From genome sequence to integrated bioprocess for succinic acid production by Mannheimia succiniciproducens

Recent advances in bio-based production of top platform chemical, succinic acid: an alternative to conventional chemistry

Succinic acid (SA) is one of the top platform chemicals with huge applications in diverse sectors. The presence of two carboxylic acid groups on the terminal carbon atoms makes SA a highly functional molecule that can be derivatized into a wide range of products. The biological route for SA production is a cleaner, greener, and promising technological option with huge potential to sequester the potent greenhouse gas, carbon dioxide. The recycling of renewable carbon of biomass (an indirect form of CO2), along with fixing CO2 in the form of SA, offers a carbon-negative SA manufacturing route to reduce atmospheric CO2 load. These attractive attributes compel a paradigm shift from fossil-based to microbial SA manufacturing, as evidenced by several commercial-scale bio-SA production in the last decade. The current review article scrutinizes the existing knowledge and covers SA production by the most efficient SA producers, including several bacteria and yeast strains. The review starts with the biochemistry of the major pathways accumulating SA as an end product. It discusses the SA production from a variety of pure and crude renewable sources by native as well as engineered strains with details of pathway/metabolic, evolutionary, and process engineering approaches for enhancing TYP (titer, yield, and productivity) metrics. The review is then extended to recent progress on separation technologies to recover SA from fermentation broth. Thereafter, SA derivatization opportunities via chemo-catalysis are discussed for various high-value products, which are only a few steps away. The last two sections are devoted to the current scenario of industrial production of bio-SA and associated challenges, along with the author's perspective.

Response mechanisms of different Saccharomyces cerevisiae strains to succinic acid

BACKGROUND

The production of succinic acid (SA) from biomass has attracted worldwide interest. Saccharomyces cerevisiae is preferred for SA production due to its strong tolerance to low pH conditions, ease of genetic manipulation, and extensive application in industrial processes. However, when compared with bacterial producers, the SA titers and productivities achieved by engineered S. cerevisiae strains were relatively low. To develop efficient SA-producing strains, it's necessary to clearly understand how S. cerevisiae cells respond to SA.

RESULTS

In this study, we cultivated five S. cerevisiae strains with different genetic backgrounds under different concentrations of SA. Among them, KF7 and NBRC1958 demonstrated high tolerance to SA, whereas NBRC2018 displayed the least tolerance. Therefore, these three strains were chosen to study how S. cerevisiae responds to SA. Under a concentration of 20 g/L SA, only a few differentially expressed genes were observed in three strains. At the higher concentration of 60 g/L SA, the response mechanisms of the three strains diverged notably. For KF7, genes involved in the glyoxylate cycle were significantly downregulated, whereas genes involved in gluconeogenesis, the pentose phosphate pathway, protein folding, and meiosis were significantly upregulated. For NBRC1958, genes related to the biosynthesis of vitamin B6, thiamin, and purine were significantly downregulated, whereas genes related to protein folding, toxin efflux, and cell wall remodeling were significantly upregulated. For NBRC2018, there was a significant upregulation of genes connected to the pentose phosphate pathway, gluconeogenesis, fatty acid utilization, and protein folding, except for the small heat shock protein gene HSP26. Overexpression of HSP26 and HSP42 notably enhanced the cell growth of NBRC1958 both in the presence and absence of SA.

CONCLUSIONS

The inherent activities of small heat shock proteins, the levels of acetyl-CoA and the strains' potential capacity to consume SA all seem to affect the responses and tolerances of S. cerevisiae strains to SA. These factors should be taken into consideration when choosing host strains for SA production. This study provides a theoretical basis and identifies potential host strains for the development of robust and efficient SA-producing strains.

A Review on the Production of C4 Platform Chemicals from Biochemical Conversion of Sugar Crop Processing Products and By-Products

The development and commercialization of sustainable chemicals from agricultural products and by-products is necessary for a circular economy built on renewable natural resources. Among the largest contributors to the final cost of a biomass conversion product is the cost of the initial biomass feedstock, representing a significant challenge in effective biomass utilization. Another major challenge is in identifying the correct products for development, which must be able to satisfy the need for both low-cost, drop-in fossil fuel replacements and novel, high-value fine chemicals (and/or commodity chemicals). Both challenges can be met by utilizing wastes or by-products from biomass processing, which have very limited starting cost, to yield platform chemicals. Specifically, sugar crop processing (e.g., sugarcane, sugar beet) is a mature industry that produces high volumes of by-products with significant potential for valorization. This review focuses specifically on the production of acetoin (3-hydroxybutanone), 2,3-butanediol, and C4 dicarboxylic (succinic, malic, and fumaric) acids with emphasis on biochemical conversion and targeted upgrading of sugar crop products/by-products. These C4 compounds are easily derived from fermentations and can be converted into many different final products, including food, fragrance, and cosmetic additives, as well as sustainable biofuels and other chemicals. State-of-the-art literature pertaining to optimization strategies for microbial conversion of sugar crop byproducts to C4 chemicals (e.g., bagasse, molasses) is reviewed, along with potential routes for upgrading and valorization. Directions and opportunities for future research and industrial biotechnology development are discussed.

Insights on the Advancements of In Silico Metabolic Studies of Succinic Acid Producing Microorganisms: A Review with Emphasis on Actinobacillus succinogenes

Succinic acid (SA) is one of the top candidate value-added chemicals that can be produced from biomass via microbial fermentation. A considerable number of cell factories have been proposed in the past two decades as native as well as non-native SA producers. Actinobacillus succinogenes is among the best and earliest known natural SA producers. However, its industrial application has not yet been realized due to various underlying challenges. Previous studies revealed that the optimization of environmental conditions alone could not entirely resolve these critical problems. On the other hand, microbial in silico metabolic modeling approaches have lately been the center of attention and have been applied for the efficient production of valuable commodities including SA. Then again, literature survey results indicated the absence of up-to-date reviews assessing this issue, specifically concerning SA production. Hence, this review was designed to discuss accomplishments and future perspectives of in silico studies on the metabolic capabilities of SA producers. Herein, research progress on SA and A. succinogenes, pathways involved in SA production, metabolic models of SA-producing microorganisms, and status, limitations and prospects on in silico studies of A. succinogenes were elaborated. All in all, this review is believed to provide insights to understand the current scenario and to develop efficient mathematical models for designing robust SA-producing microbial strains.

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

BACKGROUND

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

RESULTS

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

CONCLUSIONS

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

Photochemistry and mechanism of designed pyrenyl probe towards promoted cleavage of proteins

A new photochemical reagent, succinic acid-1(1-pyrene)methylamide (PMA-SUC), was developed to recognize the specific binding sites on model proteins, egg-white lysozyme and avidin. The interaction of the photochemical reagent with the proteins was studied by UV-Vis, fluorescence spectroscopic methods and docking description. PMA-SUC was found to bind to lysozyme and avidin with binding constants (K_b) of 2.4×10⁵ and 6.7×10⁵ (M^-1), respectively. The fluorescence intensity of PMA-SUC decreased with increasing concentration of both proteins. Quenching of PMA-SUC fluorescence, in the absence and presence of the protein by an electron acceptor (Hexaamminecobalt(III) chloride, Co(NH₃)₆Cl₃) showed no significant changes in the K_sv values (Stern-Volmer quenching constant), indicating that PMA-SUC bound to the hydrophilic sites or near the surface of the proteins. Irradiation of protein-PMA-SUC mixture, at 342nm for a period of time, in the presence of Co(NH₃)₆Cl₃ as an electron acceptor, resulted in the cleavage of both proteins with high specificity. Binding mechanisms were studied using Molecular docking method. Molecular docking study indicated the position of PMA-SUC upon binding to the proteins by hydrogen bonding interaction with donor-acceptor within the distance of less than 5Å in the minimum of binding free energy. The docking results have supported the results obtained from the spectroscopic methods and cleavage studies.

Production of succinic acid from sugarcane molasses supplemented with a mixture of corn steep liquor powder and peanut meal as nitrogen sources by Actinobacillus succinogenes

The potential of using corn steep liquor powder (CSLP), peanut meal (PM), soybean meal (SM), cotton meal (CM) and urea as the substitute of yeast extract (YE) as the nitrogen source was investigated for producing succinic acid (SA). Actinobacillus succinogenes GXAS137 was used as the fermenting bacterium and sugarcane molasses was used as the main substrate. None of these materials were able to produce SA as high as YE did. The CSLP could still be considered as a feasible and inexpensive alternate for YE as the yield of SA produced using CSLP was second only to the yield of SA obtained by YE. The use of CSLP-PM mixed formulation (CSLP to PM ratio = 2·6) as nitrogen source produced SA up to 59·2 g l(-1) with a productivity of 1·2 g l(-1) h(-1). A batch fermentation using a stirred bioreactor produced up to 60·7 g l(-1) of SA at the same formulation. Fed-batch fermentation that minimized the substrate inhibition produced 64·7 g l(-1) SA. These results suggest that sugarcane molasses supplemented with a mixture of CSLP and PM as the nitrogen source could be used to produce SA more economically using A. succinogenes. Significance and impact of the study: Succinic acid (SA) is commonly used as a platform chemical to produce a number of high value derivatives. Yeast extract (YE) is used as a nitrogen source to produce SA. The high cost of YE is currently the limiting factor for industrial production of SA. This study reports the use of a mixture of corn steep liquor powder (CSLP) and peanut meal (PM) as an inexpensive nitrogen source to substitute YE. The results showed that this CSLP-PM mixed formulation can be used as an effective and economic nitrogen source for the production of SA.

Advances in metabolic engineering of yeast Saccharomyces cerevisiae for production of chemicals

Yeast Saccharomyces cerevisiae is an important industrial host for production of enzymes, pharmaceutical and nutraceutical ingredients and recently also commodity chemicals and biofuels. Here, we review the advances in modeling and synthetic biology tools and how these tools can speed up the development of yeast cell factories. We also present an overview of metabolic engineering strategies for developing yeast strains for production of polymer monomers: lactic, succinic, and cis,cis-muconic acids. S. cerevisiae has already firmly established itself as a cell factory in industrial biotechnology and the advances in yeast strain engineering will stimulate development of novel yeast-based processes for chemicals production.

Evolution of pyruvate kinase-deficient Escherichia coli mutants enables glycerol-based cell growth and succinate production

AIMS

The aim of this study was to engineer Escherichia coli strains that efficiently produce succinate from glycerol under anaerobic conditions after an aerobic growth phase.

METHODS AND RESULTS

We constructed E. coli strain ss195 with deletions of pykA and pykF, which resulted in slow growth on glycerol as sole carbon source. This growth defect was overcome by the selection of fast-growing mutants. Whole-genome resequencing of the evolved mutant ss251 identified the mutation A595S in PEP carboxylase (Ppc). Reverse metabolic engineering by introducing the wild-type allele revealed that this mutation is crucial for the described phenotype. Strain ss251 and derivatives thereof produced succinate with high yields above 80% mol mol(-1) from glycerol under nongrowth conditions.

CONCLUSIONS

The results show that during the aerobic growth of ss251, the formation of pyruvate proceeds via the proposed POMP pathway, starting with the carboxylation of PEP by Ppc. The resulting oxaloacetate is reduced by malate dehydrogenase (Mdh) to malate, which is then decarboxylated back to pyruvate by a malic enzyme (MaeA or MaeB). Mutation of ppc is crucial for fast growth of pykAF mutants on glycerol.

SIGNIFICANCE AND IMPACT OF STUDY

An E. coli mutant that is capable of achieving high yields of succinate (a top valued-added chemical) from glycerol (an abundant carbon source) was constructed. The identified ppc mutation could be applied to other production strains that require strong PEP carboxylation fluxes.

Systems-wide analysis and engineering of metabolic pathway fluxes in bio-succinate producing Basfia succiniciproducens

Basfia succiniciproducens has been recently isolated as novel producer for succinate, an important platform chemical. In batch culture, the wild type exhibited a high natural yield of 0.75 mol succinate (mol glucose)⁻¹. Systems-wide ¹³C metabolic flux analysis identified undesired fluxes through pyruvate-formate lyase (PflD) and lactate dehydrogenase (LdhA). The double deletion strain B. succiniciproducens ΔldhA ΔpflD revealed a 45% improved product yield of 1.08 mol mol⁻¹. In addition, metabolic flux analysis unraveled the parallel in vivo activity of the oxidative and reductive branch of the TCA cycle in B. succiniciproducens, whereby the oxidative part mainly served for anabolism. The wild type re-directed surplus NADH via a cycle involving malic enzyme or via transhydrogenase, respectively, to supply NADPH for anabolism, because the fluxes through the oxidative PPP and isocitrate dehydrogenase, that also provide this cofactor, were not sufficient. This was not observed for the deletion mutants, B. succiniciproducens ΔpflD and ΔldhA ΔpflD, where PPP and isocitrate dehydrogenase flux alone matched with the reduced anabolic NADPH demand. The integration of the production performance into the theoretical flux space, computed by elementary flux mode analysis, revealed that B. succiniciproducens ΔldhA ΔpflD reached 62% of the theoretical maximum yield.

Applying systems biology methods to the study of human physiology in extreme environments

Systems biology is defined in this review as ‘an iterative process of computational model building and experimental model revision with the aim of understanding or simulating complex biological systems’. We propose that, in practice, systems biology rests on three pillars: computation, the omics disciplines and repeated experimental perturbation of the system of interest. The number of ethical and physiologically relevant perturbations that can be used in experiments on healthy humans is extremely limited and principally comprises exercise, nutrition, infusions (e.g. Intralipid), some drugs and altered environment. Thus, we argue that systems biology and environmental physiology are natural symbionts for those interested in a system-level understanding of human biology. However, despite excellent progress in high-altitude genetics and several proteomics studies, systems biology research into human adaptation to extreme environments is in its infancy. A brief description and overview of systems biology in its current guise is given, followed by a mini review of computational methods used for modelling biological systems. Special attention is given to high-altitude research, metabolic network reconstruction and constraint-based modelling.

Production of succinic acid by engineered E. coli strains using soybean carbohydrates as feedstock under aerobic fermentation conditions

Escherichia coli strains HL2765 and HL27659k harboring pRU600 and pKK313 were examined for succinate production under aerobic conditions using galactose, sucrose, raffinose, stachyose, and mixtures of these sugars extracted from soybean meal and soy solubles. HL2765(pKK313)(pRU600) and HL27659k(pKK313)(pRU600) consumed 87mM and 98mM hexose of soybean meal extract and produced 83mM and 95mM succinate, respectively. While using soy solubles extract, HL2765(pKK313)(pRU600) and HL27659k(pKK313)(pRU600) consumed 160mM and 187mM hexose and produced 158mM and 183mM succinate, respectively. Succinate yield of HL2765(pKK313)(pRU600) was low as compared to that of HL27659k(pKK313)(pRU600) while using acid hydrolysate of soybean meal or soy solubles extracts. Maximum succinate production of 312mM with a molar yield of 0.82mol/mol hexose was obtained using soy solubles hydrolysate by HL27659k(pKK313)(pRU600). This study demonstrated the use of soluble carbohydrates of the renewable feedstock, soybean as an inexpensive carbon source to produce succinate by fermentation.

Significance of CO2 donor on the production of succinic acid by Actinobacillus succinogenes ATCC 55618

Background

Succinic acid is a building-block chemical which could be used as the precursor of many industrial products. The dissolved CO₂ concentration in the fermentation broth could strongly regulate the metabolic flux of carbon and the activity of phosphoenolpyruvate (PEP) carboxykinase, which are the important committed steps for the biosynthesis of succinic acid by Actinobacillus succinogenes. Previous reports showed that succinic acid production could be promoted by regulating the supply of CO₂ donor in the fermentation broth. Therefore, the effects of dissolved CO₂ concentration and MgCO₃ on the fermentation process should be investigated. In this article, we studied the impacts of gaseous CO₂ partial pressure, dissolved CO₂ concentration, and the addition amount of MgCO₃ on succinic acid production by Actinobacillus succinogenes ATCC 55618. We also demonstrated that gaseous CO₂ could be removed when MgCO₃ was fully supplied.

Results

An effective CO₂ quantitative mathematical model was developed to calculate the dissolved CO₂ concentration in the fermentation broth. The highest succinic acid production of 61.92 g/L was obtained at 159.22 mM dissolved CO₂ concentration, which was supplied by 40 g/L MgCO₃ at the CO₂ partial pressure of 101.33 kPa. When MgCO₃ was used as the only CO₂ donor, a maximal succinic acid production of 56.1 g/L was obtained, which was just decreased by 7.03% compared with that obtained under the supply of gaseous CO₂ and MgCO₃.

Conclusions

Besides the high dissolved CO₂ concentration, the excessive addition of MgCO₃ was beneficial to promote the succinic acid synthesis. This was the first report investigating the replaceable of gaseous CO2 in the fermentation of succinic acid. The results obtained in this study may be useful for reducing the cost of succinic acid fermentation process.

Succinate production in Escherichia coli

Succinate has been recognized as an important platform chemical that can be produced from biomass. While a number of organisms are capable of succinate production naturally, this review focuses on the engineering of Escherichia coli for the production of four-carbon dicarboxylic acid. Important features of a succinate production system are to achieve an optimal balance of reducing equivalents generated by consumption of the feedstock, while maximizing the amount of carbon channeled into the product. Aerobic and anaerobic production strains have been developed and applied to production from glucose and other abundant carbon sources. Metabolic engineering methods and strain evolution have been used and supplemented by the recent application of systems biology and in silico modeling tools to construct optimal production strains. The metabolic capacity of the production strain, the requirement for efficient recovery of succinate, and the reliability of the performance under scaleup are important in the overall process. The costs of the overall biorefinery-compatible process will determine the economic commercialization of succinate and its impact in larger chemical markets.

Simultaneous saccharification and fermentation of acid-pretreated rapeseed meal for succinic acid production using Actinobacillus succinogenes

Rapeseed meal was evaluated for succinic acid production by simultaneous saccharification and fermentation using Actinobacillus succinogenes ATCC 55618. Diluted sulfuric acid pretreatment and subsequent hydrolysis with pectinase was used to release sugars from rapeseed meal. The effects of culture pH, pectinase loading and yeast extract concentration on succinic acid production were investigated. When simultaneous saccharification and fermentation of diluted acid pretreated rapeseed meal with a dry matter content of 12.5% (w/v) was performed at pH 6.4 and a pectinase loading of 2% (w/w, on dry matter) without supplementation of yeast extract, a succinic acid concentration of 15.5 g/L was obtained at a yield of 12.4 g/100g dry matter. Fed-batch simultaneous saccharification and fermentation was carried out with supplementation of concentrated pretreated rapeseed meal and pectinase at 18 and 28 h to yield a final dry matter content of 20.5% and pectinase loading of 2%, with the succinic acid concentration enhanced to 23.4 g/L at a yield of 11.5 g/100g dry matter and a productivity of 0.33 g/(Lh). This study suggests that rapeseed meal may be an alternative substrate for the efficient production of succinic acid by A. succinogenes without requiring nitrogen source supplementation.

Long-term continuous adaptation of Escherichia coli to high succinate stress and transcriptome analysis of the tolerant strain

To understand the responses of Escherichia coli to high succinate stress and to determine the roles of upregulated genes in high succinate tolerance, a continuous culture of wild-type E. coli W3110 was performed for 268 days in a gradually increasing concentration of succinate. Growth of the final adapted strain, designated DST160, proceeded growth rate of 0.20 h(-1) without a lag phase in medium containing 0.592 M succinate, while the wild-type strain showed 0.02 h(-1) in 38 h. The growth rates of DST160 in media containing either 0.61 M NaCl, 0.61 M KCl, or at pH 4.5 were 25% higher, 18% lower, and 57% higher than those of wild-type, respectively, implying DST160 acquired salt tolerance and pH shock tolerance as well as succinate tolerance. DNA microarray and real-time PCR results indicated that genes controlling active transport and biosynthesis of osmoprotectants were upregulated in DST160 compared to W3110. When ygjE, encoding a putative tartrate/succinate antiporter, and betA, encoding betaine biosynthesis, were expressed in a wild-type E. coli as represent genes for active transport and osmoprotectant synthesis, respectively, greater growth rates were achieved under 0.592 M succinate stress conditions (seven times higher due to ygjE expression and six times higher due to betA expression) than wild-type. The potential to design a metabolic engineering for microbial succinate production is suggested based on the transcriptional regulation of the long-term adapted DST160.

Kinetic evaluation of products inhibition to succinic acid producers Escherichia coli NZN111, AFP111, BL21, and Actinobacillus succinogenes 130Z T

Succinic acid is one of the platform compounds and its production via natural feedstocks has drawn worldwide concerns. To evaluate the inhibitory effects of fermentation products on the growth of Actinobacillus succinogenes 130Z(T) and Escherichia coli NZN111, AFP111, BL21, fermentations with addition of individual products in medium were carried out. The cell growth was inhibited when the concentrations of formate, acetate, lactate, and succinate were at range of 8.8-17.6 g/L, 10-40 g/L, 9-18 g/L, and 10-80 g/L, respectively. For these two species of bacteria, E. coli was more resistant to acid products than A. succinogenes, while both endured succinate rather than by-products. As a result of end product inhibition, succinate production yield by A. succinogenes decreased from 1.11 to 0.49 g/g glucose. Logistic and Monod mathematical models were presented to simulate the inhibition kinetics. The Logistic model was found more suitable for describing the overall synergistic inhibitory effects.

Applications of genome-scale metabolic reconstructions

The availability and utility of genome-scale metabolic reconstructions have exploded since the first genome-scale reconstruction was published a decade ago. Reconstructions have now been built for a wide variety of organisms, and have been used toward five major ends: (1) contextualization of high-throughput data, (2) guidance of metabolic engineering, (3) directing hypothesis-driven discovery, (4) interrogation of multi-species relationships, and (5) network property discovery. In this review, we examine the many uses and future directions of genome-scale metabolic reconstructions, and we highlight trends and opportunities in the field that will make the greatest impact on many fields of biology.

Conversion of MDCK cell line to suspension culture by transfecting with human siat7e gene and its application for influenza virus production

MDCK cells are currently being considered as an alternative to embryonated eggs for influenza virus propagation and hemagglutinin (HA) production intended for vaccine manufacturing. MDCK cells were found suitable for the virus production but their inability to grow in suspension burdens the process of scale up and hence their production capability. Anchorage-dependent MDCK cells were converted to anchorage-independent cells, capable of growing in suspension as a result of transfection with the human siat7e gene (ST6GalNac V). This gene was previously identified as having an important role in cellular adhesion when the transcriptions of genes from anchorage-dependent and anchorage-independent HeLa cells were compared. Unlike the parental MDCK cells, the siat7e-expressing cells were capable of growing in shake flasks as suspension cultures, achieving maximum concentration of 7 x 10(5) cells/mL while keeping close to 100% viability throughout the growth phase. In production experiments, the siat7e-expressing cells were infected with the Influenza B/Victoria/504/2000 strain. It was determined that the cell-derived viruses retained similar antigenic properties as those obtained from egg-derived viruses and their nucleotide sequences were identical. The specific production of hemagglutinin (expressed in hemagglutination units per 10(6) cells) from the siat7e-expressing cells was approximately 20 times higher than the specific production from the parental MDCK cells. If this suspension process scales up, the production potential of HA from 10 L of siat7e-expressing cells at a concentration of 10(6) cells/mL would be equivalent to the amount of HA obtained from 10,000 embryonated eggs.

Kinetic study on succinic acid and acetic acid formation during continuous cultures of Anaerobiospirillum succiniciproducens grown on glycerol

Succinic acid-producing Anaerobiospirillum succiniciproducens was anaerobically grown in a glycerol-fed continuous bioreactor in order to investigate the physiological responses of the cell to different pH values (5.9, 6.2, or 6.5) and various dilution rates, D. In these experiments, A. succiniciproducens showed a pH-dependent glycerol consumption behavior. When pH was maintained at 5.9 or 6.5, glycerol started to accumulate even at a very low D of 0.027 h(-1). Succinic acid yield was not significantly affected by the pH of the culture or the Ds. However, more acetic acid formation was observed when the growth rate of A. succiniciproducens was fast on glycerol at pH 6.2 (at D > or = 0.15 h(-1)). The highest obtainable succinic acid/acetic acid ratio was 40:1, which was 10 times higher than that obtained by batch cultures grown on glucose. The maximum obtainable productivity of succinic acid was 2.1 g L(-1) h(-1)), which was 14 times higher than that obtained by batch culture.

In silico analysis of the effects of H2 and CO2 on the metabolism of a capnophilic bacterium Mannheimia succiniciproducens

This study presents an in-depth study on the physiological behavior of Mannheimia succiniciproducens, a capnophilic bacterium and an efficient succinic acid producer, under varying gas conditions as H(2) and CO(2) play important roles in the production of succinic acid. Constraints-based flux analysis of the genome-scale metabolic model of M. succiniciproducens was performed to estimate the production patterns of several organic acids in response to varying H(2), CO(2), and glucose uptake rates. Results from controlled cultivations performed previously and constraints-based flux analyses of M. succiniciproducens in this study revealed that there is an optimal range of CO(2) level in the medium for enhancing cell growth and succinic acid production at a given glucose uptake rate. Furthermore, the uptake rates of H(2) and CO(2) from the medium have a direct relationship with each other, significantly influencing the rates of cell growth and succinic acid production. Predictions made in this study quantitatively describe the physiological changes of the cell in response to varying H(2), CO(2), and glucose uptake rates, which consequently allow us to identify the feasible physiological states of the cell with respect to cell growth rate and succinic acid production rate.