has been of great interest. However, rumen bacteria that naturally possess strong reductive TCA cycle have been rarely studied to produce these chemicals, except for succinic acid, due to their dependence on fumarate reduction to transport electrons for ATP synthesis. In this study, malic acid (MA), a dicarboxylic acid of industrial importance, was selected as a target chemical for mass production using Mannheimia succiniciproducens, a rumen bacterium possessing a strong reductive branch of the TCA cycle. The metabolic pathway was reconstructed by eliminating fumarase to prevent MA conversion to fumarate. The respiration system of M. succiniciproducens was reconstructed by introducing the Actinobacillus succinogenes dimethylsulfoxide (DMSO) reductase to improve cell growth using DMSO as an electron acceptor. Also, the cell membrane was engineered by employing Pseudomonas aeruginosa cis-trans isomerase to enhance MA tolerance. High inoculum fed-batch fermentation of the final engineered strain produced 61 g/L of MA with an overall productivity of 2.27 g/L/h, which is the highest MA productivity reported to date. The systems metabolic engineering strategies reported in this study will be useful for developing anaerobic bioprocesses for the production of various industrially important chemicals.

A staged representation electrochemical stimulated strategy to regulate intracellular reducing power for improving succinate production by Escherichia coli AFP111

BACKGROUND

Escherichia coli AFP111 was previously engineered for succinate production by eliminating byproducts of synthesis pathways. Still, the succinate yield is limited due to the insufficient NADH supplement, when fed with glucose. Microbial electrolysis cell (MEC) allows microorganisms to perform unbalanced fermentation by establishing polarized cathode interaction.

METHODS AND RESULTS

In this study, a cathode electrode was used as an additional electron donor to improve succinate synthesis by E. coli AFP111. In MEC with -0.65 V (vs. Ag/AgCl) poised on cathode electrode, 95.72% electrons were transferred into cells via neutral red (NR), and the ratio of NADH/NAD⁺ increased by 2.5-fold. Meanwhile, compared with the control experiment, the value of oxidation-reduction potential (ORP) changed from -240 to -265 mV in MEC, which was beneficial for NADH generation. During two-stage fermentation (no potential growth stage followed by electric stimulation) in MEC, succinate yield was increased by 29.09% (the final yield was 0.71 g g^-1 ), and glucose consumption rate was enhanced by 36.22%. In addition, the carbon flux was pumped to succinate and pyruvate metabolism was enhanced.

CONCLUSION AND IMPLICATIONS

Staged representation of electrochemical stimulated strategy is effective for succinate producing in engineered E. coli by regulating intracellular reducing power, which provides a new concept for producing reduced metabolite in unbalanced fermentation.

Microbial fuel cell-assisted utilization of glycerol for succinate production by mutant of Actinobacillus succinogenes

BACKGROUND

The global production of glycerol is increasing year by year since the demands of biodiesel is rising. It is benefit for high-yield succinate synthesis due to its high reducing property. A. succinogenes, a succinate-producing candidate, cannot grow on glycerol anaerobically, as it needs a terminal electron acceptor to maintain the balance of intracellular NADH and NAD⁺. Microbial fuel cell (MFC) has been widely used to release extra intracellular electrons. However, A. succinogenes is a non-electroactive strain which need the support of electron shuttle in MFC, and pervious research showed that acid-tolerant A. succinogenes has higher content of unsaturated fatty acids, which may be beneficial for the transmembrane transport of lipophilic electron shuttle.

RESULTS

MFC-assisted succinate production was evaluated using neutral red as an electron shuttle to recover the glycerol utilization. First, an acid-tolerant mutant JF1315 was selected by atmospheric and room temperature plasma (ARTP) mutagenesis aiming to improve transmembrane transport of neutral red (NR). Additionally, MFC was established to increase the ratio of oxidized NR to reduced NR. By combining these two strategies, ability of JF1315 for glycerol utilization was significantly enhanced, and 23.92 g/L succinate was accumulated with a yield of 0.88 g/g from around 30 g/L initial glycerol, along with an output voltage above 300 mV.

CONCLUSIONS

A novel MFC-assisted system was established to improve glycerol utilization by A. succinogenes for succinate and electricity production, making this system as a platform for chemicals production and electrical supply simultaneously.

Co-cultivation of Thermoanaerobacter strains with a methanogenic partner enhances glycerol conversion

Glycerol-rich waste streams produced by the biodiesel, bioethanol and oleochemical industries can be treated and valorized by anaerobic microbial communities to produce methane. As current knowledge of the microorganisms involved in thermophilic glycerol conversion to methane is scarce, thermophilic glycerol-degrading methanogenic communities were enriched. A co-culture of Thermoanaerobacter and Methanothermobacter species was obtained, pointing to a non-obligately syntrophic glycerol degradation. This hypothesis was further studied by incubating Thermoanaerobacter brockii subsp. finnii and T. wiegelii with glycerol (10 mM) in pure culture and with different hydrogenotrophic methanogens. The presence of the methanogen accelerated glycerol fermentation by the two Thermoanaerobacter strains up to 3.3 mM day-1 , corresponding to 12 times higher volumetric glycerol depletion rates in the methanogenic co-cultures than in the pure bacterial cultures. The catabolic pathways of glycerol conversion were identified by genome analysis of the two Thermoanaerobacter strains. NADH and reduced ferredoxin formed in the pathway are linked to proton reduction, which becomes thermodynamically favourable when the hydrogen partial pressure is kept low by the hydrogenotrophic methanogenic partner.

Opportunities, challenges, and future perspectives of succinic acid production by Actinobacillus succinogenes

Due to environmental issues and the depletion of fossil-based resources, ecofriendly sustainable biomass-based chemical production has been given more attention recently. Succinic acid (SA) is one of the top value added bio-based chemicals. It can be synthesized through microbial fermentation using various waste steam bioresources. Production of chemicals from waste streams has dual function as it alleviates environmental concerns; they could have caused because of their improper disposal and transform them into valuable products. To date, Actinobacillus succinogenes is termed as the best natural SA producer. However, few reviews regarding SA production by A. succinogenes were reported. Herewith, pathways and metabolic engineering strategies, biomass pretreatment and utilization, and process optimization related with SA fermentation by A. succinogenes were discussed in detail. In general, this review covered vital information including merits, achievements, progresses, challenges, and future perspectives in SA production using A. succinogenes. Therefore, it is believed that this review will provide platform to understand the potential of the strain and tackle existing hurdles so as to develop superior strain for industrial applications. It will also be used as a baseline for identification, isolation, and improvement of other SA-producing microbes.

Reconstruction of a genome-scale metabolic model for Actinobacillus succinogenes 130Z

BACKGROUND

Actinobacillus succinogenes is a promising bacterial catalyst for the bioproduction of succinic acid from low-cost raw materials. In this work, a genome-scale metabolic model was reconstructed and used to assess the metabolic capabilities of this microorganism under producing conditions.

RESULTS

The model, iBP722, was reconstructed based on the functional reannotation of the complete genome sequence of A. succinogenes 130Z and manual inspection of metabolic pathways, covering 1072 enzymatic reactions associated with 722 metabolic genes that involve 713 metabolites. The highly curated model was effective in capturing the growth of A. succinogenes on various carbon sources, as well as the SA production under various growth conditions with fair agreement between experimental and predicted data. Calculated flux distributions under different conditions show that a number of metabolic pathways are affected by the activity of some metabolic enzymes at key nodes in metabolism, including the transport mechanism of carbon sources and the ability to fix carbon dioxide.

CONCLUSIONS

The established genome-scale metabolic model can be used for model-driven strain design and medium alteration to improve succinic acid yields.

Long-term adaptation of Escherichia coli to methanogenic co-culture enhanced succinate production from crude glycerol

Escherichia coli can hardly grow anaerobically on glycerol without exogenous electron acceptor. The formate-consuming methanogen Methanobacterium formicicum plays a role as a living electron acceptor in glycerol fermentation of E. coli. Wild-type and mutant E. coli strains were screened for succinate production using glycerol in a co-culture with M. formicicum. Subsequently, E. coli was adapted to glycerol fermentation over 39 rounds (273 days) by successive co-culture with M. formicicum. The adapted E. coli (19.9 mM) produced twice as much succinate as non-adapted E. coli (9.7 mM) and 62% more methane. This study demonstrated improved succinate production from waste glycerol using an adapted wild-type strain of E. coli with wild-type M. formicicum, which is more useful than genetically modified strains. Crude glycerol, an economical feedstock, was used for the cultivation. Furthermore, the increase in methane production by M. formicicum during co-culture with adapted E. coli illustrated the possibility of energy-saving effects for the fermentation process.

Enterobacter sp. LU1 as a novel succinic acid producer - co-utilization of glycerol and lactose

Succinic acid is an important C4-building chemical platform for many applications. A novel succinic acid-producing bacterial strain was isolated from goat rumen. Phylogenetic analysis based on the 16S rRNA sequence and physiological analysis indicated that the strain belongs to the genus Enterobacter. This is the first report of a wild bacterial strain from the genus Enterobacter that is capable of efficient succinic acid production. Co-fermentation of glycerol and lactose significantly improved glycerol utilization under anaerobic conditions, debottlenecking the utilization pathway of this valuable biodiesel waste product. Succinic acid production reached 35 g l-1 when Enterobacter sp. LU1 was cultured in medium containing 50 g l-1 of glycerol and 25 g l-1 of lactose as carbon sources.

The pentose phosphate pathway leads to enhanced succinic acid flux in biofilms of wild-type Actinobacillus succinogenes

Increased pentose phosphate pathway flux, relative to total substrate uptake flux, is shown to enhance succinic acid (SA) yields under continuous, non-growth conditions of Actinobacillus succinogenes biofilms. Separate fermentations of glucose and xylose were conducted in a custom, continuous biofilm reactor at four different dilution rates. Glucose-6-phosphate dehydrogenase assays were performed on cell extracts derived from in situ removal of biofilm at each steady state. The results of the assays were coupled to a kinetic model that revealed an increase in oxidative pentose phosphate pathway (OPPP) flux relative to total substrate flux with increasing SA titre, for both substrates. Furthermore, applying metabolite concentration data to metabolic flux models that include the OPPP revealed similar flux relationships to those observed in the experimental kinetic analysis. A relative increase in OPPP flux produces additional reduction power that enables increased flux through the reductive branch of the TCA cycle, leading to increased SA yields, reduced by-product formation and complete closure of the overall redox balance.

Continuous succinic acid production from xylose by Actinobacillus succinogenes

Continuous, anaerobic fermentations of D-xylose were performed by Actinobacillus succinogenes 130Z in a custom, biofilm reactor at dilution rates of 0.05, 0.10 and 0.30 h(-1). Succinic acid yields on xylose (0.55-0.68 g g(-1)), titres (10.9-29.4 g L(-1)) and productivities (1.5-3.4 g L(-1) h(-1)) were lower than those of a previous study on glucose, but product ratios (succinic acid/acetic acid = 3.0-5.0 g g(-1)) and carbohydrate consumption rates were similar. Also, mass balance closures on xylose were up to 18.2 % lower than those on glucose. A modified HPLC method revealed pyruvic acid excretion at appreciable concentrations (1.2-1.9 g L(-1)) which improved the mass balance closure by up to 16.8 %. Furthermore, redox balances based on the accounted xylose consumed and the excreted metabolites, indicated an overproduction of reducing power. The oxidative pentose phosphate pathway was shown to be a plausible source of the additional reducing power.

Continuous succinic acid production by Actinobacillus succinogenes on xylose-enriched hydrolysate

BACKGROUND

Bio-manufacturing of high-value chemicals in parallel to renewable biofuels has the potential to dramatically improve the overall economic landscape of integrated lignocellulosic biorefineries. However, this will require the generation of carbohydrate streams from lignocellulose in a form suitable for efficient microbial conversion and downstream processing appropriate to the desired end use, making overall process development, along with selection of appropriate target molecules, crucial to the integrated biorefinery. Succinic acid (SA), a high-value target molecule, can be biologically produced from sugars and has the potential to serve as a platform chemical for various chemical and polymer applications. However, the feasibility of microbial SA production at industrially relevant productivities and yields from lignocellulosic biorefinery streams has not yet been reported.

RESULTS

Actinobacillus succinogenes 130Z was immobilised in a custom continuous fermentation setup to produce SA on the xylose-enriched fraction of a non-detoxified, xylose-rich corn stover hydrolysate stream produced from deacetylation and dilute acid pretreatment. Effective biofilm attachment, which serves as a natural cell retention strategy to increase cell densities, productivities and resistance to toxicity, was accomplished by means of a novel agitator fitting. A maximum SA titre, yield and productivity of 39.6 g L(-1), 0.78 g g(-1) and 1.77 g L(-1) h(-1) were achieved, respectively. Steady states were obtained at dilution rates of 0.02, 0.03, 0.04, and 0.05 h(-1) and the stirred biofilm reactor was stable over prolonged periods of operation with a combined fermentation time of 1550 h. Furthermore, it was found that a gradual increase in the dilution rate was required to facilitate adaptation of the culture to the hydrolysate, suggesting a strong evolutionary response to the toxic compounds in the hydrolysate. Moreover, the two primary suspected fermentation inhibitors, furfural and HMF, were metabolised during fermentation with the concentration of each remaining at zero across all steady states.

CONCLUSIONS

The results demonstrate that immobilised A. succinogenes has the potential for effective conversion of an industrially relevant, biomass-derived feed stream to succinic acid. Furthermore, due to the attractive yields, productivities and titres achieved in this study, the process has the potential to serve as a means for value-added chemical manufacturing in the integrated biorefinery.