Moderate expression of the transcriptional regulator ALsR enhances acetoin production by Bacillus subtilis

Construction and characterization of a promoter library with varying strengths to enhance acetoin production from xylose in Serratia marcescens

Serratia marcescens is utilized as a significant enterobacteria in the production of various high-value secondary metabolites. Acetoin serves as a crucial foundational compound of development and finds application in a broad range of fields. Furthermore, S. marcescens HBQA-7 is capable of utilizing xylose as its exclusive carbon source for acetoin production. The objective of this study was to utilize a constitutive promoter screening strategy to enhance both xylose utilization and acetoin production in S. marcescens HBQA-7. By utilizing RNA-seq, we identified the endogenous constitutive promoter P6 that is the most robust, which facilitated the overexpression of the sugar transporter protein GlfL445I, α-acetyl lactate synthase, and α-acetyl lactate decarboxylase, respectively. The resultant recombinant strains exhibited enhanced xylose utilization rates and acetoin yields. Subsequently, a recombinant plasmid, denoted as pBBR1MCS-P6-glfL445IalsSalsD, was constructed, simultaneously expressing the aforementioned three genes. The resulting recombinant strain, designated as S3, demonstrated a 1.89-fold boost in xylose consumption rate compared with the original strain during shake flask fermentation. resulting in the accumulation of 7.14 g/L acetoin in the final fermentation medium. Subsequently, in a 5 L fermenter setup, the acetoin yield reached 48.75 g/L, corresponding to a xylose-to-acetoin conversion yield of 0.375 g/g.

Bacillus sp. as a microbial cell factory: Advancements and future prospects

Bacillus sp. is one of the most distinctive gram-positive bacteria, able to grow efficiently using cheap carbon sources and secrete a variety of useful substances, which are widely used in food, pharmaceutical, agricultural and environmental industries. At the same time, Bacillus sp. is also recognized as a safe genus with a relatively clear genetic background, which is conducive to the industrial production of target metabolites. In this review, we discuss the reasons why Bacillus sp. has been so extensively studied and summarize its advances in systems and synthetic biology, engineering strategies to improve microbial cell properties, and industrial applications in several metabolic engineering applications. Finally, we present the current challenges and possible solutions to provide a reliable basis for Bacillus sp. as a microbial cell factory.

Metabolic Engineering of Microorganisms to Produce Pyruvate and Derived Compounds

Pyruvate is a hub of various endogenous metabolic pathways, including glycolysis, TCA cycle, amino acid, and fatty acid biosynthesis. It has also been used as a precursor for pyruvate-derived compounds such as acetoin, 2,3-butanediol (2,3-BD), butanol, butyrate, and L-alanine biosynthesis. Pyruvate and derivatives are widely utilized in food, pharmaceuticals, pesticides, feed additives, and bioenergy industries. However, compounds such as pyruvate, acetoin, and butanol are often chemically synthesized from fossil feedstocks, resulting in declining fossil fuels and increasing environmental pollution. Metabolic engineering is a powerful tool for producing eco-friendly chemicals from renewable biomass resources through microbial fermentation. Here, we review and systematically summarize recent advances in the biosynthesis pathways, regulatory mechanisms, and metabolic engineering strategies for pyruvate and derivatives. Furthermore, the establishment of sustainable industrial synthesis platforms based on alternative substrates and new tools to produce these compounds is elaborated. Finally, we discuss the potential difficulties in the current metabolic engineering of pyruvate and derivatives and promising strategies for constructing efficient producers.

Anode-assisted electro-fermentation with Bacillus subtilis under oxygen-limited conditions

BACKGROUND

Bacillus subtilis is generally regarded as a ubiquitous facultative anaerobe. Oxygen is the major electron acceptor of B. subtilis, and when oxygen is absent, B. subtilis can donate electrons to nitrate or perform fermentation. An anode electrode can also be used by microorganisms as the electron sink in systems called anodic electro-fermentation. The facultative anaerobic character of B. subtilis makes it an excellent candidate to explore with different electron acceptors, such as an anode. This study aimed to optimise industrial aerobic bioprocesses using alternative electron acceptors. In particular, different end product spectrum of B. subtilis with various electron acceptors, including anode from the electro-fermentation system, was investigated.

RESULTS

B. subtilis was grown using three electron acceptors, i.e. oxygen, nitrate and anode (poised at a potential of 0.7 V vs. standard hydrogen electrode). The results showed oxygen had a crucial role for cells to remain metabolically active. When nitrate or anode was applied as the sole electron acceptor anaerobically, immediate cell lysis and limited glucose consumption were observed. In anode-assisted electro-fermentation with a limited aeration rate, acetoin, as the main end product showed the highest yield of 0.78 ± 0.04 mol_product/mol_glucose, two-fold higher than without poised potential (0.39 ± 0.08 mol_product/mol_glucose).

CONCLUSIONS

Oxygen controls B. subtilis biomass growth, alternative electron acceptors utilisation and metabolites formation. Limited oxygen/air supply enabled the bacteria to donate excess electrons to nitrate or anode, leading to steered product spectrum. The anode-assisted electro-fermentation showed its potential to boost acetoin production for future industrial biotechnology applications.

Acetoin production from lignocellulosic biomass hydrolysates with a modular metabolic engineering system in Bacillus subtilis

Background

Acetoin (AC) is a vital platform chemical widely used in food, pharmaceutical and chemical industries. With increasing concern over non-renewable resources and environmental issues, using low-cost biomass for acetoin production by microbial fermentation is undoubtedly a promising strategy.

Results

This work reduces the disadvantages of Bacillus subtilis during fermentation by regulating genes involved in spore formation and autolysis. Then, optimizing intracellular redox homeostasis through Rex protein mitigated the detrimental effects of NADH produced by the glycolytic metabolic pathway on the process of AC production. Subsequently, multiple pathways that compete with AC production are blocked to optimize carbon flux allocation. Finally, the population cell density-induced promoter was used to enhance the AC synthesis pathway. Fermentation was carried out in a 5-L bioreactor using bagasse lignocellulosic hydrolysate, resulting in a final titer of 64.3 g/L, which was 89.5% of the theoretical yield.

Conclusions

The recombinant strain BSMAY-4-PsrfA provides an economical and efficient strategy for large-scale industrial production of acetoin.

Genomic and Transcriptional Characteristics of Strain Rum-meliibacillus sp. TYF-LIM-RU47 with an Aptitude of Directly Producing Acetoin from Lignocellulose

Rummeliibacillus sp. TYF-LIM-RU47, isolated from the fermentation substrate of grain vinegar, could produce acetoin using a variety of carbon sources, including pentose, hexose and lignocellulose. The draft genome of TYF-LIM-RU47 was constructed and the genomic information revealed that TYF-LIM-RU47 contains genes related to starch and sucrose metabolism, pyruvate metabolism, the oxidative phosphorylation metabolic pathway and lignocellulosic metabolism. The acetoin anabolic pathway of TYF-LIM-RU47 has been deduced from the sequencing results, and acetoin is produced from α-acetolactate via decarboxylation and diacetyl reductase catalytic steps. The results of quantitative real-time PCR tests showed that the synthesis and degradation of acetoin had a dynamic balance in acetoin metabolism, and the transcription of the α-acetolactate synthase gene might exist to the extent of feedback regulation. This study can help researchers to better understand the bioinformation of thermophilic-lignocellulosic bacteria and the mechanisms of the acetoin biosynthesis pathway.

Perception of Biocontrol Potential of Bacillus inaquosorum KR2-7 against Tomato Fusarium Wilt through Merging Genome Mining with Chemical Analysis

Simple Summary

Bacillus is a bacterial genus that is widely used as a promising alternative to chemical pesticides due to its protective activity toward economically important plant pathogens. Fusarium wilt of tomato is a serious fungal disease limiting tomato production worldwide. Recently, the newly isolated B. inaquosorum strain KR2-7 considerably suppressed Fusarium wilt of tomato plants. The present study was performed to perceive potential direct and indirect biocontrol mechanisms implemented by KR2-7 against this disease through genome and chemical analysis. The potential direct biocontrol mechanisms of KR2-7 were determined through the identification of genes involved in the synthesis of antibiotically active compounds suppressing tomato Fusarium wilt. Furthermore, the indirect mechanisms of this bacterium were perceived through recognizing genes that contributed to the resource acquisition or modulation of plant hormone levels. This is the first study that aimed at the modes of actions of B. inaquosorum against Fusarium wilt of tomatoes and the results strongly indicate that strain KR2-7 could be a good candidate for microbial biopesticide formulations to be used for biological control of plant diseases and plant growth promotion.

Abstract

Tomato Fusarium wilt, caused by Fusarium oxysporum f. sp. lycopersici (Fol), is a destructive disease that threatens the agricultural production of tomatoes. In the present study, the biocontrol potential of strain KR2-7 against Fol was investigated through integrated genome mining and chemical analysis. Strain KR2-7 was identified as B. inaquosorum based on phylogenetic analysis. Through the genome mining of strain KR2-7, we identified nine antifungal and antibacterial compound biosynthetic gene clusters (BGCs) including fengycin, surfactin and Bacillomycin F, bacillaene, macrolactin, sporulation killing factor (skf), subtilosin A, bacilysin, and bacillibactin. The corresponding compounds were confirmed through MALDI-TOF-MS chemical analysis. The gene/gene clusters involved in plant colonization, plant growth promotion, and induced systemic resistance were also identified in the KR2-7 genome, and their related secondary metabolites were detected. In light of these results, the biocontrol potential of strain KR2-7 against tomato Fusarium wilt was identified. This study highlights the potential to use strain KR2-7 as a plant-growth promotion agent.

Current Advances in Microbial Production of Acetoin and 2,3-Butanediol by Bacillus spp

The growing need for industrial production of bio-based acetoin and 2,3-butanediol (2,3-BD) is due to both environmental concerns, and their widespread use in the food, pharmaceutical, and chemical industries. Acetoin is a common spice added to many foods, but also a valuable reagent in many chemical syntheses. Similarly, 2,3-BD is an indispensable chemical on the platform in the production of synthetic rubber, printing inks, perfumes, antifreeze, and fuel additives. This state-of-the-art review focuses on representatives of the genus Bacillus as prospective producers of acetoin and 2,3-BD. They have the following important advantages: non-pathogenic nature, unpretentiousness to growing conditions, and the ability to utilize a huge number of substrates (glucose, sucrose, starch, cellulose, and inulin hydrolysates), sugars from the composition of lignocellulose (cellobiose, mannose, galactose, xylose, and arabinose), as well as waste glycerol. In addition, these strains can be improved by genetic engineering, and are amenable to process optimization. Bacillus spp. are among the best acetoin producers. They also synthesize 2,3-BD in titer and yield comparable to those of the pathogenic producers. However, Bacillus spp. show relatively lower productivity, which can be increased in the course of challenging future research.

Prospects on bio-based 2,3-butanediol and acetoin production: Recent progress and advances

The bio-based platform chemicals 2,3-butanediol (BDO) and acetoin have various applications in chemical, cosmetics, food, agriculture, and pharmaceutical industries, whereas the derivatives of BDO could be used as fuel additives, polymer and synthetic rubber production. This review summarizes the novel technological developments in adapting genetic and metabolic engineering strategies for selection and construction of chassis strains for BDO and acetoin production. The valorization of renewable feedstocks and bioprocess development for the upstream and downstream stages of bio-based BDO and acetoin production are discussed. The techno-economic aspects evaluating the viability and industrial potential of bio-based BDO production are presented. The commercialization of bio-based BDO and acetoin production requires the utilization of crude renewable resources, the chassis strains with high fermentation production efficiencies and development of sustainable purification or conversion technologies.

Metabolic Engineering of Bacillus licheniformis for Production of Acetoin

Acetoin is a potential platform compound for a variety of chemicals. Bacillus licheniformis MW3, a thermophilic and generally regarded as safe (GRAS) microorganism, can produce 2,3-butanediol with a high concentration, yield, and productivity. In this study, B. licheniformis MW3 was metabolic engineered for acetoin production. After deleting two 2,3-butanediol dehydrogenases encoding genes budC and gdh, an engineered strain B. licheniformis MW3 (ΔbudCΔgdh) was constructed. Using fed-batch fermentation of B. licheniformis MW3 (ΔbudCΔgdh), 64.2 g/L acetoin was produced at a productivity of 2.378 g/[L h] and a yield of 0.412 g/g from 156 g/L glucose in 27 h. The fermentation process exhibited rather high productivity and yield of acetoin, indicating that B. licheniformis MW3 (ΔbudCΔgdh) might be a promising acetoin producer.

Analysis of Chuanxiong Rhizoma substrate on production of ligustrazine in endophytic Bacillus subtilis by ultra high performance liquid chromatography with quadrupole time-of-flight mass spectrometry

Ligustrazine was the active ingredient of the traditional Chinese medicine Chuanxiong Rhizoma. However, the content of ligustrazine is very low. We proposed a hypothesis that ligustrazine was produced by the mutual effects between endophytic Bacillus subtilis and the Ligusticum chuanxiong Hort. This study aimed to explore whether the endophytic B. subtilis LB5 could make use of Chuanxiong Rhizoma fermentation matrix to produce ligustrazine and clarify the mechanisms of action preliminarily. Ultra high performance liquid chromatography with quadrupole time-of-flight mass spectrometry analysis showed the content of ligustrazine in Chuanxiong Rhizoma was below the detection limit (0.1 ng/mL), while B. subtilis LB5 produced ligustrazine at the yield of 1.0268 mg/mL in the Chuanxiong Rhizoma-ammonium sulfate fermentation medium. In the fermented matrix, the reducing sugar had a significant reduction from 12.034 to 2.424 mg/mL, and rough protein content increased from 2.239 to 4.361 mg/mL. Acetoin, the biosynthetic precursor of ligustrazine, was generated in the Chuanxiong Rhizoma-Ammonium sulfate (151.2 mg/mL) fermentation medium. This result showed that the endophytic bacteria B. subtilis LB5 metabolized Chuanxiong Rhizoma via secreted protein to consume the sugar in Chuanxiong Rhizoma to produce a considerable amount of ligustrazine. Collectively, our preliminary research suggested that ligustrazine was the interaction product of endophyte, but not the secondary metabolite of Chuanxiong Rhizoma itself.

Synthetic engineering of Corynebacterium crenatum to selectively produce acetoin or 2,3-butanediol by one step bioconversion method

BACKGROUND

Acetoin (AC) and 2,3-butanediol (2,3-BD) as highly promising bio-based platform chemicals have received more attentions due to their wide range of applications. However, the non-efficient substrate conversion and mutually transition between AC and 2,3-BD in their natural producing strains not only led to a low selectivity but also increase the difficulty of downstream purification. Therefore, synthetic engineering of more suitable strains should be a reliable strategy to selectively produce AC and 2,3-BD, respectively.

RESULTS

In this study, the respective AC (alsS and alsD) and 2,3-BD biosynthesis pathway genes (alsS, alsD, and bdhA) derived from Bacillus subtilis 168 were successfully expressed in non-natural AC and 2,3-BD producing Corynebacterium crenatum, and generated recombinant strains, C. crenatum SD and C. crenatum SDA, were proved to produce 9.86 g L-1 of AC and 17.08 g L-1 of 2,3-BD, respectively. To further increase AC and 2,3-BD selectivity, the AC reducing gene (butA) and lactic acid dehydrogenase gene (ldh) in C. crenatum were then deleted. Finally, C. crenatumΔbutAΔldh SD produced 76.93 g L-1 AC in one-step biocatalysis with the yield of 0.67 mol mol-1. Meanwhile, after eliminating the lactic acid production and enhancing 2,3-butanediol dehydrogenase activity, C. crenatumΔldh SDA synthesized 88.83 g L-1 of 2,3-BD with the yield of 0.80 mol mol-1.

CONCLUSIONS

The synthetically engineered C. crenatumΔbutAΔldh SD and C. crenatumΔldh SDA in this study were proved as an efficient microbial cell factory for selective AC and 2,3-BD production. Based on the insights from this study, further synthetic engineering of C. crenatum for AC and 2,3-BD production is suggested.

Engineered microbial host selection for value-added bioproducts from lignocellulose

Lignocellulose is a rich and sustainable globally available carbon source and is considered a prominent alternative raw material for producing biofuels and valuable chemical compounds. Enzymatic hydrolysis is one of the crucial steps of lignocellulose degradation. Cellulolytic and hemicellulolytic enzyme mixes produced by different microorganisms including filamentous fungi, yeasts and bacteria, are used to degrade the biomass to liberate monosaccharides and other compounds for fermentation or conversion to value-added products. During biomass pretreatment and degradation, toxic compounds are produced, and undesirable carbon catabolic repression (CCR) can occur. In order to solve this problem, microbial metabolic pathways and transcription factors involved have been investigated along with the application of protein engineering to optimize the biorefinery platform. Engineered Microorganisms have been used to produce specific enzymes to breakdown biomass polymers and metabolize sugars to produce ethanol as well other biochemical compounds. Protein engineering strategies have been used for modifying lignocellulolytic enzymes to overcome enzymatic limitations and improving both their production and functionality. Furthermore, promoters and transcription factors, which are key proteins in this process, are modified to promote microbial gene expression that allows a maximum performance of the hydrolytic enzymes for lignocellulosic degradation. The present review will present a critical discussion and highlight the aspects of the use of microorganisms to convert lignocellulose into value-added bioproduct as well combat the bottlenecks to make the biorefinery platform from lignocellulose attractive to the market.

Enhancing 2-Ketogluconate Production of Pseudomonas plecoglossicida JUIM01 by Maintaining the Carbon Catabolite Repression of 2-Ketogluconate Metabolism

2-Ketogluconate (2KGA) is an organic acid that is important for pharmaceutical, cosmetic, and environmental applications. Pseudomonas plecoglossicida JUIM01 strain is an important industrial 2KGA producer in China. In this paper, we found that P. plecoglossicida JUIM01 could convert glucose to 2KGA extracellularly, and the formed 2KGA was subsequently consumed after glucose was exhausted during the fermentation process. Experiments of glucose and 2KGA supplementation during fermentation process revealed that, only when glucose was exhausted, the strain started to consume the product 2KGA. Then, the mechanism of this phenomenon was investigated at transcription and protein levels, and the results indicated that P. plecoglossicida JUIM01 possesses carbon catabolite repression of 2KGA metabolism by glucose. Next, increasing the supply of glucose could attenuate 2KGA consumption and enhance the 2KGA yield from glucose. Finally, fed-batch fermentation of P. plecoglossicida JUIM01 resulted in 205.67 g/L of 2KGA with a productivity of 6.86 g/L/h and yield of 0.953 g/g glucose. These results can provide references for the industrial fermentation production of 2KGA and other fermentation products.

Metabolic engineering of Serratia marcescens MG1 for enhanced production of (3R)-acetoin

Background

Optically pure acetoin (AC) is an important platform chemical which has been widely used to synthesize novel optically active α-hydroxyketone derivatives and liquid crystal composites.

Results

In this study, slaC and gldA encoding meso-2,3-butanediol dehydrogenase (meso-2,3-BDH) and glycerol dehydrogenase (GDH), respectively, in S. marcescens MG1 were knocked out to block the conversion from AC to 2,3-butanediol (2,3-BD). The resulting strain MG14 was found to produce a large amount of optically pure (3R)-AC with a little 2,3-BD, indicating that another enzyme responsible for 2,3-BD formation except meso-2,3-BDH and GDH existed in the strain MG1. Furthermore, SlaR protein, a transcriptional activator of AC cluster, was overexpressed using P_C promoter in the strain MG14, leading to enhancement of the (3R)-AC yield by 29.91%. The recombinant strain with overexpression of SlaR, designated as S. marcescens MG15, was used to perform medium optimization for improving (3R)-AC production.

Conclusion

Under the optimized conditions, 39.91 ± 1.35 g/l (3R)-AC was produced by strain MG15 with the productivity of 1.11 g/l h and the conversion rate of 80.13%.

Efficient 9α-hydroxy-4-androstene-3,17-dione production by engineered Bacillus subtilis co-expressing Mycobacterium neoaurum 3-ketosteroid 9α-hydroxylase and B. subtilis glucose 1-dehydrogenase with NADH regeneration

3-Ketosteroid 9α-hydroxylase (KSH, consisting of KshA and KshB), a key enzyme in steroid metabolism, can catalyze the transformation of 4-androstene-3,17-dione (AD) to 9α-hydroxy-4-androstene-3,17-dione (9OHAD) with NADH as coenzyme. In this work, KSH from Mycobacterium neoaurum JC-12 was successfully cloned and overexpressed in Bacillus subtilis 168. The expression and purification of KSH was analyzed by SDS-PAGE and KSH activity assay. Preliminary characterization of KSH was performed using purified KshA and KshB. The results showed that KSH was very unstable, and its activity was inhibited by most metal ions, especially Zn(2+). The whole-cells of recombinant B. subtilis, co-expression of KSH and glucose 1-dehydrogenase (GDH), were used as biocatalyst to convert AD to 9OHAD. The biocatalyst, in which the intracellular NADH was regenerated, efficiently catalyzed the bioconversion of AD to 9OHAD with a conversion rate of 90.4 % and productivity of 0.45 g (L h)(-1), respectively. This work proposed a strategy for efficiently producing 9OHAD by using B. subtilis as a promising whole-cell biocatalyst host and co-expressing KSH and GDH to construct a NADH regeneration system.

Production of Acetoin through Simultaneous Utilization of Glucose, Xylose, and Arabinose by Engineered Bacillus subtilis

Glucose, xylose and arabinose are the three most abundant monosaccharide found in lignocellulosic biomass. Effectively and simultaneously utilization of these sugars by microorganisms for production of the biofuels and bio-chemicals is essential toward directly fermentation of the lignocellulosic biomass. In our previous study, the recombinant Bacillus subtilis 168ARSRCPΔacoAΔbdhA strain was already shown to efficiently utilize xylose for production of acetoin, with a yield of 0.36 g/g xylose. In the current study, the Bacillus subtilis168ARSRCPΔacoAΔbdhA strain was further engineered to produce acetoin from a glucose, xylose, and arabinose mixtures. To accomplish this, the endogenous xylose transport protein AraE, the exogenous xylose isomerase gene xylA and the xylulokinase gene xylB from E. coli were co-overexpressed in the Bacillus subtilis 168ARSRCPΔacoAΔbdhA strain, which enabled the resulting strain, denoted ZB02, to simultaneously utilize glucose and xylose. Unexpectedly, the ZB02 strain could simultaneously utilize glucose and arabinose also. Further results indicated that the transcriptional inhibition of the arabinose transport protein gene araE was the main limiting factor for arabinose utilization in the presence of glucose. Additionally, the arabinose operon in B. subtilis could be activated by the addition of arabinose, even in the presence of glucose. Through fed-batch fermentation, strain ZB02 could simultaneously utilize glucose, xylose, and arabinose, with an average sugar consumption rate of 3.00 g/l/h and an average production of 62.2 g/l acetoin at a rate of 0.864 g/l/h. Finally, the strain produced 11.2 g/l acetoin from lignocellulosic hydrolysate (containing 20.6g/l glucose, 12.1 g/l xylose and 0.45 g/l arabinose) in flask cultivation, with an acetoin yield of 0.34 g/g total sugar. The result demonstrates that this strain has good potential for the utilization of lignocellulosic hydrolysate for production of acetoin.

Improvement of the intracellular environment for enhancing l-arginine production of Corynebacterium glutamicum by inactivation of H2O2-forming flavin reductases and optimization of ATP supply

l-arginine, a semi essential amino acid, is an important amino acid in food flavoring and pharmaceutical industries. Its production by microbial fermentation is gaining more and more attention. In previous work, we obtained a new l-arginine producing Corynebacterium crenatum (subspecies of Corynebacterium glutamicum) through mutation breeding. In this work, we enhanced l-arginine production through improvement of the intracellular environment. First, two NAD(P)H-dependent H₂O₂-forming flavin reductases Frd181 (encoded by frd1 gene) and Frd188 (encoded by frd2) in C. glutamicum were identified for the first time. Next, the roles of Frd181 and Frd188 in C. glutamicum were studied by overexpression and deletion of the encoding genes, and the results showed that the inactivation of Frd181 and Frd188 was beneficial for cell growth and l-arginine production, owing to the decreased H₂O₂ synthesis and intracellular reactive oxygen species (ROS) level, and increased intracellular NADH and ATP levels. Then, the ATP level was further increased by deletion of noxA (encoding NADH oxidase) and amn (encoding AMP nucleosidase), and overexpression of pgk (encoding 3-phosphoglycerate kinase) and pyk (encoding pyruvate kinase), and the l-arginine production and yield from glucose were significantly increased. In fed-batch fermentation, the l-arginine production and yield from glucose of the final strain reached 57.3g/L and 0.326g/g, respectively, which were 49.2% and 34.2% higher than those of the parent strain, respectively. ROS and ATP are important elements of the intracellular environment, and l-arginine biosynthesis requires a large amount of ATP. For the first time, we enhanced l-arginine production and yield from glucose through reducing the H₂O₂ synthesis and increasing the ATP supply.

Regulation of the NADH pool and NADH/NADPH ratio redistributes acetoin and 2,3-butanediol proportion in Bacillus subtilis

Bacillus subtilis produces acetoin as a major product along with several NADH-dependent byproducts, especially 2,3-butanediol. In this study, the down-regulation of the NADH pool and the redistribution of NADH/NADPH were targeted using external and genetic processes, as a means by which to redistribute the metabolic flux in favor of acetoin synthesis. First, it was found that the use of carbon sources of different oxidation states resulted in very different intracellular NADH/NAD(+) ratios that dictated the total process yield of acetoin. A mixture of glucose and gluconate as substrate produced a relatively low NADH/NAD(+) ratio, and resulted in an increase in acetoin production while byproducts significantly decreased. Metabolic engineering methods using glucose as a substrate could yield a similar effect. Acetoin production was significantly enhanced by overexpression of the oxidative pentose phosphate pathway: increased expression of glucose-6-phosphate dehydrogenase resulted in a decrease in the intracellular NADH/NADPH ratio (1.9-fold) and NADH/NAD(+) ratio (1.7-fold). In fed-batch culture the engineered strain yielded an acetoin concentration of 43.3 g L(-1) , while the production of 2,3-butanediol was only 1.7 g L(-1) . The concept of the manipulation of cofactor levels to redistribute carbon flux by external and genetic means as explored in this paper provides a novel strategy for improving industrial acetoin fermentation.

Synthetic operon for (R,R)-2,3-butanediol production in Bacillus subtilis and Escherichia coli

To reduce dependence on petroleum, an alternative route to production of the chemical feedstock 2,3-butanediol (2,3-BD) from renewable lignocellulosic sources is desirable. In this communication, the genes encoding the pathway from pyruvate to 2,3-BD (alsS, alsD, and bdhA encoding acetolactate synthase, acetolactate decarboxylase, and butanediol dehydrogenase, respectively) from Bacillus subtilis were engineered into a single tricistronic operon under control of the isopropyl β-D-1-thiogalactopyranoside (IPTG)-inducible Pspac promoter in a shuttle plasmid capable of replication and expression in either B. subtilis or Escherichia coli. We describe the construction and performance of a shuttle plasmid carrying the IPTG-inducible synthetic operon alsSDbdhA coding for 2,3-BD pathway capable of (i) expression in two important representative model microorganisms, the gram-positive B. subtilis and the gram-negative E. coli; (ii) increasing 2,3-BD production in B. subtilis; and (iii) successfully introducing the B. subtilis 2,3-BD pathway into E. coli. The synthetic alsSDbdhA operon constructed using B. subtilis native genes not only increased the 2,3-BD production in its native host but also efficiently expressed the pathway in the heterologous organism E. coli. Construction of an efficient shuttle plasmid will allow investigation of 2,3-BD production performance in related organisms with industrial potential for production of bio-based chemicals.

Enhanced 2,3-butanediol production from biodiesel-derived glycerol by engineering of cofactor regeneration and manipulating carbon flux in Bacillus amyloliquefaciens

BACKGROUND

Bacillus amyloliquefaciens B10-127 exhibited an excellent ability for industrial-scale microbial fermentation of 2,3-butanediol (2,3-BD) from biodiesel-derived glycerol. However, the accumulation of by-products (acetoin, acetoin, lactate and succinate) and the 2,3-BD yield remains prohibitively low for commercial production.

RESULTS

Several strategies were developed to manipulate the carbon flux to 2,3-BD branch in a designed B. amyloliquefaciens. Firstly, extra copies of NADH/NAD(+) regeneration system were introduced into B. amyloliquefaciens by co-overproduction of glycerol dehydrogenase and acetoin reductase, which resulting in improvement of 2,3-BD production and suppression of by-products accumulation. Subsequently, the transcriptional regulator ALsR under the control of a moderate promoter PbdhA was introduced into B. amyloliquefaciens, which increased carbon flux to 2,3-BD branch. Finally, a three-stage dissolved oxygen control strategy were proposed based on analysis of the characteristic of 2,3-BD fermentation, and a two-stage pH control strategy were proposed based on different pH preferences of ACR for reduction and oxidation. Following these strategies, a high titer (102.3 g/L), yield (0.44 g/g), and productivity (1.16 g/L/h) of 2,3-BD were achieved.

CONCLUSIONS

To our knowledge, this is the highest reported 2,3-BD production using biodiesel-derived glycerol as substrate, and this designed B. amyloliquefaciens should be an excellent candidate for producing 2,3-BD on an industrial scale.

Efficient whole-cell biocatalyst for acetoin production with NAD+ regeneration system through homologous co-expression of 2,3-butanediol dehydrogenase and NADH oxidase in engineered Bacillus subtilis

Acetoin (3-hydroxy-2-butanone), an extensively-used food spice and bio-based platform chemical, is usually produced by chemical synthesis methods. With increasingly requirement of food security and environmental protection, bio-fermentation of acetoin by microorganisms has a great promising market. However, through metabolic engineering strategies, the mixed acid-butanediol fermentation metabolizes a certain portion of substrate to the by-products of organic acids such as lactic acid and acetic acid, which causes energy cost and increases the difficulty of product purification in downstream processes. In this work, due to the high efficiency of enzymatic reaction and excellent selectivity, a strategy for efficiently converting 2,3-butandiol to acetoin using whole-cell biocatalyst by engineered Bacillus subtilis is proposed. In this process, NAD⁺ plays a significant role on 2,3-butanediol and acetoin distribution, so the NADH oxidase and 2,3-butanediol dehydrogenase both from B. subtilis are co-expressed in B. subtilis 168 to construct an NAD⁺ regeneration system, which forces dramatic decrease of the intracellular NADH concentration (1.6 fold) and NADH/NAD⁺ ratio (2.2 fold). By optimization of the enzymatic reaction and applying repeated batch conversion, the whole-cell biocatalyst efficiently produced 91.8 g/L acetoin with a productivity of 2.30 g/(L·h), which was the highest record ever reported by biocatalysis. This work indicated that manipulation of the intracellular cofactor levels was more effective than the strategy of enhancing enzyme activity, and the bioprocess for NAD⁺ regeneration may also be a useful way for improving the productivity of NAD⁺-dependent chemistry-based products.

Two-stage pH control strategy based on the pH preference of acetoin reductase regulates acetoin and 2,3-butanediol distribution in Bacillus subtilis

Acetoin reductase/2,3-butanediol dehydrogenase (AR/BDH), which catalyzes the interconversion between acetoin and 2,3-butanediol, plays an important role in distribution of the products pools. This work characterized the Bacillus subtilis AR/BDH for the first time. The enzyme showed very different pH preferences of pH 6.5 for reduction and pH 8.5 for oxidation. Based on these above results, a two-stage pH control strategy was optimized for acetoin production, in which the pH was controlled at 6.5 for quickly converting glucose to acetoin and 2,3-butanediol, and then 8.0 for reversely transforming 2,3-butanediol to acetoin. By over-expression of AR/BDH in the wild-type B. subtilis JNA 3-10 and applying fed-batch fermentation based on the two-stage pH control strategy, acetoin yield of B. subtilis was improved to a new record of 73.6 g/l, with the productivity of 0.77 g/(l·h). The molar yield of acetoin was improved from 57.5% to 83.5% and the ratio of acetoin/2,3-butanediol was switched from 2.7∶1 to 18.0∶1.

The rebalanced pathway significantly enhances acetoin production by disruption of acetoin reductase gene and moderate-expression of a new water-forming NADH oxidase in Bacillus subtilis

Bacillus subtilis produces acetoin as a major extracellular product. However, the by-products of 2,3-butanediol, lactic acid and ethanol were accompanied in the NADH-dependent pathways. In this work, metabolic engineering strategies were proposed to redistribute the carbon flux to acetoin by manipulation the NADH levels. We first knocked out the acetoin reductase gene bdhA to block the main flux from acetoin to 2,3-butanediol. Then, among four putative candidates, we successfully screened an active water-forming NADH oxidase, YODC. Moderate-expression of YODC in the bdhA disrupted B. subtilis weakened the NADH-linked pathways to by-product pools of acetoin. Through these strategies, acetoin production was improved to 56.7g/l with an increase of 35.3%, while the production of 2,3-butanediol, lactic acid and ethanol were decreased by 92.3%, 70.1% and 75.0%, respectively, simultaneously the fermentation duration was decreased 1.7-fold. Acetoin productivity by B. subtilis was improved to 0.639g/(lh).

Efficient bioconversion of 2,3-butanediol into acetoin using Gluconobacter oxydans DSM 2003

BACKGROUND

2,3-Butanediol is a platform and fuel biochemical that can be efficiently produced from biomass. However, a value-added process for this chemical has not yet been developed. To expand the utilization of 2,3-butanediol produced from biomass, an improved derivative process of 2,3-butanediol is desirable.

RESULTS

In this study, a Gluconobacter oxydans strain DSM 2003 was found to have the ability to transform 2,3-butanediol into acetoin, a high value feedstock that can be widely used in dairy and cosmetic products, and chemical synthesis. All three stereoisomers, meso-2,3-butanediol, (2R,3R)-2,3-butanediol, and (2S,3S)-2,3-butanediol, could be transformed into acetoin by the strain. After optimization of the bioconversion conditions, the optimum growth temperature for acetoin production by strain DSM 2003 was found to be 30°C and the medium pH was 6.0. With an initial 2,3-butanediol concentration of 40 g/L, acetoin at a high concentration of 89.2 g/L was obtained from 2,3-butanediol by fed-batch bioconversion with a high productivity (1.24 g/L · h) and high yield (0.912 mol/mol).

CONCLUSIONS

G. oxydans DSM 2003 is the first strain that can be used in the direct production of acetoin from 2,3-butanediol. The product concentration and yield of the novel process are both new records for acetoin production. The results demonstrate that the method developed in this study could provide a promising process for efficient acetoin production and industrially produced 2,3-butanediol utilization.