Regulation of extracellular oxidoreduction potential enhanced (R,R)-2,3-butanediol production by Paenibacillus polymyxa CJX518

Metabolic engineering of Paenibacillus polymyxa for effective production of 2,3-butanediol from poplar hydrolysate

2,3-Butanediol is an essential renewable fuel. The synthesis of 2,3-butanediol using Paenibacillus polymyxa has attracted increasing attention. In this study, the glucose-derived 2,3-butanediol pathway and its related genes were identified in P. polymyxa using combined transcriptome and metabolome analyses. The functions of two distinct genes ldh1 and ldh3 encoding lactate dehydrogenase, the gene bdh encoding butanediol dehydrogenase, and the spore-forming genes spo0A and spoIIE were studied and directly knocked out or overexpressed in the genome sequence to improve the production of 2,3-butanediol. A raw hydrolysate of poplar wood containing 27 g/L glucose and 15 g/L xylose was used to produce 2,3-butanediol with a maximum yield of 0.465 g/g and 93 % of the maximum theoretical value, and the total production of 2,3-butanediol and ethanol reached 21.7 g/L. This study provides a new scheme for engineered P. polymyxa to produce renewable fuels using raw poplar wood hydrolysates.

Production of highly pure R,R-2,3-butanediol for biological plant growth promoting agent using carbon feeding control of Paenibacillus polymyxa MDBDO

BACKGROUND

Chemical fertilizers have greatly contributed to the development of agriculture, but alternative fertilizers are needed for the sustainable development of agriculture. 2,3-butanediol (2,3-BDO) is a promising biological plant growth promoter.

RESULTS

In this study, we attempted to develop an effective strategy for the biological production of highly pure R,R-2,3-butanediol (R,R-2,3-BDO) by Paenibacillus polymyxa fermentation. First, gamma-ray mutagenesis was performed to obtain P. polymyxa MDBDO, a strain that grew faster than the parent strain and had high production of R,R-2,3-BDO. The activities of R,R-2,3-butanediol dehydrogenase and diacetyl reductase of the mutant strain were increased by 33% and decreased by 60%, respectively. In addition, it was confirmed that the carbon source depletion of the fermentation broth affects the purity of R,R-2,3-BDO through batch fermentation. Fed-batch fermentation using controlled carbon feeding led to production of 77.3 g/L of R,R-2,3-BDO with high optical purity (> 99% of C₄ products) at 48 h. Additionally, fed-batch culture using corn steep liquor as an alternative nitrogen source led to production of 70.3 g/L of R,R-2,3-BDO at 60 h. The fed-batch fermentation broth of P. polymyxa MDBDO, which contained highly pure R,R-2,3-BDO, significantly stimulated the growth of soybean and strawberry seedlings.

CONCLUSIONS

This study suggests that P. polymyxa MDBDO has potential for use in biological plant growth promoting agent applications. In addition, our fermentation strategy demonstrated that high-purity R,R-2,3-BDO can be produced at high concentrations using P. polymyxa.

Medium composition and aeration to high (R,R)-2,3-butanediol and acetoin production by Paenibacillus polymyxa in fed-batch mode

Concerning the potential application of the optically active isomer (R,R)-2,3-butanediol, and its production by a non-pathogenic bacterium Paenibacillus polymyxa ATCC 842, the present study evaluated the use of a commercial crude yeast extract Nucel®, as an organic nitrogen and vitamin source, at different medium composition and two airflows (0.2 or 0.5 vvm). The medium formulated (M4) with crude yeast extract carried out with the airflow of 0.2 vvm (experiment R6) allowed for a reduction in the cultivation time and kept the dissolved oxygen values at low levels until the total glucose consumption. Thus, the experiment R6 led to a fermentation yield of 41% superior when compared to the standard medium (experiment R1), which was conducted at airflow of 0.5 vvm. The maximum specific growth rate at R6 (0.42 h^-1) was lower than R1 (0.60 h^-1), however, the final cell concentration was not affected. Moreover, this condition (medium formulated-M4 and low airflow-0.2 vvm) was a great alternative to produce (R,R)-2,3-BD at fed-batch mode, resulting in 30 g.L^-1 of the isomer at 24 h of cultivation, representing the main product in the broth (77%) and with a fermentation yield of 80%. These results showed that both medium composition and oxygen supply have an important role to produce 2,3-BD by P. polymyxa.

Enzymes and pathways in microbial production of 2,3-butanediol and 3-acetoin isomers

2,3-Butanediol (BD) and acetoin (AC) are products of the non-oxidative metabolism of microorganisms, presenting industrial importance due to their wide range of applications and high market value. Their optical isomers have particular applications, justifying the efforts on the selective bioproduction. Each microorganism produces different isomer mixtures, as a consequence of having different butanediol dehydrogenase (BDH) enzymes. However, the whole scene of the isomer bioproduction, considering the several enzymes and conditions, has not been completely elucidated. Here we show the BDH classification as R, S or meso by bioinformatics analysis uncovering the details of the isomers production. The BDH was compared to diacetyl reductases (DAR) and the new enoyl reductases (ER). We observed that R-BDH is the most singular BDH, while meso and S-BDHs are similar and may be better distinguished through their stereo-selective triad. DAR and ER showed distinct stereo-triads from those described for BDHs, agreeing with kinetic data from the literature and our phylogenetic analysis. The ER family probably has meso-BDH like activity as already demonstrated for a single sequence from this group. These results are of great relevance, as they organize BD producing enzymes, to our known, never shown before in the literature. This review also brings attention to nontraditional enzymes/pathways that can be involved with BD/AC synthesis, as well as oxygen conditions that may lead to the differential production of their isomers. Together, this information can provide helpful orientation for future studies in the field of BD/AC biological production, thus contributing to achieve their production on an industrial scale.

Efficient conversion of hemicellulose into 2, 3-butanediol by engineered psychrotrophic Raoultella terrigena: mechanism and efficiency

Low-temperature biorefineries inhibit the multiplication of undesired microorganisms, improve product purity and reduce economic costs. Herein, to improve the 2,3-butanediol (2,3-BD) bioconversion efficiency from hemicellulose, a psychrotrophic hemicellulose-degrading strain Raoultella terrigena HC6 with high β-xylosidase activity 1520 U/mL was isolated and genetically modified. Xylan (hemicellulose replacement) was depolymerized into xylooligosaccharides (XOS) and xylose by HC6, which were further converted into 2,3-BD. Transcriptomic analysis revealed that β-xylosidase gene (xynB) and xylose isomerase gene (xylA), which are beneficial for increasing the carbon flux from xylan to 2,3-BD, were significantly upregulated 56.9-fold and 234-fold, respectively. A recombinant strain was constructed by overexpressing xynB in HC6, which obtained 0.389 g/g yield of 2,3-BD from hemicellulose extracted from corn straw at 15 °C. This study proposed a promised strategy for the bioconversion of agricultural waste into 2,3-BD at low temperatures and provides a basis for future efforts in the achievement of carbon neutrality.

Production of Different Biochemicals by Paenibacillus polymyxa DSM 742 From Pretreated Brewers' Spent Grains

Brewers' spent grains (BSG) are a by-product of the brewing industry that is mainly used as feedstock; otherwise, it has to be disposed according to regulations. Due to the high content of glucose and xylose, after pretreatment and hydrolysis, it can be used as a main carbohydrate source for cultivation of microorganisms for production of biofuels or biochemicals like 2,3-butanediol or lactate. 2,3-Butanediol has applications in the pharmaceutical or chemical industry as a precursor for varnishes and paints or in the food industry as an aroma compound. So far, Klebsiella pneumoniae, Serratia marcescens, Clostridium sp., and Enterobacter aerogenes are being used and investigated in different bioprocesses aimed at the production of 2,3-butanediol. The main drawback is bacterial pathogenicity which complicates all production steps in laboratory, pilot, and industrial scales. In our study, a gram-positive GRAS bacterium Paenibacillus polymyxa DSM 742 was used for the production of 2,3-butanediol. Since this strain is very poorly described in literature, bacterium cultivation was performed in media with different glucose and/or xylose concentration ranges. The highest 2,3-butanediol concentration of 18.61 g l-1 was achieved in medium with 70 g l-1 of glucose during 40 h of fermentation. In contrast, during bacterium cultivation in xylose containing medium there was no significant 2,3-butanediol production. In the next stage, BSG hydrolysates were used for bacterial cultivation. P. polymyxa DSM 742 cultivated in the liquid phase of pretreated BSG produced very low 2,3-butanediol and ethanol concentrations. Therefore, this BSG hydrolysate has to be detoxified in order to remove bacterial growth inhibitors. After detoxification, bacterium cultivation resulted in 30 g l-1 of lactate, while production of 2,3-butanediol was negligible. The solid phase of pretreated BSG was also used for bacterium cultivation after its hydrolysis by commercial enzymes. In these cultivations, P. polymyxa DSM 742 produced 9.8 g l-1 of 2,3-butanediol and 3.93 g l-1 of ethanol. On the basis of the obtained results, it can be concluded that different experimental setups give the possibility of directing the metabolism of P. polymyxa DSM 742 toward the production of either 2,3-butanediol and ethanol or lactate.

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.

Metabolic engineering of non-pathogenic microorganisms for 2,3-butanediol production

2,3-Butanediol (2,3-BDO) is a promising commodity chemical with various industrial applications. While petroleum-based chemical processes currently dominate the industrial production of 2,3-BDO, fermentation-based production of 2,3-BDO provides an attractive alternative to chemical-based processes with regards to economic and environmental sustainability. The achievement of high 2,3-BDO titer, yield, and productivity in microbial fermentation is a prerequisite for the production of 2,3-BDO at large scales. Also, enantiopure production of 2,3-BDO production is desirable because 2,3-BDO stereoisomers have unique physicochemical properties. Pursuant to these goals, many metabolic engineering strategies to improve 2,3-BDO production from inexpensive sugars by Klebsiella oxytoca, Bacillus species, and Saccharomyces cerevisiae have been developed. This review summarizes the recent advances in metabolic engineering of non-pathogenic microorganisms to enable efficient and enantiopure production of 2,3-BDO. KEY POINTS: • K. oxytoca, Bacillus species, and S. cerevisiae have been engineered to achieve efficient 2,3-BDO production. • Metabolic engineering of non-pathogenic microorganisms enabled enantiopure production of 2,3-BDO. • Cost-effective 2,3-BDO production can be feasible by using renewable biomass.

Process optimization for mass production of 2,3-butanediol by Bacillus subtilis CS13

BACKGROUND

Bacillus subtilis CS13 was previously isolated for 2,3-butanediol (2,3-BD) and poly-γ-glutamic acid (γ-PGA) co-production. When culturing this strain without L-glutamic acid in the medium, 2,3-BD is the main metabolic product. 2,3-BD is an important substance and fuel with applications in the chemical, food, and pharmaceutical industries. However, the yield and productivity for the B. subtilis strain should be improved for more efficient production of 2,3-BD.

RESULTS

The medium composition, which contained 281.1 g/L sucrose, 21.9 g/L ammonium citrate, and 3.6 g/L MgSO₄·7H₂O, was optimized by response surface methodology for 2,3-BD production using B. subtilis CS13. The maximum amount of 2,3-BD (125.5 ± 3.1 g/L) was obtained from the optimized medium after 96 h. The highest concentration and productivity of 2,3-BD were achieved simultaneously at an agitation speed of 500 rpm and aeration rate of 2 L/min in the batch cultures. A total of 132.4 ± 4.4 g/L 2,3-BD was obtained with a productivity of 2.45 ± 0.08 g/L/h and yield of 0.45 g_2,3-BD/g_sucrose by fed-batch fermentation. The meso-2,3-BD/2,3-BD ratio of the 2,3-BD produced by B. subtilis CS13 was 92.1%. Furthermore, 89.6 ± 2.8 g/L 2,3-BD with a productivity of 2.13 ± 0.07 g/L/h and yield of 0.42 g_2,3-BD/g_sugar was achieved using molasses as a carbon source.

CONCLUSIONS

The production of 2,3-BD by B. subtilis CS13 showed a higher concentration, productivity, and yield compared to the reported generally recognized as safe 2,3-BD producers. These results suggest that B. subtilis CS13 is a promising strain for industrial-scale production of 2,3-BD.

Bioreactor production of 2,3-butanediol by Pantoea agglomerans using soybean hull acid hydrolysate as substrate

Production of 2,3-butanediol (2,3-BD) by Pantoea agglomerans strain BL1 was investigated using soybean hull hydrolysate as substrate in batch reactors. The cultivation media consisted of a mixture of xylose, arabinose, and glucose, obtained from the hemicellulosic fraction of the soybean hull biomass. We evaluated the influence of oxygen supply, pH control, and media supplementation on the growth kinetics of the microorganism and on 2,3-BD production. P. agglomerans BL1 was able to simultaneously metabolize all three monosaccharides present in the broth, with average conversions of 75% after 48 h of cultivation. The influence of aeration conditions employed demonstrated the mixed acid pathway of 2,3-BD formation by enterobacteria. Under fully aerated conditions (2 vvm of air), up to 14.02 g L-1 of 2.3-BD in 12 h of cultivation were produced, corresponding to yields of 0.53 g g-1 and a productivity of 1.17 g L-1 h-1, the best results achieved. These results suggest the production potential of 2,3-BD by P. agglomerans BL1, which has been recently isolated from an environmental consortium. The present work proposes a solution for the usage of the hemicellulosic fraction of agroindustry biomasses, carbohydrates whose utilization are not commonly addressed in bioprocess.

Control of the polymyxin analog ratio by domain swapping in the nonribosomal peptide synthetase of Paenibacillus polymyxa

Polymyxins are used as the last-line therapy against multidrug-resistant bacteria. However, their further clinical development needs to solve problems related to the presence of heterogeneous analogs, but there is still no platform or methods that can regulate the biosynthesis of polymyxin analogs. In this study, we present an approach to swap domains in the polymyxin gene cluster to regulate the production of different analogs. Following adenylation domain swapping, the proportion of polymyxin B1 increased from 41.36 to 52.90%, while that of B1-1 decreased from 18.25 to 3.09%. The ratio of polymyxin B1 and B3 following starter condensation domain swapping changed from 41.36 and 16.99 to 55.03 and 6.39%, respectively. The two domain-swapping strains produced 62.96% of polymyxin B1, 6.70% of B3 and 3.32% of B1-1. This study also revealed the presence of overflow fluxes between acetoin, 2,3-butanediol and polymyxin. To our best knowledge, this is the first report of engineering the polymyxin synthetase gene cluster in situ to regulate the relative proportions of polymyxin analogs. This research paves a way for regulating lipopeptide analogs and will facilitate the development of novel lipopeptide derivatives.

Importance of consideration of oxidoreduction potential as a critical quality parameter in food industries

There are many intrinsic and extrinsic factors affecting the nutritional, organoleptic, microbial-enzymatic and physicochemical characteristics of food products. Some of these factors are commonly considered by food processors such as the temperature, water activity, pH, dissolved oxygen and chemical composition, while others are less considered such as the oxidoreduction potential (E_h). This latter factor is an intrinsic electrochemical parameter expressing the tendency of the substance/medium to give or receive electrons. Contrary to what is expected, the important role of E_h is not limited to inorganic chemistry, metallic chemistry, natural water, and wastewater treatment fields but it also covers many domains in biology such as metabolic engineering, enzymatic functions, food safety, and biotechnology. Unfortunately, although the critical roles of E_h in several key reactions occurred in biological media such as food and biotechnological products, its application or controlling is still uncommon or mis-considered by food processors. The lack of specific studies and reviews concerning the E_h and its influences on the quality parameters of products could be a reason for this lack of interest from the side of food processors. Recent studies reported the potential application of this parameter in novel food processing techniques such as reducing atmosphere drying (RAD) of food products and reducing atmosphere packaging (RAP) of fresh food products for preserving the quality attributes and extending the shelf-life of food products. This paper aims to help the technical and operational personnel working in food industry sectors as well as the scientific community to have an updated and a comprehensible review about the E_h parameter permitting its consideration for potential applications in food industries.

High-Efficient Production of (S)-1-[3,5-Bis(trifluoromethyl)phenyl]ethanol via Whole-Cell Catalyst in Deep-Eutectic Solvent-Containing Micro-Aerobic Medium System

The ratio of substrate to catalyst (S/C) is a prime target for the application of asymmetric production of enantiomerically enriched intermediates by whole-cell biocatalyst. In the present study, an attractive increase in S/C was achieved in a natural deep-eutectic solvent (NADES) containing reaction system under microaerobic condition for high production of (S)-1-[3,5-bis(trifluoromethyl)phenyl]ethanol ((S)-3,5-BTPE) with Candida tropicalis 104. In PBS buffer (0.2 M, pH 8.0) at 200 rpm and 30 °C, 79.5 g (Dry Cell Weight, DCW)/L C. tropicalis 104 maintained the same yield of 73.7% for the bioreduction of 3,5-bis(trifluoromethyl)acetophenone (BTAP) under an oxygen-deficient environment compared with oxygen-sufficient conditions, while substrate load increased 4.0-fold (from 50 mM to 200 mM). Furthermore, when choline chloride:trehalose (ChCl:T, 1:1 molar ratio) was introduced into the reaction system for its versatility of increasing cell membrane permeability and declining BTAP cytotoxicity to biocatalyst, the yields were further increased to 86.2% under 200 mM BTAP, or 72.9% at 300 mM BTAP. After the optimization of various reaction parameters involved in the bioreduction, and the amount of biocatalyst and maltose co-substrate remained 79.5 g (DCW)/L and 50 g/L, the S/C for the reduction elevated 6.3 times (3.8 mM/g versus 0.6 mM/g). By altering the respiratory pattern of the whole-cell biocatalyst and exploiting the ChCl:T-containing reaction system, the developed strategy exhibits an attractive potential for enhancing catalytic efficiency of whole-cell-mediated reduction, and provides valuable insight for the development of whole-cell catalysis.

Shake flask methodology for assessing the influence of the maximum oxygen transfer capacity on 2,3-butanediol production

BACKGROUND

Production of 2,3-butanediol from renewable resources is a promising measure to decrease the consumption of fossil resources in the chemical industry. One of the most influential parameters on biotechnological 2,3-butanediol production is the oxygen availability during the cultivation. As 2,3-butanediol is produced under microaerobic process conditions, a well-controlled oxygen supply is the key parameter to control biomass formation and 2,3-butanediol production. As biomass is on the one hand not the final product, but on the other hand the essential biocatalyst, the optimal compromise between biomass formation and 2,3-butanediol production has to be defined.

RESULTS

A shake flask methodology is presented to evaluate the effects of oxygen availability on 2,3-butanediol production with Bacillus licheniformis DSM 8785 by variation of the filling volume. A defined two-stage cultivation strategy was developed to investigate the metabolic response to different defined maximum oxygen transfer capacities at equal initial growth conditions. The respiratory quotient was measured online to determine the point of glucose depletion, as 2,3-butanediol is consumed afterwards. Based on this strategy, comparable results to stirred tank reactors were achieved. The highest space-time yield (1.3 g/L/h) and a 2,3-butanediol concentration of 68 g/L combined with low acetoin concentrations and avoided glycerol formation were achieved at a maximum oxygen transfer capacity of 13 mmol/L/h. The highest overall 2,3-butanediol concentration of 78 g/L was observed at a maximum oxygen transfer capacity of 4 mmol/L/h.

CONCLUSIONS

The presented shake flask approach reduces the experimental effort and costs providing a fast and reliable methodology to investigate the effects of oxygen availability. This can be applied especially on product and by-product formation under microaerobic conditions. Utilization of the maximum oxygen transfer capacity as measure for the oxygen availability allows for an easy adaption to other bioreactor setups and scales.

Identification of a native promoter PLH-77 for gene expression in Paenibacillus polymyxa

Paenibacillus polymyxa is a rhizobacterium that has attracted substantial attention due to its ability to produce functional metabolites and promote plant growth. Metabolic and genetic improvements in this species will benefit research and other applications of the bacterium. However, a suitable gene expression system has not been established in this species. In this study, a promoter trap system based on a green fluorescent protein and a chloramphenicol-resistance gene was developed to isolate native promoters of P. polymyxa SC2-M1 to regulate gene expression. Through high-throughput screening, the novel promoter P_LH-77 was identified, sequenced, and subsequently characterized. Promoter P_LH-77 is a strong, continuous expression system containing the typical -10 and -35 motifs regions. Its effective sequence was evaluated and then cascaded to improve the promotion efficiency. To further verify the existence of P_LH-77, a heterogenous xylose isomerase was expressed by P_LH-77 in P. polymyxa SC2-M1. In the resulting strain, the amount of xylose consumed was increased by 2.5 g/L during the 78 h fermentation period. Meanwhile, the production levels of lactate and acetate increased. It was confirmed that promoter P_LH-77 could effectively mediate gene expression in P. polymyxa SC2-M1 and will further benefit the quantitative monitoring of gene expression in P. polymyxa.

Two different restriction-modification systems for degrading exogenous DNA in Paenibacillus polymyxa

Accompanied by benefits from horizontally transferred genes, bacteria have to face the risk of the invasion of dangerous genes. Bacteria often use the restriction-modification (R-M) system, which is consisted of methyl transferase (MEase) and restrictase (REase), to protect self-DNA and defend against foreign DNA. Paenibacillus polymyxa, widely used as growth promoting rhizobacteria in agriculture, can also produce compounds of medical and industrial interests. It is unclear whether R-M systems exist in P. polymyxa. In this study, we used a shuttle plasmid with epigenetic modification from different bacteria to explore R-M systems in P. polymyxa. We found that DNA which is methylated by DNA adenine methyltransferase (Dam) in E. coli was strongly restricted, indicating the presence of a Dam-methylation-dependent R-M system in P. polymyxa. Whereas, DNA from a dam^-E. coli strain was also moderately restricted, indicating the presence of a Dam-methylation-independent R-M system. Degradation of plasmid DNA with Dam methylation by cell-free protein extract of P. polymyxa provides additional evidence for the presence of Dam-methylation-dependent R-M system. Taken together, our work showed that there are two different types of R-M system in P. polymyxa, providing a foundation for the study of innate immunity in P. polymyxa and for the development of genetic engineering tools in P. polymyxa.

Production of R,R-2,3-butanediol of ultra-high optical purity from Paenibacillus polymyxa ZJ-9 using homologous recombination

The present study describes the use of metabolic engineering to achieve the production of R,R-2,3-butanediol (R,R-2,3-BD) of ultra-high optical purity (>99.99%). To this end, the diacetyl reductase (DAR) gene (dud A) of Paenibacillus polymyxa ZJ-9 was knocked out via homologous recombination between the genome and the previously constructed targeting vector pRN5101-L'C in a process based on homologous single-crossover. PCR verification confirmed the successful isolation of the dud A gene disruption mutant P. polymyxa ZJ-9-△dud A. Moreover, fermentation results indicated that the optical purity of R,R-2,3-BD increased from about 98% to over 99.99%, with a titer of 21.62 g/L in Erlenmeyer flasks. The latter was further increased to 25.88 g/L by fed-batch fermentation in a 5-L bioreactor.

Improvement of sulfamethoxazole (SMX) elimination and inhibition of formations of hydroxylamine-SMX and N4-acetyl-SMX by membrane bioreactor systems

Sulfamethoxazole (SMX) has frequently been detected in aquatic environments. In natural environment, not only individual microorganism but also microbial consortia are involved in some biotransformation of pollutants. The competition for space under consortia causing cell-cell contact inhibition changes the cellular behaviors. Herein, the membrane bioreactor system (MBRS) was applied to improve SMX elimination thorough exchanging the cell-free broths (CFB). The removal efficiency of SMX was increased by more than 24% whether under the pure culture of A. faecalis or under the co-culture of A. faecalis and P. denitrificans with MBRS. Meanwhile, MBRS significantly inhibited the formation of HA-SMX, and Ac-SMX from parent compound. Additionally, the cellular growth under MBRS was obviously enhanced, indicating that the increases in the cellular growth under MBRS are possibly related to the decreases in the levels of HA-SMX and Ac-SMX compared to that without MBRS. The intracellular NADH/NAD⁺ ratios of A. faecalis under MBRS were increased whether thorough itself-recycle of CFB or exchanging CFB between the pure cultures of A. faecalis and P. denitrificans, suggesting that the enhancement in the bioremoval efficiencies of SMX under MBRS by A. faecalis is likely related to the increases in the NADH/NAD⁺ ratio. Taken together, the regulation of cell-to-cell communication is preferable strategy to improve the bioremoval efficiency of SMX.

Enhanced production of 2,3-butanediol from xylose by combinatorial engineering of xylose metabolic pathway and cofactor regeneration in pyruvate decarboxylase-deficient Saccharomyces cerevisiae

The aim of this study was to produce 2,3-butanediol (2,3-BDO) from xylose efficiently by modulation of the xylose metabolic pathway in engineered Saccharomyces cerevisiae. Expression of the Scheffersomyces stipitis transaldolase and NADH-preferring xylose reductase in S. cerevisiae improved xylose consumption rate by a 2.1-fold and 2,3-BDO productivity by a 1.8-fold. Expression of the Lactococcus lactis noxE gene encoding NADH oxidase also increased 2,3-BDO yield by decreasing glycerol accumulation. Additionally, the disadvantage of C₂-dependent growth of pyruvate decarboxylase-deficient (Pdc^-) S. cerevisiae was overcome by expression of the Candida tropicalis PDC1 gene. A fed-batch fermentation of the BD5X-TXmNP strain resulted in 96.8g/L 2,3-BDO and 0.58g/L-h productivity from xylose, which were 15.6- and 2-fold increases compared with the corresponding values of the BD5X strain. It was concluded that facilitation of the xylose metabolic pathway, oxidation of NADH and relief of C₂-dependency synergistically triggered 2,3-BDO production from xylose in Pdc^-S. cerevisiae.

Artificial consortium that produces riboflavin regulates distribution of acetoin and 2,3-butanediol by Paenibacillus polymyxa CJX518

The introduction of an NADH/NAD⁺ regeneration system can regulate the distribution between acetoin and 2,3-butanediol. NADH regeneration can also enhance butanol production in coculture fermentation. In this work, a novel artificial consortium of Paenibacillus polymyxa CJX518 and recombinant Escherichia coli LS02T that produces riboflavin (VB₂) was used to regulate the NADH/NAD⁺ ratio and, consequently, the distribution of acetoin and 2,3-butanediol by P. polymyxa. Compared with a pure culture of P. polymyxa, the level of acetoin was increased 76.7% in the P. polymyxa and recombinant E. coli coculture. Meanwhile, the maximum production and yield of acetoin in an artificial consortium with fed-batch fermentation were 57.2 g/L and 0.4 g/g glucose, respectively. Additionally, the VB₂ production of recombinant E. coli could maintain a relatively low NADH/NAD⁺ ratio by changing NADH dehydrogenase activity. It was also found that 2,3-butanediol dehydrogenase activity was enhanced and improved acetoin production by the addition of exogenous VB₂ or by being in the artificial consortium that produces VB₂. These results illustrate that the coculture of P. polymyxa and recombinant E. coli has enormous potential to improve acetoin production. It was also a novel strategy to regulate the NADH/NAD⁺ ratio to improve the acetoin production of P. polymyxa.

Exogenous cofactors for the improvement of bioremoval and biotransformation of sulfamethoxazole by Alcaligenes faecalis

Sulfamethoxazole (SMX), an extensively prescribed or administered antibiotic pharmaceutical product, is usually detected in aquatic environments, because of its incomplete metabolism and elimination. This study investigated the effects of exogenous cofactors on the bioremoval and biotransformation of SMX by Alcaligenes faecalis. High concentration (100mg·L(-1)) of exogenous vitamin C (VC), vitamin B6 (VB6) and oxidized glutathione (GSSG) enhanced SMX bioremoval, while the additions of vitamin B2 (VB2) and vitamin B12 (VB12) did not significantly alter the SMX removal efficiency. Globally, cellular growth of A. faecalis and SMX removal both initially increased and then gradually decreased, indicating that SMX bioremoval is likely dependent on the primary biomass activity of A. faecalis. The decreases in the SMX removal efficiency indicated that some metabolites of SMX might be transformed into parent compound at the last stage of incubation. Two transformation products of SMX, N-hydroxy sulfamethoxazole (HO-SMX) and N4-acetyl sulfamethoxazole (Ac-SMX), were identified by a high-performance liquid chromatograph coupled with mass spectrometer. High concentrations of VC, nicotinamide adenine dinucleotide hydrogen (NADH, 7.1mg·L(-1)), and nicotinamide adenine dinucleotide (NAD(+), 6.6mg·L(-1)), and low concentrations of reduced glutathione (GSH, 0.1 and 10mg·L(-1)) and VB2 (1mg·L(-1)) remarkably increased the formation of HO-SMX, while VB12 showed opposite effects on HO-SMX formation. In addition, low concentrations of GSH and NADH enhanced Ac-SMX formation by the addition of A. faecalis, whereas cofactors (VC, VB2, VB12, NAD(+), and GSSG) had no obvious impact on the formation of Ac-SMX compared with the controls. The levels of Ac-SMX were stable when biomass of A. faecalis gradually decreased, indicating the direct effect of biomass on the formation of Ac-SMX by A. faecalis. In sum, these results help us understand the roles played by exogenous cofactors in eliminating SMX by A. faecalis and provide potential strategies for improving SMX biodegradation.

Nicotinamide cofactor ratios in engineered strains of Clostridium thermocellum and Thermoanaerobacterium saccharolyticum

Clostridium thermocellum and Thermoanaerobacterium saccharolyticum are bacteria under investigation for production of biofuels from plant biomass. Thermoanaerobacterium saccharolyticum has been engineered to produce ethanol at high yield (>90% of theoretical) and titer (>70 g/l). Efforts to engineer C. thermocellum have not, to date, been as successful, and efforts are underway to transfer the ethanol production pathway from T. saccharolyticum to C. thermocellum One potential challenge in transferring metabolic pathways is the possibility of incompatible levels of nicotinamide cofactors. These cofactors (NAD(+), NADH, NADP(+) and NADPH) and their oxidation state are important in the context of microbial redox metabolism. In this study we directly measured the concentrations and reduced oxidized ratios of these cofactors in a number of strains of C. thermocellum and T. saccharolyticum by using acid/base extraction and enzymatic assays. We found that cofactor ratios are maintained in a fairly narrow range, regardless of the metabolic network modifications considered. We have found that the ratios are similar in both organisms, which is a relevant observation in the context of transferring the T. saccharolyticum ethanol production pathway to C. thermocellum.

Introduction of the exogenous NADH coenzyme regeneration system and its influence on intracellular metabolic flux of Paenibacillus polymyxa

The NAD(+)-dependent formate dehydrogenase (FDH) gene from Candida boidinii was introduced into Paenibacillus polymyxa ZJ-9. The effects of this exogenous gene on the growth of the recombinant strain P. polymyxa XG-1, FDH activity, intracellular NADH and NAD(+) level and the synthesis of R,R-2,3-butanediol (R,R-2,3-BD) were determined. Results from the fermentation in the 7.5L bioreactor showed that the exogenous FDH was highly expressed in the recombinant strain. The titers of NADH, lactic acid, ethanol, NADH/NAD(+), and CO2 excretion rate (CER) of the recombinant strain increased considerably, while acetoin and formic acid decreased significantly. The highest titers of R,R-2,3-BD by the recombinant strain in batch and fed-batch fermentation were 36.8g/L and 51.3g/L, increased 10.2% and 8.0% compared with the parent strain, respectively. This study confirmed that coenzyme regeneration system can manipulate substance metabolism in bacteria, and is an efficient way for promoting the synthesis of NADH-dependent products.

Phenolic compounds: Strong inhibitors derived from lignocellulosic hydrolysate for 2,3-butanediol production by Enterobacter aerogenes

Lignocellulosic biomass are attractive feedstocks for 2,3-butanediol production due to their abundant supply and low price. During the hydrolysis of lignocellulosic biomass, various byproducts are formed and their effects on 2,3-butanediol production were not sufficiently studied compared to ethanol production. Therefore, the effects of compounds derived from lignocellulosic biomass (weak acids, furan derivatives and phenolics) on the cell growth, the 2,3-butanediol production and the enzymes activity involved in 2,3-butanediol production were evaluated using Enterobacter aerogenes ATCC 29007. The phenolic compounds showed the most toxic effects on cell growth, 2,3-butanediol production and enzyme activity, followed by furan derivatives and weak acids. The significant effects were not observed in the presence of acetic acid and formic acid. Also, feasibility of 2,3-butanediol production from lignocellulosic biomass was evaluated using Miscanthus as a feedstock. In the fermentation of Miscanthus hydrolysate, 11.00 g/L of 2,3-butanediol was obtained from 34.62 g/L of reducing sugar. However, 2,3-butanediol was not produced when the concentration of total phenolic compounds in the hydrolysate increased to more than 1.5 g/L. The present study provides useful information to develop strategies for biological production of 2,3-butanediol and to establish biorefinery for biochemicals from lignocellulosic biomass.

Constructing a synthetic constitutive metabolic pathway in Escherichia coli for (R, R)-2,3-butanediol production

Many microorganisms could naturally produce (R, R)-2,3-butanediol ((R, R)-2,3-BD), which has unique applications due to its special chiral group and spatial configuration. But the low enantio-purity of the product hindered the development of large-scale production. In this work, a synthetic constitutive metabolic pathway for enantiomerically pure (R, R)-2,3-BD biosynthesis was constructed in Escherichia coli with vector pUC6S, which does not contain any lac sequences. The expression of this artificial constructed gene cluster was optimized by using two different strength of promoters (AlperPLTet01 (P01) and AlperBB (PBB)). The strength of P01 is twice stronger than PBB. The fermentation results suggested that the yield of (R, R)-2,3-BD was higher when using the stronger promoter. Compared with the wild type, the recombinant strain E. coli YJ2 produced a small amount of acetic acid and showed higher glucose consumption rate and higher cell density, which indicated a protection against acetic acid inhibition. In order to further increase the (R, R)-2,3-BD production by reducing the accumulation of its precursor acetoin, the synthetic operon was reconstructed by adding the strong promoter P01 in front of the gene ydjL coding for the enzyme of (R, R)-2,3-BD dehydrogenase which catalyzes the conversion of acetoin to (R, R)-2,3-BD. The engineered strain E. coli YJ3 showed a 20 % decrease in acetoin production compared with that of E. coli YJ2. After optimization the fermentation conditions, 30.5 g/L of (R, R)-2,3-BD and 3.2 g/L of acetoin were produced from 80 g/L of glucose within 18 h, with an enantio-purity over 99 %.

Metabolic engineering of Bacillus subtilis for redistributing the carbon flux to 2,3-butanediol by manipulating NADH levels

BACKGROUND

Acetoin reductase (Acr) catalyzes the conversion of acetoin to 2,3-butanediol (2,3-BD) with concomitant oxidation of NADH to NAD(+). Therefore, intracellular 2,3-BD production is likely governed by the quantities of rate-limiting factor(s) Acr and/or NADH. Previously, we showed that a high level of Acr was beneficial for 2,3-BD accumulation.

RESULTS

Metabolic engineering strategies were proposed to redistribute carbon flux to 2,3-BD by manipulating NADH levels. The disruption of NADH oxidase (YodC, encoded by yodC) by insertion of a formate dehydrogenase gene in Bacillus subtilis was more efficient for enhancing 2,3-BD production and decreasing acetoin formation than the disruption of YodC by the insertion of a Cat expression cassette. This was because the former resulted in the recombinant strain AFY in which an extra NADH regeneration system was introduced and NADH oxidase was disrupted simultaneously. On fermentation by strain AFY, the highest 2,3-BD concentration increased by 19.9 % while the acetoin titer decreased by 71.9 %, relative to the parental strain. However, the concentration of lactate, the main byproduct, increased by 47.2 %. To further improve carbon flux and NADH to 2,3-BD, the pathway to lactate was blocked using the insertional mutation technique to disrupt the lactate dehydrogenase gene ldhA. The resultant engineered strain B. subtilis AFYL could efficiently convert glucose into 2,3-BD with little acetoin and lactate accumulation.

CONCLUSIONS

Through increasing the availability of NADH and decreasing the concentration of unwanted byproducts, this work demonstrates an important strategy in the metabolic engineering of 2,3-BD production by integrative recombinant hosts.

A metabolic-based approach to improve xylose utilization for fumaric acid production from acid pretreated wheat bran by Rhizopus oryzae

In this work, wheat bran (WB) was utilized as feedstock to synthesize fumaric acid by Rhizopus oryzae. Firstly, the pretreatment process of WB by dilute sulfuric acid hydrolysis undertaken at 100°C for 30min offered the best performance for fumaric acid production. Subsequently, through optimizing the seed culture medium, a suitable morphology (0.55mm pellets diameter) of R. oryzae was obtained. Furthermore, a metabolic-based approach was developed to profile the differences of intracellular metabolites concentration of R. oryzae between xylose (the abundant sugar in wheat bran hydrolysate (WBH)) and glucose metabolism. The xylitol, sedoheptulose 7-phosphate, ribulose 5-phosphate, glucose 6-phosphate, proline and serine were responsible for fumaric acid biosynthesis limitation in xylose fermentation. Consequently, regulation strategies were proposed, leading to a 149% increase in titer (up to 15.4g/L). Finally, by combinatorial regulation strategies the highest production was 20.2g/L from WBH, 477% higher than that of initial medium.

Metabolic engineering of Escherichia coli for production of (2S,3S)-butane-2,3-diol from glucose

BACKGROUND

Butane-2,3-diol (2,3-BD) is a fuel and platform biochemical with various industrial applications. 2,3-BD exists in three stereoisomeric forms: (2R,3R)-2,3-BD, meso-2,3-BD and (2S,3S)-2,3-BD. Microbial fermentative processes have been reported for (2R,3R)-2,3-BD and meso-2,3-BD production.

RESULTS

The production of (2S,3S)-2,3-BD from glucose was acquired by whole cells of recombinant Escherichia coli coexpressing the α-acetolactate synthase and meso-butane-2,3-diol dehydrogenase of Enterobacter cloacae subsp. dissolvens strain SDM. An optimal biocatalyst for (2S,3S)-2,3-BD production, E. coli BL21 (pETDuet-PT7-budB-PT7-budC), was constructed and the bioconversion conditions were optimized. With the addition of 10 mM FeCl3 in the bioconversion system, (2S,3S)-2,3-BD at a concentration of 2.2 g/L was obtained with a stereoisomeric purity of 95.0 % using the metabolically engineered strain from glucose.

CONCLUSIONS

The engineered E. coli strain is the first one that can be used in the direct production of (2S,3S)-2,3-BD from glucose. The results demonstrated that the method developed here would be a promising process for efficient (2S,3S)-2,3-BD production.