Cell immobilization for microbial production of 1,3-propanediol

Effect of loading rate and pH on glycerol fermentation and microbial population in an upflow anaerobic filter reactor

A reactor with silicone tubes as support medium was used for glycerol fermentation. The experimental set-up consisted of three phases. In P1, the applied glycerol loading rate (gly-LR) was in the range of 6-10 g.L-1.d-1 at an influent pH of 7.9 ± 0.4. In P2, gly-LR was kept constant (18.0 ± 1.8 g.L-1.d-1) with different doses of NaHCO3. Finally in P3, two different gly-LR (9 and 18 g.L-1.d-1) were evaluated, dosing 1 g-NaHCO3 per g-COD of glycerol. Glycerol consumption was close 90%. The main end-product was 1,3-propanediol (1,3-PDO) (0.40 mol.mol-gly-1), but ethanol was also generated, particularly at pH above 8 and low gly-LR (0.20 mol.mol-gly-1). After 1-year operation with glycerol as the only carbon source, a drastic shift in the bacterial community was observed. The 1,3-PDO producers Lacrimispora and Clostridium became dominant, although non-glycerol-degrading fermentative genera, e.g., Actinomyces and Eubacterium, thrived at the expense of cellular breakdown products.

Metabolic Engineering and Regulation of Diol Biosynthesis from Renewable Biomass in Escherichia coli

As bulk chemicals, diols have wide applications in many fields, such as clothing, biofuels, food, surfactant and cosmetics. The traditional chemical synthesis of diols consumes numerous non-renewable energy resources and leads to environmental pollution. Green biosynthesis has emerged as an alternative method to produce diols. Escherichia coli as an ideal microbial factory has been engineered to biosynthesize diols from carbon sources. Here, we comprehensively summarized the biosynthetic pathways of diols from renewable biomass in E. coli and discussed the metabolic-engineering strategies that could enhance the production of diols, including the optimization of biosynthetic pathways, improvement of cofactor supplementation, and reprogramming of the metabolic network. We then investigated the dynamic regulation by multiple control modules to balance the growth and production, so as to direct carbon sources for diol production. Finally, we proposed the challenges in the diol-biosynthesis process and suggested some potential methods to improve the diol-producing ability of the host.

Carbon nanomaterial properties help to enhance xylanase production from recombinant Kluyveromyces lactis through a cell immobilization method

Carbon nanomaterials, due to their catalytic activity and high surface area, have potential as cell immobilization supports to increase the production of xylanase. Recombinant Kluyveromyces lactis used for xylanase production was integrated into a polymeric gel network with carbon nanomaterials. Carbon nanomaterials were pretreated before cell immobilization with hydrochloric acid (HCl) treatment and glutaraldehyde (GA) crosslinking, which contributes to cell immobilization performance. Carbon nanotubes (CNTs) and graphene oxide (GO) were further screened using a Plackett-Burman experimental design. Cell loading and agar concentration were the most important factors in xylanase production with low cell leakage. Under optimized conditions, xylanase production was increased by more than 400% compared to free cells. Immobilized cell material containing such high cell densities may exhibit new and unexplored beneficial properties because the cells comprise a large fraction of the component. The use of carbon nanomaterials as a cell immobilization support along with the entrapment method successfully enhances the production of xylanase, providing a new route to improved bioprocessing, particularly for the production of enzymes. KEY POINTS: • Carbon nanomaterials (CNTs, GO) have potential as cell immobilization supports. • Entrapment in a polymeric gel network provides space for xylanase production. • Plackett-Burman design screen for the most important factor for cell immobilization.

Hydrogen Production by Immobilized Cells of Clostridium intestinale Strain URNW Using Alginate Beads

Biological hydrogen (H₂) is a promising candidate for production of renewable hydrogen. Using entrapped cells rather than conventional suspended cell cultures for the production of H₂ offers several advantages, such as improved production yields related to higher cell density, and enhanced resistance to substrate and end-product inhibition. In this study, H₂ production by a novel isolate of Clostridium intestinale (strain URNW) was evaluated using cells entrapped within 2% calcium-alginate beads under strictly anaerobic conditions. Both immobilized cells and suspended cultures were studied in sequential batch-mode anaerobic fermentation over 192 h. The production of H₂ in the headspace was examined for four different initial cellobiose concentrations (5, 10, 20, and 40 mM). Although a lag period for initiation of the fermentation process was observed for bacteria entrapped within hydrogel beads, the immobilized cells achieved both higher volumetric production rates (mmol H₂/(L culture h)) and molar yields (mol H₂/mol glucose equivalent) of H₂ compared with suspended cultures. In the current study, the maximum cellobiose consumption rate of 0.40 mM/h, corresponding to 133.3 mg/(L h), was achieved after 72 h of fermentation by immobilized cells, generating a high hydrogen yield of 3.57 mol H₂/mol cellobiose, whereas suspended cultures only yielded 1.77 mol H₂/mol cellobiose. The results suggest that cells remain viable within the hydrogels and proliferated with a slow rate over the course of fermentation. The stable productivity of immobilized cells over 8 days with four changes of medium depicted that the immobilized cells of the isolated strain can successfully yield higher hydrogen and lower soluble metabolites than suspended cells suggesting a feasible process for future applications for bioH₂ production.

1,3-Propanediol production from glycerol in polyurethane foam containing anaerobic reactors: performance and biomass cultivation and retention

The use of batch and upflow anaerobic reactors filled with polyurethane foam for pure glycerol fermentation was evaluated. The best reactor operational conditions to obtain high yield and productivity of 1,3-propanediol (1,3-PDO) as the main product and the role of the polyurethane foam in the growth and retention of suspended and attached biomass in the reactors were investigated. In the experiment at 30 °C with a batch reactor (700 mL), biomass growth was mostly as immobilized attached cells, and the achieved 1,3-PDO yield was up to 0.58 mol mol-gly^-1. In the experiment (30 °C) with an upflow anaerobic reactor (717 mL), glycerol loading rates (gly-LR) ranging from 6.94 to 15.47 g gly L^-1 day^-1 were applied during a 102-day period. During the operation, average 1,3-PDO yield was 0.47 mol mol-gly^-1, reaching a maximum of 0.51 mol mol-gly^-1 at gly-LR of 13.57 g gly L^-1 day^-1. High 1,3-PDO productivity (5.35 to 5.44 g L^-1 day^-1) was obtained when gly-LR was 13.57 to 15.47 g gly L^-1 day^-1. Comparing the close yield values in both batch and continuous reactors and based on microbial evaluation, it is concluded that most of the 1,3-PDO generated in the continuous reactor was due to the suspended biomass retained by the foam cubes. The Clostridium genus was the predominant 1,3-PDO producer. Good yields and productivities with packed reactors were attributed to polyurethane foam used for mixed culture growth and retention. Consequently, they are worth considering for 1,3-PDO production from pure glycerol.

Analysis of the Growth and Metabolites of a Pyruvate Dehydrogenase Complex- Deficient Klebsiella pneumoniae Mutant in a Glycerol-Based Medium

To determine the role of pyruvate dehydrogenase complex (PDHC) in Klebsiella pneumoniae, the growth and metabolism of PDHC-deficient mutant in glycerol-based medium were analyzed and compared with those of other strains. Under aerobic conditions, the PDHC activity was fourfold higher than that of pyruvate formate lyase (PFL), and blocking of PDHC caused severe growth defect and pyruvate accumulation, indicating that the carbon flux through pyruvate to acetyl coenzyme A mainly depended on PDHC. Under anaerobic conditions, although the PDHC activity was only 50% of that of PFL, blocking of PDHC resulted in more growth defect than blocking of PFL. Subsequently, combined with the requirement of CO2 and intracellular redox status, it was presumed that the critical role of PDHC was to provide NADH for the anaerobic growth of K. pneumoniae. This presumption was confirmed in the PDHC-deficient mutant by further blocking one of the formate dehydrogenases, FdnGHI. Besides, based on our data, it can also be suggested that an improvement in the carbon flux in the PFL-deficient mutant could be an effective strategy to construct highyielding 1,3-propanediol-producing K. pneumoniae strain.

Random mutagenesis of Clostridium butyricum strain and optimization of biosynthesis process for enhanced production of 1,3-propanediol

The aim of this work was to study the random mutagenesis of Clostridium butyricum strain. A high 1,3-PD tolerant mutant strain, designated as C. butyricum YP855, was developed from the wild strain C. butyricum XYB11, using combined chemical (NTG, N-methyl-N'-nitro-N-nitrosoguanidine,) and plasma-based mutagenesis (ARTP, atmospheric and room temperature plasma). The YP855 showed a maximum tolerance of 85 g/L to 1,3-PD (up to 30.8% increase) when compared with the tolerance exhibited by the wild strain. Under the optimum conditions as established by the response surface methodology (RSM), the mutant strain produced 37.20 g/L of 1,3-PD, which is 29.48% higher than the concentration obtained from the wild strain (28.73 g/L). This research would offer information for further development of the biosynthesis of 1,3-PD by the mutant strain of C. butyricum.

Haloferax volcanii as immobilised whole cell biocatalyst: new applications for halophilic systems

Enzyme-mediated synthesis of pharmaceutical compounds is a 'green' alternative to traditional synthetic chemistry, and microbial engineering opens up the possibility of using whole cells as mini-factories. Whole-cell biocatalysis reduces cost by eliminating expensive enzyme purification and cofactor addition steps, as well as resulting in increased enzyme stability. Haloferax volcanii is a model halophilic archaeon encoding highly salt and organic solvent tolerant enzymes such as alcohol dehydrogenase (HvADH2), which catalyses the reduction of aldehydes and ketone in the presence of NADPH/NADH cofactor. A H. volcanii strain for constitutive HvADH2 expression was generated using a strong synthetic promoter (p.syn). The strain was immobilised in calcium alginate beads and repeatedly used as a whole-cell biocatalyst. The reduction of acetophenone, used as test substrate, was very successful and high yields were detected from immobilised whole cells over repeated biotransformation cycles. The immobilised H. volcanii retained stability and high product yields after 1 month of storage at room temperature. This newly developed system offers halophilic enzyme expression in its native environment, high product yield, stability and reusability without the addition of any expensive NADPH/NADH cofactor. This is the first report of whole cell-mediated biocatalysis by the halophilic archaeon H. volcanii.

Immobilized Cells of Bacillus circulans ATCC 21783 on Palm Curtain for Fermentation in 5 L Fermentation Tanks

Palm curtain was selected as carrier to immobilize Bacillus circulans ATCC 21783 to produce β-cyclodextrin (β-CD). The influence for immobilization to CGTase activity was analyzed to determine the operation stability. 83.5% cyclodextrin glycosyltransferases (CGTase) of the 1st cycle could be produced in the 7th cycle for immobilized cells, while only 28.90% CGTase was produced with free cells. When palm curtain immobilized cells were reused at the 2th cycle, enzyme activities were increased from 5003 to 5132 U/mL, which was mainly due to physical adsorption of cells on palm curtain with special concave surface structure. Furthermore, conditions for expanded culture of immobilized cells in a 5 L fermentation tank were optimized through specific rotation speed procedure (from 350 r/min to 450 r/min with step size of 50 r/min) and fixed ventilation capacity (4.5 L/min), relations between biomass, enzyme activity, pH, and oxygen dissolution was investigated, and the fermentation periods under the two conditions were both 4 h shorter. Compared with free cell, immobilized cell was more stable, effective, and had better application potential in industries.

Genetically Engineered Strains: Application and Advances for 1,3-Propanediol Production from Glycerol

1,3-Propanediol (1,3-PD) is one of the most important chemicals widely used as monomers for synthesis of some commercially valuable products, including cosmetics, foods, lubricants and medicines. Although 1,3-PD can be synthesized both chemically and biosynthetically, the latter offers more merits over chemical approach as it is economically viable, environmentally friendly and easy to carry out. The biosynthesis of 1,3-PD can be done by transforming glycerol or other similar substrates using some bacteria, such as Clostridium butyricum and Klebsiella pneumoniae. However, these natural microorganisms pose some bottlenecks like low productivity and metabolite inhibition. To overcome these problems, recent research efforts have been focused more on the development of new strains by modifying the genome through different techniques, such as mutagenesis and genetic engineering. Genetically engineered strains obtained by various strategies cannot only gain higher yield than wild types, but also overcome some of the barriers in production by the latter. This review paper presents an overview on the recent advances in the technological approaches to develop genetically engineered microorganisms for efficient biosynthesis of 1,3-PD.

Key enzymes catalyzing glycerol to 1,3-propanediol

Biodiesel can replace petroleum diesel as it is produced from animal fats and vegetable oils, and it produces about 10 % (w/w) glycerol, which is a promising new industrial microbial carbon, as a major by-product. One of the most potential applications of glycerol is its biotransformation to high value chemicals such as 1,3-propanediol (1,3-PD), dihydroxyacetone (DHA), succinic acid, etc., through microbial fermentation. Glycerol dehydratase, 1,3-propanediol dehydrogenase (1,3-propanediol-oxydoreductase), and glycerol dehydrogenase, which were encoded, respectively, by dhaB, dhaT, and dhaD and with DHA kinase are encompassed by the dha regulon, are the three key enzymes in glycerol bioconversion into 1,3-PD and DHA, and these are discussed in this review article. The summary of the main research direction of these three key enzyme and methods of glycerol bioconversion into 1,3-PD and DHA indicates their potential application in future enzymatic research and industrial production, especially in biodiesel industry.