Increased Production of Food-Grade d-Tagatose from d-Galactose by Permeabilized and Immobilized Cells of Corynebacterium glutamicum, a GRAS Host, Expressing d-Galactose Isomerase from Geobacillus thermodenitrificans

Efficient biosynthesis of creatine by whole-cell catalysis from guanidinoacetic acid in Corynebacterium glutamicum

Creatine is a naturally occurring derivative of an amino acid commonly utilized in functional foods and pharmaceuticals. Nevertheless, the current industrial synthesis of creatine relies on chemical processes, which may hinder its utilization in certain applications. Therefore, a biological approach was devised that employs whole-cell biocatalysis in the bacterium Corynebacterium glutamicum, which is considered safe for use in food production, to produce safe-for-consumption creatine. The objective of this study was to identify a guanidinoacetate N-methyltransferase (GAMT) with superior catalytic activity for creatine production. Through employing whole-cell biocatalysis, a gamt gene from Mus caroli (Mcgamt) was cloned and expressed in C. glutamicum ATCC 13032, resulting in a creatine titer of 3.37 g/L. Additionally, the study employed a promoter screening strategy that utilized nine native strong promoters in C. glutamicum to enhance the expression level of GAMT. The highest titer was achieved using the P1676 promoter, reaching 4.14 g/L. The conditions of whole-cell biocatalysis were further optimized, resulting in a creatine titer of 5.42 g/L. This is the first report of successful secretory creatine expression in C. glutamicum, which provides a safer and eco-friendly approach for the industrial production of creatine.

Isopropanol production using engineered Corynebacterium glutamicum from waste rice straw biomass

Isopropanol, a well-known biofuel, is a widely used precursor for chemical products that can replace nonrenewable petroleum energy. Here, engineered Corynebacterium glutamicum that can effectively utilize all xylose and glucose in agricultural waste rice straw to produce isopropanol was described. First, codon mutations were introduced into transporters and glycolytic-related genes to decrease the glucose preference of C. glutamicum. A more energetically favorable xylose oxidative pathway was constructed that replaced traditional xylose isomerization pathways, saving twice the number of enzymatic steps. A succinate auxiliary module was incorporated into the tricarboxylic acid cycle (TCA), connecting the xylose-utilized pathway with the isopropanol pathway to maximize xylose orientation towards the product. The final engineered strain successfully consumed 100 % of the xylose from NaOH-pretreated, enzyme-hydrolyzed rice straw and effectively synthesized 4.91 g/L isopropanol. This study showcases the successful conversion of agricultural waste into renewable energy, unveiling new possibilities for advancing biological fermentation technology.

Synthesis of a Healthy Sweetener d-Tagatose from Starch Catalyzed by Semiartificial Cell Factories

d-Tagatose is one of the several healthy sweeteners that can be a substitute for sucrose and fructose in our daily life. Whole cell-catalyzed phosphorylation and dephosphorylation previously reported by our group afford a thermodynamic-driven strategy to achieve tagatose production directly from starch with high product yields. Nonetheless, the poor structural stability of cells and difficulty in biocatalyst recycling restrict its practical application. Herein, an efficient and stable semiartificial cell factory (SACF) was developed by constructing an organosilica network (OSN) artificial shell on the cells bearing five thermophilic enzymes to produce tagatose. The OSN artificial shell, the thickness of which can be regulated by changing the tetraethyl silicate concentration, exhibited tunable permeability and superior mechanical strength. In contrast with cells, SACFs showed a relative activity of 99.5% and an extended half-life from 33.3 to 57.8 h. Over 50% of initial activity was retained after 20 reuses. The SACFs can catalyze seven consecutive reactions with tagatose yields of over 40.7% in field applications.

A Novel Salt-Tolerant L-Glutaminase: Efficient Functional Expression, Computer-Aided Design, and Application

The low productivity in long fermentation duration and high-salt working conditions limit the application of L-glutaminase in soy sauce brewing. In this study, a novel L-glutaminase (LreuglsA) with eminent salt tolerance was mined and achieved more than 70% activity with 30% NaCl. To improve the robustness of the enzyme at different fermentation strategies, mutation LreuglsAH105K was built by a computer-aided design, and the recombinant protein expression level, an essential parameter in industrial applications, was increased 5.61-fold with the synthetic biology strategy by improving the mRNA stability. Finally, the LreuglsAH105K functional expression box was contributed to Bacillus subtilis 168 by auxotrophic complementation, and the production in a 5-L bioreactor was improved to 2516.78 ± 20.83 U mL−1, the highest production ever reported. When the immobilized cells were applied to high-salt dilute-state soy sauce brewing, the L-glutamate level was increased by 45.9%. This work provides insight into the salt-tolerant enzyme for improving the efficiency of industrial applications.

Integrated Whole-Cell Biocatalysis for Trehalose Production from Maltose Using Permeabilized Pseudomonas monteilii Cells and Bioremoval of Byproduct

Trehalose is a non-conventional sugar with potent applications in the food, healthcare and biopharma industries. In this study, trehalose was synthesized from maltose using whole-cell Pseudomonas monteilii TBRC 1196 producing trehalose synthase (TreS) as the biocatalyst. The reaction condition was optimized using 1% Triton X-100 permeabilized cells. According to our central composite design (CCD) experiment, the optimal process was achieved at 35°C and pH 8.0 for 24 h, resulting in the maximum trehalose yield of 51.60 g/g after 12 h using an initial cell loading of 94 g/l. Scale-up production in a lab-scale bioreactor led to the final trehalose concentration of 51.91 g/l with a yield of 51.60 g/g and productivity of 4.37 g/l/h together with 8.24 g/l glucose as a byproduct. A one-pot process integrating trehalose production and byproduct bioremoval showed 53.35% trehalose yield from 107.4 g/l after 15 h by permeabilized P. moteilii cells. The residual maltose and glucose were subsequently removed by Saccharomyces cerevisiae TBRC 12153, resulting in trehalose recovery of 99.23% with 24.85 g/l ethanol obtained as a co-product. The present work provides an integrated alternative process for trehalose production from maltose syrup in bio-industry.

Sustainable isomaltulose production in Corynebacterium glutamicum by engineering the thermostability of sucrose isomerase coupled with one-step simplified cell immobilization

Sucrose isomerase (SI), catalyzing sucrose to isomaltulose, has been widely used in isomaltulose production, but its poor thermostability is still resisted in sustainable batches production. Here, protein engineering and one-step immobilized cell strategy were simultaneously coupled to maintain steady state for long-term operational stabilities. First, rational design of Pantoea dispersa SI (PdSI) for improving its thermostability by predicting and substituting the unstable amino acid residues was investigated using computational analysis. After screening mutagenesis library, two single mutants (PdSIV280L and PdSIS499F) displayed favorable characteristics on thermostability, and further study found that the double mutant PdSIV280L/S499F could stabilize PdSIWT better. Compared with PdSIWT, PdSIV280L/S499F displayed a 3.2°C-higher T m , and showed a ninefold prolonged half-life at 45°C. Subsequently, a one-step simplified immobilization method was developed for encapsulation of PdSIV280L/S499F in food-grade Corynebacterium glutamicum cells to further enhance the recyclability of isomaltulose production. Recombinant cells expressing combinatorial mutant (RCSI2) were successfully immobilized in 2.5% sodium alginate without prior permeabilization. The immobilized RCSI2 showed that the maximum yield of isomaltulose by batch conversion reached to 453.0 g/L isomaltulose with a productivity of 41.2 g/l/h from 500.0 g/L sucrose solution, and the conversion rate remained 83.2% after 26 repeated batches.

Engineering Corynebacterium glutamicum for synthesis of poly(3-hydroxybutyrate) from lignocellulose biomass

Lignocellulose is the only feasible carbohydrates feedstock for commercial scale and carbon neutral production of poly(3-hydroxybutyrate) (PHB) biopolymer by its great abundance and availability. Microbial cell factories for fermentative PHB synthesis are highly restricted by the growth suppression of inhibitors from lignocellulose pretreatment. This study targeted on a potential PHB-producing cell factory Corynebacterium glutamicum owing to its strong inhibitors tolerance. A systematic metabolic engineering was conducted starting with the stable PHB synthesis pathway construction from glucose and xylose, followed by the enhancement of PHB synthesis on PHA synthase activity and stability, cell morphology modification, and growth factors regulation. The relocation of the PHA synthase on the cell membrane guided by secrete signal peptides and cell membrane display motifs increased the PHB content by 2.4 folds. Excessive nitrogen preferentially promoted the PHB synthesis capacity and resulted in the PHB content increased by 13.3 folds. Modification of the genes responsible for cell division changed the cell morphology but the cell size was not enlarged to a PHB accumulation favorable environment. The metabolic engineering of C. glutamicum resulted in a high fermentative production of PHB using wheat straw as feedstock. This study provided an important microbial cell factory choice for PHB production using lignocellulose feedstock. This article is protected by copyright. All rights reserved.

Direct conversion of cheese whey to polymalic acid by mixed culture of Aureobasidium pullulans and permeabilized Kluyveromyces marxianus

Cheese whey is an abundant and low-cost feedstock with lactose as its main component, but the inability to metabolize lactose prevents Aureobasidium pullulans from using cheese whey directly. In this study, Kluyveromyces marxianus was permeabilized to obtain nonviable but biocatalytic cells for lactose hydrolysis, and the mixed culture of A. pullulans and permeabilized K. marxianus was conducted for polymalic acid (PMA) production from cheese whey. In the mixed culture, PMA titer varied directly to β-galactosidase activity of K. marxianus, but inversely to cell viability of K. marxianus, and ethanol permeabilized K. marxianus was the most compatible with A. pullulans for PMA production. 37.8 g/L PMA was produced in batch fermentation, and PMA titer was increased to 97.3 g/L in fed-batch fermentation, with a productivity of 0.51 g/(L·h) and a yield of 0.56 g/g. This study paved an economical and environmentally friendly way for PMA production from cheese whey.

Production of trans-cinnamic acid by whole-cell bioconversion from L-phenylalanine in engineered Corynebacterium glutamicum

Background

trans-cinnamic acid (t-CA) is a phenylpropanoid with a broad spectrum of biological activities including antioxidant and antibacterial activities, and it also has high potential in food and cosmetic applications. Although significant progress has been made in the production of t-CA using microorganisms, its relatively low product titers still need to be improved. In this study, we engineered Corynebacterium glutamicum as a whole-cell catalyst for the bioconversion of l-phenylalanine (l-Phe) into t-CA and developed a repeated bioconversion process.

Results

An expression module based on a phenylalanine ammonia lyase-encoding gene from Streptomyces maritimus (SmPAL), which mediates the conversion of l-Phe into t-CA, was constructed in C. glutamicum. Using the strong promoter P_H36 and ribosome binding site (RBS) (in front of gene 10 of the T7 phage), and a high-copy number plasmid, SmPAL could be expressed to levels as high as 39.1% of the total proteins in C. glutamicum. Next, to improve t-CA production at an industrial scale, reaction conditions including temperature and pH were optimized; t-CA production reached up to 6.7 mM/h in a bioreactor under optimal conditions (50 °C and pH 8.5, using NaOH as base solution). Finally, a recycling system was developed by coupling membrane filtration with the bioreactor, and the engineered C. glutamicum successfully produced 13.7 mM of t-CA (24.3 g) from 18.2 mM of l-Phe (36 g) and thus with a yield of 75% (0.75 mol/mol) through repetitive supplementation.

Conclusions

We developed a highly efficient bioconversion process using C. glutamicum as a biocatalyst and a micromembrane-based cell recycling system. To the best of our knowledge, this is the first report on t-CA production in C. glutamicum, and this robust platform will contribute to the development of an industrially relevant platform for the production of t-CA using microorganisms.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01631-1.

An integrative approach to improving the biocatalytic reactions of whole cells expressing recombinant enzymes

Biotransformation is a selective, stereospecific, efficient, and environment friendly method, compared to chemical synthesis, and a feasible tool for industrial and pharmaceutical applications. The design of biocatalysts using enzyme engineering and metabolic engineering tools has been widely reviewed. However, less importance has been given to the biocatalytic reaction of whole cells expressing recombinant enzymes. Along with the remarkable development of biotechnology tools, a variety of techniques have been applied to improve the biocatalytic reaction of whole cell biotransformation. In this review, techniques related to the biocatalytic reaction are examined, reorganized, and summarized via an integrative approach. Moreover, equilibrium-shifted biotransformation is reviewed for the first time.

Whole-cell biosynthesis of d-tagatose from maltodextrin by engineered Escherichia coli with multi-enzyme co-expression system

d-tagatose is a functional sweetener that occurs in small quantity in nature. It is mainly produced through the isomerization of d-galactose by l-arabinose isomerase (l-AI; EC 5.3.1.4). However, the cost of d-galactose is much higher than those commonly used for the production of functional sweeteners such as glucose, maltodextrin, or starch. Here, a multi-enzyme catalytic system consists of five enzymes that utilizes maltodextrin as substrate to synthesize d-tagatose were co-expressed in E. coli, resulting in recombinant cells harboring the plasmids pETDuet-αgp-pgm and pCDFDuet-pgi-gatz-pgp. The activity of this whole-cell catalyst was optimal at 60 °C and pH 7.5, and 1 mM Mg²⁺ and 50 mM phosphate were the optimal cofactors for activity. Under the optimal reaction conditions, 2.08 and 3.2 g L^-1d-tagatose were produced by using 10 and 20 g L^-1 maltodextrin as substrates with recombinant cells for 24 h. This co-expression system provides a one-pot synthesis approach for the production of d-tagatose using inexpensive substrate, avoiding enzymes purification steps and supplementation of expensive cofactors.

No more cleaning up - Efficient lactic acid bacteria cell catalysts as a cost-efficient alternative to purified lactase enzymes

β-galactosidases, commonly referred to as lactases, are used for producing lactose-free dairy products. Lactases are usually purified from microbial sources, which is a costly process. Here, we explored the potential that lies in using whole cells of a food-grade dairy lactic acid bacterium, Streptococcus thermophilus, as a substitute for purified lactase. We found that S. thermophilus cells, when treated with the antimicrobial peptide nisin, were able to hydrolyze lactose efficiently. The rate of hydrolysis increased with temperature; however, above 50 °C, stability was compromised. Different S. thermophilus strains were tested, and the best candidate was able to hydrolyze 80% of the lactose in a 50 g/L solution in 4 h at 50 °C, using only 0.1 g/L cells (dry weight basis). We demonstrated that it was possible to grow the cell catalyst on dairy waste, and furthermore, that a cell-free supernatant of a culture of a nisin-producing Lactococcus lactis strain could be used instead of purified nisin, which reduced cost of use significantly. Finally, we tested the cell catalysts in milk, where lactose also was efficiently hydrolyzed. The method presented is natural and low-cost, and allows for production of clean-label and lactose-free dairy products without using commercial enzymes from recombinant microorganisms. KEY POINTS: • Nisin-permeabilized Streptococcus thermophilus cells can hydrolyze lactose efficiently. • A low-cost and more sustainable alternative to purified lactase enzymes. • Reduction of overall sugar content. • Clean-label production of lactose-free dairy products.

Efficient production of α-acetolactate by whole cell catalytic transformation of fermentation-derived pyruvate

Background

Diacetyl provides the buttery aroma in products such as butter and margarine. It can be made via a harsh set of chemical reactions from sugarcane bagasse, however, in dairy products it is normally formed spontaneously from α-acetolactate, a compound generated by selected lactic acid bacteria in the starter culture used. Due to its bacteriostatic properties, it is difficult to achieve high levels of diacetyl by fermentation. Here we present a novel strategy for producing diacetyl based on whole-cell catalysis, which bypasses the toxic effects of diacetyl.

Results

By expressing a robust α-acetolactate synthase (ALS) in a metabolically optimized Lactococcus lactis strain we obtained a whole-cell biocatalyst that efficiently converted pyruvate into α-acetolactate. After process optimization, we achieved a titer for α-acetolactate of 172 ± 2 mM. Subsequently we used a two-stage production setup, where pyruvate was produced by an engineered L. lactis strain and subsequently used as the substrate for the biocatalyst. Using this approach, 122 ± 5 mM and 113 ± 3 mM α-acetolactate could be made from glucose or lactose in dairy waste, respectively. The whole-cell biocatalyst was robust and fully active in crude fermentation broth containing pyruvate.

Conclusions

An efficient approach for converting sugar into α-acetolactate, via pyruvate, was developed and tested successfully. Due to the anaerobic conditions used for the biotransformation, little diacetyl was generated, and this allowed for efficient biotransformation of pyruvate into α-acetolactate, with the highest titers reported to date. The use of a two-step procedure for producing α-acetolactate, where non-toxic pyruvate first is formed, and subsequently converted into α-acetolactate, also simplified the process optimization. We conclude that whole cell catalysis is suitable for converting lactose in dairy waste into α-acetolactate, which favors resource utilization.

High-density immobilization of a ginsenoside-transforming β-glucosidase for enhanced food-grade production of minor ginsenosides

Use of recombinant glycosidases is a promising approach for the production of minor ginsenosides, e.g., Compound K (CK) and F₁, which have potential applications in the food industry. However, application of these recombinant enzymes for food-grade preparation of minor ginsenosides are limited by the lack of suitable expression hosts and low productivity. In this study, Corynebacterium glutamicum ATCC13032, a GRAS strain that has been used extensively for the industrial-grade production of additives for foodstuffs, was employed to express a novel β-glucosidase (MT619) from Microbacterium testaceum ATCC 15829 with high ginsenoside-transforming activity. A cellulose-binding module was additionally fused to the N-terminus of MT619 for immobilization on cellulose, which is an abundant and safe material. Via one-step immobilization, the fusion protein in cell lysates was efficiently immobilized on regenerated amorphous cellulose at a high density (maximum 984 mg/g cellulose), increasing the enzyme concentration by 286-fold. The concentrated and immobilized enzyme showed strong conversion activities against protopanaxadiol- and protopanaxatriol-type ginsenosides for the production of CK and F₁. Using gram-scale ginseng extracts as substrates, the immobilized enzyme produced 7.59 g/L CK and 9.42 g/L F₁ in 24 h. To the best of our knowledge, these are the highest reported product concentrations of CK and F₁, and this is the first time that a recombinant enzyme has been immobilized on cellulose for the preparation of minor ginsenosides. This safe, convenient, and efficient production method could also be effectively exploited in the preparation of food-processing recombinant enzymes in the pharmaceutical, functional food, and cosmetics industries.

Sweet As Sugar-Efficient Conversion of Lactose into Sweet Sugars Using a Novel Whole-Cell Catalyst

Lactose, the sugar contained in milk, has a low sweetness. We have constructed an efficient whole-cell catalyst (WCC) that can be grown on dairy waste and that is able to convert lactose into a mixture of sugars as sweet as sucrose. The WCC is based on Corynebacterium glutamicum ATCC13032, which has been engineered to metabolize lactose, to express xylose and arabinose isomerase, and to eliminate byproduct formation. When introduced in concentrated cheese whey permeate, its content of 98 g/L lactose was completely hydrolyzed and the liberated sugars partially isomerized into 23.5 g/L fructose and 20.4 g/L tagatose, which corresponds to a 49% conversion of the glucose and a 44% conversion of galactose. The latter is similar to what can be obtained using purified enzymes. The new technology enables better resource utilization and allows for dairy waste to be converted into a valuable food sweetener with many potential uses.

Optimizing a CRISPR-Cpf1-based genome engineering system for Corynebacterium glutamicum

Background

Corynebacterium glutamicum is an important industrial strain for the production of a diverse range of chemicals. Cpf1 nucleases are highly specific and programmable, with efficiencies comparable to those of Cas9. Although the Francisella novicida (Fn) CRISPR-Cpf1 system has been adapted for genome editing in C. glutamicum, the editing efficiency is currently less than 15%, due to false positives caused by the poor targeting efficiency of the crRNA.

Results

To address this limitation, a screening strategy was developed in this study to systematically evaluate crRNA targeting efficiency in C. glutamicum. We quantitatively examined various parameters of the C. glutamicum CRISPR-Cpf1 system, including the protospacer adjacent motif (PAM) sequence, the length of the spacer sequence, and the type of repair template. We found that the most efficient C. glutamicum crRNA contained a 5′-NYTV-3′ PAM and a 21 bp spacer sequence. Moreover, we observed that linear DNA could be used to repair double strand breaks.

Conclusions

Here, we identified optimized PAM-related parameters for the CRISPR-Cpf1 system in C. glutamicum. Our study sheds light on the function of the FnCpf1 endonuclease and Cpf1-based genome editing. This optimized system, with higher editing efficiency, could be used to increase the production of bulk chemicals, such as isobutyrate, in C. glutamicum.

Electronic supplementary material

The online version of this article (10.1186/s12934-019-1109-x) contains supplementary material, which is available to authorized users.

Overcoming the thermodynamic equilibrium of an isomerization reaction through oxidoreductive reactions for biotransformation

Isomerases perform biotransformations without cofactors but often cause an undesirable mixture of substrate and product due to unfavorable thermodynamic equilibria. We demonstrate the feasibility of using an engineered yeast strain harboring oxidoreductase reactions to overcome the thermodynamic limit of an isomerization reaction. Specifically, a yeast strain capable of consuming lactose intracellularly is engineered to produce tagatose from lactose through three layers of manipulations. First, GAL1 coding for galactose kinase is deleted to eliminate galactose utilization. Second, heterologous xylose reductase (XR) and galactitol dehydrogenase (GDH) are introduced into the ∆gal1 strain. Third, the expression levels of XR and GDH are adjusted to maximize tagatose production. The resulting engineered yeast produces 37.69 g/L of tagatose from lactose with a tagatose and galactose ratio of 9:1 in the reaction broth. These results suggest that in vivo oxidoreaductase reactions can be employed to replace isomerases in vitro for biotransformation.

A desired product cannot be obtained at higher concentration than its equilibrium concentration when isomerases are used for biotransformation. Here, the authors engineer in vivo oxidoreductive reactions in yeast to overcome the equilibrium limitation of in vitro isomerases-based tagatose production.

Development of food-grade expression system for d-allulose 3-epimerase preparation with tandem isoenzyme genes in Corynebacterium glutamicum and its application in conversion of cane molasses to D-allulose

D-Allulose 3-epimerase (DAE) has been applied to produce D-allulose, a low-calorie and functional sweetener. In this study, a new DAE from Paenibacillus senegalensis was characterized in Escherichia coli. Furthermore, we presented a tandem isoenzyme gene expression strategy to express multiple DAEs in one cell and construct food-grade expression systems based on Corynebacterium glutamicum. Seventeen expression cassettes based on three DAE genes from different organisms were constructed. Among all recombinant strains, DAE16 harboring three DAE genes in an expression vector exhibited the highest enzyme activity with 22.7 U/mg. Whole-cell transformation of DAE16 produced 225 g/L D-allulose with a volumetric productivity of 353 g·g ^-1 ·hr ^-1 . The catalytic efficiency of strain C-DAE9 integrating total 11 DAE genes in chromosome was 16.4-fold higher than strains carrying one DAE. Fed-batch culture of C-DAE9 gave enzyme activity of 44,700 U/L. We also expressed a thermostable invertase in C. glutamicum and obtained enzyme activity of 29 U/mg. Immobilized cells expressing DAE or invertase exhibited 80% of retained activity after 30 cycles of catalytic reactions. Those immobilized cells were coupled to produce 61.2 g/L D-allulose from cane molasses in a two-step reaction process. This study provided an efficient approach for enzyme preparation and allowed access to produce D-allulose from other abundant and low-cost feedstock enriched with sucrose.

Highly efficient production of Clostridium cellulolyticum H10 D-psicose 3-epimerase in Bacillus subtilis and use of these cells to produce D-psicose

Background

d-Psicose 3-epimerase (DPEase) catalyzes the isomerization of d-fructose to the rare sugar d-psicose, which may help prevent obesity, reduce blood sugar and blood fat, and inhibit intra-abdominal fat accumulation.

Results

In this study, the DPEase of Clostridium cellulolyticum H10 was expressed in the food-grade host Bacillus subtilis. Optimization of the culture medium during shake-flask experiments yielded a DPEase activity of 314 U/mL. The optimal medium included 20 g/L peptone, 15 g/L corn steep powder, 5 g/L glycerol, and 1 mM Ca²⁺. Controlling the carbon source concentration was important because elevated concentrations can result in catabolite metabolic suppression (CCR). To avoid CCR and increase DPEase expression, we developed a fed-batch strategy in a 3.6-L fermenter. We altered the ratio of carbon source to nitrogen source (C/N) in the feeding medium and employed a constant feeding rate (6 g/L/h). This strategy improved the DPEase activity to 2246 U/mL (7.8 g/L), which is almost 15 times higher than that observed in the original shake-flask cultures. Finally, we used the DPEase-expressing B. subtilis cells to produce d-psicose from d-fructose, and a 28% conversion yield was achieved with these cells, demonstrating their potential use in d-psicose production.

Conclusions

This is the first report to enhance recombinant DPEase production in B. subtilis using efficient and convenient fermentation strategy, and the DPEase yield is three times higher than the highest yield reported to date. The recombinant B. subtilis cells were further used in the efficient synthesis of d-psicose. This study provides a basis for the industrial production of d-psicose.

Recombinant Protein Expression System in Corynebacterium glutamicum and Its Application

Corynebacterium glutamicum, a soil-derived gram-positive actinobacterium, has been widely used for the production of biochemical molecules such as amino acids (i.e., L-glutamate and L-lysine), nucleic acids, alcohols, and organic acids. The metabolism of the bacterium has been engineered to increase the production of the target biochemical molecule, which requires a cytosolic enzyme expression. As recent demand for new proteinaceous biologics (such as antibodies, growth factors, and hormones) increase, C. glutamicum is attracting industrial interest as a recombinant protein expression host for therapeutic protein production due to the advantages such as low protease activity without endotoxin activity. In this review, we have summarized the recent studies on the heterologous expression of the recombinant protein in C. glutamicum for metabolic engineering, expansion of substrate availability, and recombinant protein secretion. We have also outlined the advances in genetic components such as promoters, surface anchoring systems, and secretory signal sequences in C. glutamicum for effective recombinant protein expression.

Efficient gene editing in Corynebacterium glutamicum using the CRISPR/Cas9 system

BACKGROUND

Corynebacterium glutamicum (C. glutamicum) has traditionally been used as a microbial cell factory for the industrial production of many amino acids and other industrially important commodities. C. glutamicum has recently been established as a host for recombinant protein expression; however, some intrinsic disadvantages could be improved by genetic modification. Gene editing techniques, such as deletion, insertion, or replacement, are important tools for modifying chromosomes.

RESULTS

In this research, we report a CRISPR/Cas9 system in C. glutamicum for rapid and efficient genome editing, including gene deletion and insertion. The system consists of two plasmids: one containing a target-specific guide RNA and a homologous sequence to a target gene, the other expressing Cas9 protein. With high efficiency (up to 100%), this system was used to disrupt the porB, mepA, clpX and Ncgl0911 genes, which affect the ability to express proteins. The porB- and mepA-deletion strains had enhanced expression of green fluorescent protein, compared with the wild-type stain. This system can also be used to engineer point mutations and gene insertions.

CONCLUSIONS

In this study, we adapted the CRISPR/Cas9 system from S. pyogens to gene deletion, point mutations and insertion in C. glutamicum. Compared with published genome modification methods, methods based on the CRISPR/Cas9 system can rapidly and efficiently achieve genome editing. Our research provides a powerful tool for facilitating the study of gene function, metabolic pathways, and enhanced productivity in C. glutamicum.

d-Tagatose production by permeabilized and immobilized Lactobacillus plantarum using whey permeate

The aim of the work is to produce d-Tagatose by direct addition of alginate immobilized Lactobacillus plantarum cells to lactose hydrolysed whey permeate. The cells were untreated and immobilized (UIC), permeabilized and immobilized (PIC) and the relative activities were compared with purified l-arabinose isomerase (l-AI) for d-galactose isomerization. Successive lactose hydrolysis by β-galactosidase from Escherichia coli and d-galactose isomerization using l-AI from Lactobacillus plantarum was performed to investigate the in vivo production of d-tagatose in whey permeate. In whey permeate, maximum conversion of 38% and 33% (w/w) d-galactose isomerization by PIC and UIC has been obtained. 162mg/g and 141mg/g of d-tagatose production was recorded in a 48h reaction time at 50°C, pH 7.0 with 5mM Mn²⁺ ion concentration in the initial substrate mixture.

Highly Efficient Malolactic Fermentation of Red Wine Using Encapsulated Bacteria in a Robust Biocomposite of Silica-Alginate

Bacteria encapsulation to develop malolactic fermentation emerges as a biotechnological strategy that provides significant advantages over the use of free cells. Two encapsulation methods have been proposed embedding Oenococcus oeni, (i) interpenetrated polymer networks of silica and Ca-alginate and (ii) Ca-alginate capsules coated with hydrolyzed 3-aminopropyltriethoxysilane (hAPTES). On the basis of our results, only the first method was suitable for bacteria encapsulation. The optimized silica-alginate capsules exhibited a negligible bacteria release and an increase of 328% and 65% in L-malic acid consumption and mechanical robustness, respectively, compared to untreated alginate capsules. Moreover, studies of capsule stability at different pH and ethanol concentrations in water solutions and in wine indicated a better behavior of silica-alginate capsules than untreated ones. The inclusion of silicates and colloidal silica in alginate capsules containing O. oeni improved markedly their capacity to deplete the levels of L-malic acid in red wines and their mechanical robustness and stability.