Butyric acid production from lignocellulosic biomass hydrolysates by engineered Clostridium tyrobutyricum overexpressing Class I heat shock protein GroESL

Systems Metabolic Engineering of Clostridium tyrobutyricum for 1,3-Propanediol Production From Crude Glycerol

Clostridium tyrobutyricum has emerged as a non-pathogenic microbial cell factory capable of anaerobic production of various value-added products, such as butyrate, butanol, and butyl butyrate. This study reports the first systematic engineering of C. tyrobutyricum for the heterologous production of 1,3-propanediol (1,3-PDO) from industrial by-product crude glycerol. Initially, the glycerol reductive pathway for 1,3-PDO production was constructed, and the unique glycerol oxidation pathway in C. tyrobutyricum was elucidated. Subsequently, the glycerol metabolism and 1,3-PDO synthesis pathways were enhanced. Furthermore, the intracellular reducing power supply and the fermentation process were optimized to improve 1,3-PDO production. Consequently, 54.06 g/L 1,3-PDO with a yield of 0.64 mol/mol and a productivity of 1.13 g/L·h was obtained using crude glycerol and fish meal. The strategies described herein could facilitate the engineering of C. tyrobutyricum as a robust host for synthesizing valuable chemicals.

Lignocellulose-derived inhibitors can extend residence of Clostridium beijerinckii in active solventogenic state

Lignocellulose is a promising renewable resource for producing platform chemicals, such as acetone, butanol, and ethanol, via ABE fermentation by solventogenic clostridia. This study investigates the effects of common lignocellulose derived inhibitory compounds: ferulic acid, coumaric acid, and furfural on Clostridium beijerinckii. Dual-staining with propidium iodide and CFDA, combined with flow cytometry, was employed to assess physiological variability. The results showed that phenolic acid-induced stress helped maintain a higher proportion of viable cells during the production phase, enhancing solvent yields and reducing sporulation. At 0.4 g/L, ferulic and coumaric acids did not reduce cell viability; however, coumaric acid exposure led to an acid-crash profile. Conversely, a more robust inoculum exposed to both phenolic acids simultaneously exhibited effects similar to ferulic acid alone, including slower viability decline, reduced growth and sporulation, and improved solvent production. Furfural exposure at 1.5 g/L resulted in immediate viability loss in 20% of the population, though the overall decline accompanied by the highest sporulation rate occurred later than in the control. Additionally, furfural transformation was slower, suppressing butyrate production and reducing solvent production by 13%. This study suggests that delaying cell death mechanism may explain the stimulatory effects of inhibitors, advancing lignocellulose use in the future.

Enhanced tolerance of Clostridium tyrobutyricum to lignin-derived phenolic acids by overexpressing native reductases

Ferulic acid (Fer) and p-coumaric acid (Coum) are major phenolic inhibitors in lignocellulosic hydrolysates that severely hinder the growth and metabolism of Clostridia species. This study demonstrates that the reduction of Fer and Coum to dihydroferulic acid and phloretic acid by Clostridium tyrobutyricum significantly alleviates their toxicity. Overexpression of the dho1 and sdr1 genes, encoding Fer and Coum reductases, respectively, in C. tyrobutyricum can significantly enhance tolerance to these phenolic acids. As a result, the recombinant strain ATCC 25755/ds, which co-overexpresses dho1 and sdr1, exhibited a marked increase in butyrate production compared to the wild-type strain under phenolic acid stress. In fed-batch fermentation with a 1.0 g/L mixture of Fer and Coum (1:1, w/w), ATCC 25755/ds showed a 35.1 % increase in butyrate production and a 61.1 % higher productivity. These results indicate that enhancing phenolic acid reduction can significantly improve Clostridia's tolerance to phenolic acids, thereby strengthening the biotransformation of lignocellulose hydrolysates.

Synthetic biology approaches to improve tolerance of inhibitors in lignocellulosic hydrolysates

Increasing attention is being focused on using lignocellulose for valuable products. Microbial decomposition can convert lignocellulose into renewable biofuels and other high-value bioproducts, contributing to sustainable development. However, the presence of inhibitors in lignocellulosic hydrolysates can negatively affect microorganisms during fermentation. Improving microbial tolerance to these hydrolysates is a major focus in metabolic engineering. Traditional detoxification methods increase costs, so there is a need for cheap and efficient cell-based detoxification strategies. Synthetic biology approaches offer several strategies for improving microbial tolerance, including redox balancing, membrane engineering, omics-guided technologies, expression of protectants and transcription factors, irrational engineering, cell flocculation, and other novel technologies. Advances in molecular biology, high-throughput sequencing, and artificial intelligence (AI) allow for precise strain modification and efficient industrial production. Developing AI-based computational models to guide synthetic biology efforts and creating large-scale heterologous libraries with automation and high-throughput technologies will be important for future research.

Efficient production of butyric acid from lignocellulosic biomass by revealing the mechanisms of Clostridium tyrobutyricum tolerance to phenolic inhibitors

Phenolic compounds (PCs) generated during pretreatment of lignocellulosic biomass severely hinder the biorefinery by Clostridia. As a hyperbutyrate-producing strain, Clostridium tyrobutyricum has excellent tolerance to PCs, but its tolerance mechanism is poorly understood. In this study, a comprehensive transcriptome analysis was applied to elucidate the response of C. tyrobutyricum to four typical PCs. The findings revealed that the expression levels of genes associated with PC reduction, HSPs, and membrane transport were significantly altered under PC stress. Due to PCs being reduced to low-toxicity alcohols/acids by C. tyrobutyricum, enhancing the reduction of PCs by overexpressing reductase genes could enhance the strain's tolerance to PCs. Under 1.0 g/L p-coumaric acid stress, compared with the wild-type strain, ATCC 25755/sdr1 exhibited a 31.2 % increase in butyrate production and a 38.5 % increase in productivity. These insights contribute to the construction of PC-tolerant Clostridia, which holds promise for improving biofuel and chemical production from lignocellulosic biomass.

Engineering of Clostridium tyrobutyricum for butyric acid and butyl butyrate production from cassava starch

Clostridium tyrobutyricum has been successfully engineered to produce butyrate, butanol, butyl butyrate, and γ-aminobutyric acid. It would be interesting to produce bio-chemicals and bio-fuels directly using starch from non-food crop, e.g., cassava, by engineered C. tyrobutyricum. In this study, heterologous α-amylases were screened and expressed in C. tyrobutyricum, resulting in successfully starch hydrolyzation. Furthermore, α-glucosidase (AgluI) was co-expressed with α-amylases, resulting in enhancement in the capacity of starch hydrolyzation and butyrate production. When increasing the cassava starch concentration to 100 g/L, the engineered strain CTAA05 produced 27.0 g/L butyrate. In addition, when introducing butyl butyrate synthetic pathway, strain MU3-AAV produced 26.8 g/L butyl butyrate with 100 g/L cassava starch as substrate. This study showed a generalizable framework to engineered anaerobes for anaerobic production of bio-chemicals and bio-fuels from starchy biomass.

Advance of tolerance engineering on microbes for industrial production

Industrial microbes have become the core of biological manufacturing, which utilized as the cell factory for production of plenty of chemicals, fuels and medicine. However, the challenge that the extreme stress conditions exist in production is unavoidable for cell factory. Consequently, to enhance robustness of the chassis cell lays the foundation for development of bio-manufacturing. Currently, the researches on cell tolerance covered various aspects, involving reshaping regulatory network, cell membrane modification and other stress response. In fact, the strategies employed to improve cell robustness could be summarized into two directions, irrational engineering and rational engineering. In this review, the metabolic engineering technologies on enhancement of microbe tolerance to industrial conditions are summarized. Meanwhile, the novel thoughts emerged with the development of biological instruments and synthetic biology are discussed.

De novo biosynthesis of butyl butyrate in engineered Clostridium tyrobutyricum

Butyl butyrate has broad applications in foods, cosmetics, solvents, and biofuels. Microbial synthesis of bio-based butyl butyrate has been regarded as a promising approach recently. Herein, we engineered Clostridium tyrobutyricum ATCC 25755 to achieve de novo biosynthesis of butyl butyrate from fermentable sugars. Through introducing the butanol synthetic pathway (enzyme AdhE2), screening alcohol acyltransferases (AATs), adjusting transcription of VAAT and adhE2 (i.e., optimizing promoter), and efficient supplying butyryl-CoA, an excellent engineered strain, named MUV3, was obtained with ability to produce 4.58 g/L butyl butyrate at 25 °C with glucose in serum bottles. More NADH is needed for butyl butyrate synthesis, thus mannitol (the more reduced substrate) was employed to produce butyl butyrate. Ultimately, 62.59 g/L butyl butyrate with a selectivity of 95.97%, and a yield of 0.21 mol/mol was obtained under mannitol with fed-batch fermentation in a 5 L bioreactor, which is the highest butyl butyrate titer reported so far. Altogether, this study presents an anaerobic fermentative platform for de novo biosynthesis of butyl butyrate in one step, which lays the foundation for butyl butyrate biosynthesis from renewable biomass feedstocks.

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.

Deciphering of the Mannitol Metabolism Pathway in Clostridium tyrobutyricum ATCC 25755 by Comparative Transcriptome Analysis

Clostridium tyrobutyricum has great potential for bio-based chemicals and biofuel production from mannitol; however, the mannitol metabolic pathway and its metabolic regulatory mechanism have not been elucidated. To this end, the RNA-seq analysis on the mid-log growth phase of C. tyrobutyricum grown on mannitol or xylose was performed. Comparative transcriptome analysis and co-transcription experiment indicated that mtlARFD, which encodes the mannitol-specific IIA component, transcription activator, mannitol-specific IIBC components, and mannitol-1-phosphate 5-dehydrogenase, respectively, formed a polycistronic operon and could be responsible for mannitol uptake and metabolism. In addition, comparative genomic analysis of the mtlARFD organization and the MtlR protein structural domain among various Firmicutes strains identified the putative cre (catabolite-responsive element) sites and conserved phosphorylation sites, but whether the expression of mannitol operon was affected by CcpA- and MtlR-mediated metabolic regulation during mixed substrate fermentation needs to be further verified experimentally. Based on the gene knockout and complementation results, the predicted mannitol operon mtlARFD was confirmed to be responsible for mannitol utilization in C. tyrobutyricum. The results of this study could be used to enhance the mannitol metabolic pathway and explore the potential metabolic regulation mechanism of mannitol during mixed substrate fermentation.

Elimination of carbon catabolite repression in Clostridium tyrobutyricum for enhanced butyric acid production from lignocellulosic hydrolysates

Clostridium tyrobutyricum, a gram-positive anaerobic bacterium, is recognized as the promising butyric acid producer. But, the existence of carbon catabolite repression (CCR) is the major drawback for C. tyrobutyricum to efficiently use the lignocellulosic biomass. In this study, the xylose pathway genes were first identified and verified. Then, the potential regulatory mechanisms of CCR in C. tyrobutyricum were proposed and the predicted engineering targets were experimental validated. Inactivation of hprK blocked the CcpA-mediated CCR and resulted in simultaneous conversion of glucose and xylose, although xylose consumption was severe lagging behind. Deletion of xylR further shortened the lag phase of xylose utilization. When hprK and xylR were inactivated together, the CCR in C. tyrobutyricum was completely eliminated. Consequently, ATCC 25755/ΔhprKΔxylR showed significant increase in butyrate productivity (1.8 times faster than the control) and excellent butyric acid fermentation performance using both mixed sugars (11.0-11.9 g/L) and undetoxified lignocellulosic hydrolysates (12.4-13.4 g/L).

Consolidated microbial production of four-, five-, and six-carbon organic acids from crop residues: Current status and perspectives

The production of platform organic acids has been heavily dependent on petroleum-based industries. However, petrochemical-based industries that cannot guarantee a virtuous cycle of carbons released during various processes are now facing obsolescence because of the depletion of finite fossil fuel reserves and associated environmental pollutions. Thus, the transition into a circular economy in terms of the carbon footprint has been evaluated with the development of efficient microbial cell factories using renewable feedstocks. Herein, the recent progress on bio-based production of organic acids with four-, five-, and six-carbon backbones, including butyric acid and 3-hydroxybutyric acid (C4), 5-aminolevulinic acid and citramalic acid (C5), and hexanoic acid (C6), is discussed. Then, the current research on the production of C4-C6 organic acids is illustrated to suggest future directions for developing crop-residue based consolidated bioprocessing of C4-C6 organic acids using host strains with tailor-made capabilities.

Soybean hulls as carbohydrate feedstock for medium to high-value biomolecule production in biorefineries: A review

Soybean is one of the major world crops, with an annual production of 359 million tons. Each ton of processed soybean generates 50-80 kg of soybean hulls (SHs), representing 5-8% of the whole seed. Due to environmental concerns and great economic potential, the search of SHs re-use solutions are deeply discussed. The lignocellulosic composition of SHs has attracted the attention of the scientific and productive sector. Recently, some studies have reported the use of SHs in the production of medium to high value-added molecules, with potential applications in food and feed, agriculture, bioenergy, and other segments. This review presents biotechnological approaches and processes for the management and exploitation of SHs, including pre-treatment methods and fermentation techniques, for the production of different biomolecules. Great potentialities and innovations were found concerning SH exploration and valorisation of the soybean chain under a biorefinery and circular bioeconomy optic.

Butanol production from Saccharina japonica hydrolysate by engineered Clostridium tyrobutyricum: The effects of pretreatment method and heat shock protein overexpression

Macroalgal biomass is currently considered as a potential candidate for biofuel production. In this study, the effects of pretreatment method and heat shock protein overexpression were investigated for efficient butanol production from Saccharina japonica using engineered Clostridium tyrobutyricum. First, various pretreatment methods including acid hydrolysis, acid hydrolysis and enzymatic saccharification, and ultrasonic-assisted acid hydrolysis were employed to obtain the fermentable sugars, and the resulted hydrolysates were evaluated for butanol fermentation. The results showed that ultrasonic-assisted acid hydrolysate obtained the highest butanol yield (0.26 g/g) and productivity (0.19 g/L⋅h). Then, the effects of homologous or heterologous heat shock protein overexpression on butanol production and tolerance were examined. Among all the engineered strains, Ct-pMA12G exhibited improved butanol tolerance and enhanced butanol production (12.15 g/L butanol with a yield of 0.34 g/g and productivity of 0.15 g/L⋅h) from 1.8-fold concentrated S. japonica hydrolysate, which was the highest level ever reported for macroalgal biomass.

Integrating chemical and biological catalysis for simultaneous production of polyphenolics and butyric acid from waste pomegranate peels

Pomegranate peels are an abundant agricultural waste material with a high content of carbohydrates and bioactive compounds. The aim of this study was to efficiently convert waste pomegranate peels (WPP) into high-value-added products. First, high yields of phenolics (12.2%) and bioactive pectin (24.8%) were obtained via enzymatic pretreatment. The lignin was subsequently degraded using an integrated method combining heteropolyacids as catalyst and biomass-derived γ-valerolactone as sustainable solvent and cellulase-catalyzed hydrolysis. The optimal degradation conditions were found to encompass a temperature of 293 K, reaction time of 3 h and catalyst loading with 30 mM heteropolyacids. Under these conditions, the enzymatic hydrolysis efficiency was enhanced significantly, leading to a yield of 93.3% glucose from the obtained cellulosic feedstock. Finally, the fermentable sugars together with the previously recovered pectin from WPP were firstly used as carbon source to evaluate their suitability as feedstock for butyric acid production using Clostridium tyrobutyricum.

Metal-organic frameworks coupling simultaneous saccharication and fermentation for enhanced butyric acid production from rice straw under visible light by Clostridium tyrobutyricum CtΔack::cat1

Here, Metal-Organic Frameworks (MOFs) coupling simultaneous saccharification and fermentation for butyric acid production using rice straw was constructed. Clostridium tyrobutyricum Δack::cat1, with deleted ack gene and overexpressed cat1 gene, was used as the butyric-acid-fermentation strain. MOFs was employed as a photocatalyst to improve butyric acid production, as well as a cytoprotective exoskeleton with immobilized cellulase for the hydrolysis of rice straw. Thus, the survival of MOFs-coated strain, the thermostability and pH stability of cellulase both remarkably increased. As a result, 55% of rice straw was hydrolyzed in 24 h, and the final concentration of butyric acid in visible light was increased by 14.23% and 29.16% compared to uncoated and coated strain without visible light, respectively. Finally, 26.25 g/L of butyric acid with a productivity of 0.41 g/L·h in fed-batch fermentation was obtained. This novel process inspires green approach of abundant low-cost feedstocks utilization for chemical production.

Molecular Characterization of Xyloglucanase cel74a from Trichoderma reesei

BACKGROUND

The filamentous fungus Trichoderma reesei is used on an industrial scale to produce enzymes of biotechnological interest. This fungus has a complex cellulolytic system involved in the degradation of lignocellulosic biomass. However, several aspects related to the regulation of the expression of holocellulolytic genes and the production of cellulases by this fungus are still understood.

METHODS

Here, we constructed a null mutant strain for the xyloglucanase cel74a gene and performed the characterization of the Δcel74a strain to evaluate the genetic regulation of the holocellulases during sugarcane bagasse (SCB) cultivation.

RESULTS

Our results demonstrate that the deletion of xyloglucanase cel74a may impact the regulation of holocellulase expression during SCB cultivation. The expression of cellulases cel7a, cel7b, and cel6a was reduced in Δcel74a strain, while the hemicellulases xyn1 and xyn2 were increased in the presence of SCB. The cel74a mutation also affected the xyloglucan hydrolysis patterns. In addition, CEL74A activity was modulated in the presence of calcium, suggesting that this ion may be required for efficient degradation of xyloglucan.

CONCLUSIONS

CEL74A affects the regulation of holocellulolytic genes and the efficient degradation of SCB in T. reesei. This data makes a significant contribution to our understanding of the carbon utilization of fungal strains as a whole.

The “Zero Miles Product” Concept Applied to Biofuel Production: A Case Study

To make biofuel production feasible from an economic point of view, several studies have investigated the main associated bottlenecks of the whole production process through approaches such as the “cradle to grave” approach or the Life Cycle Assessment (LCA) analysis, being the main constrains the feedstock collection and transport. Whilst several feedstocks are interesting because of their high sugar content, very few of them are available all year around and moreover do not require high transportation’ costs. This work aims to investigate if the “zero miles” concept could bring advantages to biofuel production by decreasing all the associated transport costs on a locally established production platform. In particular, a specific case study applied to the Technical University of Denmark (DTU) campus is used as example to investigate the advantages and feasibility of using the spent coffee grounds generated at the main cafeteria for the production of bioethanol on site, which can be subsequently used to (partially) cover the campus’ energy demands.

Effects of benzyl viologen on increasing NADH availability, acetate assimilation, and butyric acid production by Clostridium tyrobutyricum

Clostridium tyrobutyricum produces butyric and acetic acids from glucose. The butyric acid yield and selectivity in the fermentation depend on NADH available for acetate reassimilation to butyric acid. In this study, benzyl viologen (BV), an artificial electron carrier that inhibits hydrogen production, was used to increase NADH availability and butyric acid production while eliminating acetic acid accumulation by facilitating its reassimilation. To better understand the mechanism of and find the optimum condition for BV effect on enhancing acetate assimilation and butyric acid production, BV at various concentrations and addition times during the fermentation were studied. Compared with the control without BV, the addition of 1 μM BV increased butyric acid production from glucose by ∼50% in yield and ∼29% in productivity while acetate production was completely inhibited. Furthermore, BV also increased the coutilization of glucose and exogenous acetate for butyric acid production. At a concentration ratio of acetate (g/L) to BV (mM) of 4, both acetate assimilation and butyrate biosynthesis increased with increasing the concentrations of BV (0-6.25 μM) and exogenous acetate (0-25 g/L). In a fed-batch fermentation with glucose and ∼15 g/L acetate and 3.75 μM BV, butyrate production reached 55.9 g/L with productivity 0.93 g/L/h, yield 0.48 g/g, and 97.4% purity, which would facilitate product purification and reduce production cost. Manipulating metabolic flux and redox balance via BV and acetate addition provided a simple to implement metabolic process engineering approach for butyric acid production from sugars and biomass hydrolysates.

A heterogeneous microbial consortium producing short-chain fatty acids from lignocellulose

Microbial consortia are a promising alternative to monocultures of genetically modified microorganisms for complex biotransformations. We developed a versatile consortium-based strategy for the direct conversion of lignocellulose to short-chain fatty acids, which included the funneling of the lignocellulosic carbohydrates to lactate as a central intermediate in engineered food chains. A spatial niche enabled in situ cellulolytic enzyme production by an aerobic fungus next to facultative anaerobic lactic acid bacteria and the product-forming anaerobes. Clostridium tyrobutyricum, Veillonella criceti, or Megasphaera elsdenii were integrated into the lactate platform to produce 196 kilograms of butyric acid per metric ton of beechwood. The lactate platform demonstrates the benefits of mixed cultures, such as their modularity and their ability to convert complex substrates into valuable biochemicals.

Recent advances in n-butanol and butyrate production using engineered Clostridium tyrobutyricum

Acidogenic clostridia naturally producing acetic and butyric acids has attracted high interest as a novel host for butyrate and n-butanol production. Among them, Clostridium tyrobutyricum is a hyper butyrate-producing bacterium, which re-assimilates acetate for butyrate biosynthesis by butyryl-CoA/acetate CoA transferase (CoAT), rather than the phosphotransbutyrylase-butyrate kinase (PTB-BK) pathway widely found in clostridia and other microbial species. To date, C. tyrobutyricum has been engineered to overexpress a heterologous alcohol/aldehyde dehydrogenase, which converts butyryl-CoA to n-butanol. Compared to conventional solventogenic clostridia, which produce acetone, ethanol, and butanol in a biphasic fermentation process, the engineered C. tyrobutyricum with a high metabolic flux toward butyryl-CoA produced n-butanol at a high yield of > 0.30 g/g and titer of > 20 g/L in glucose fermentation. With no acetone production and a high C4/C2 ratio, butanol was the only major fermentation product by the recombinant C. tyrobutyricum, allowing simplified downstream processing for product purification. In this review, novel metabolic engineering strategies to improve n-butanol and butyrate production by C. tyrobutyricum from various substrates, including glucose, xylose, galactose, sucrose, and cellulosic hydrolysates containing the mixture of glucose and xylose, are discussed. Compared to other recombinant hosts such as Clostridium acetobutylicum and Escherichia coli, the engineered C. tyrobutyricum strains with higher butyrate and butanol titers, yields and productivities are the most promising hosts for potential industrial applications.

Selective Enrichment of Clostridium Spp. by Nutrition Control from Sihe Coal Geological Microbial Communities

In the coal biogasification, butyric acid is an important intermediate product. The enrichment of butyric acid-producing bacteria in coal geological methanogens is critical to confirm this assertion. Therefore, to study a method for enrichment of butyric acid-producing bacteria and to explore characteristic factors for evaluating the enrichment effect would be the basis for further strain isolation and metabolomics research. In this study, the nutrition control method was used for the butyric acid-producing bacteria enrichment from concentrated bacteria solution in Sihe coal seam. The characteristic factors' changes in gas production, gas composition, butyric acid concentration, and pH were observed and analyzed in the experiment. High-throughput sequencing was used as a verification method to validate the medium and genera enrichment effect that can be used for the butyric acid-producing bacteria. Through experimental research and analysis, it was identified that the glucose-sucrose-maltose medium was the beneficial medium to the enrichment of butyric acid-producing bacteria, and the high-throughput sequencing determined that the enriched genera were Clostridium spp. Glucose-sucrose-maltose medium experimental data confirmed that the decrease of CO₂ and H₂ daily yield, the increase of butyric acid concentration, and the decrease of pH value had a significant positive correlation with the enrichment of Clostridium spp.

Butyric acid production from spent coffee grounds by engineered Clostridium tyrobutyricum overexpressing galactose catabolism genes

Clostridium tyrobutyricum cannot utilize galactose, which is abundant in lignocellulose and red algae, as a carbon source for butyric acid production. Hence, when using galactose-rich coffee ground hydrolysate as the substrate, the fermentation performance of C. tyrobutyricum is poor. In this work, a recombinant strain, C. tyrobutyricum ATCC 25755/ketp, overexpressing galactose catabolism genes (galK, galE, galT, and galP) from Clostridium acetobutylicum ATCC 824 was constructed for the co-utilization of glucose and galactose. Batch fermentation in the bioreactor showed that ATCC 25755/ketp could efficiently utilize galactose without glucose-induced carbon catabolite repression and consume nearly 100% of the galactose present in the spent coffee ground hydrolysate. Correspondingly, the butyric acid concentration and productivity of ATCC 25755/ketp reached 34.3 g/L and 0.36 g/L·h, respectively, an increase of 78.6% and 56.5% compared with the wild-type strain, indicating its potential for butyric acid production from hydrolysates of inexpensive and galactose-rich biomass.

Direct conversion of untreated cane molasses into butyric acid by engineered Clostridium tyrobutyricum

The sucrose metabolic genes (scrA, scrB and scrK) from C. acetobutylicum ATCC 824 were successfully overexpressed in C. tyrobutyricum ATCC 25755, endowing it with the ability to co-utilize sucrose, fructose and glucose in the cane molasses. As a result, the engineering strain C. tyrobutyricum ATCC 25755/scrBAK produced 18.07 g/L and 18.98 g/L butyric acid when sucrose and cane molasses were used as the carbon source, respectively. Furthermore, the medium composition and initial cane molasses concentration were optimized to make full use of the untreated cane molasses. Based on these results, 45.71 g/L butyric acid with a yield of 0.39 g/g was obtained in fed-batch fermentation, and the feedstock cost of using untreated cane molasses was decreased by ~47% when compared with the conventional glucose fermentation. This study demonstrated the potential application of C. tyrobutyricum ATCC 25755/scrBAK for economic butyric acid production from untreated cane molasses.

Efficient bio-butanol production from lignocellulosic waste by elucidating the mechanisms of Clostridium acetobutylicum response to phenolic inhibitors

Lignocellulosic biomass is considered abundant renewable feedstock to constitute a green and environmentally friendly approach for biofuels (bio-butanol) production as an effective substitute for fossil resources. However, a variety of fermentable inhibitors can be generated in hydrolysates during the biomass pretreatment process. Among them, phenolics including phenolic acids and phenolic aldehydes are the most toxic inhibitors to solventogenic clostridia for bio-butanol production. This study elucidates the physiological mechanism of Clostridium acetobutylicum ATCC 824 response to phenolic inhibitors by the integration of kinetics and transcriptional analysis. Butanol fermentations were stressed by 0.4 g/L phenolic acids or 0.4 g/L phenolic aldehydes at 12 h at the beginning of solventogenesis. With post-stress for 12 h, butanol titer was 7.01 g/L in fermentation with phenolic acid stress, while only 5.82 g/L butanol was produced in the case of phenolic aldehydes stress. Reductions in the two fermentations were 27.6% and 40.0% in comparison with the control (without stress), indicated that phenolic aldehydes had a stronger inhibitory effect on solvents synthesis in C. acetobutylicum than phenolic acids. Additionally, the transcriptional analysis revealed that phenolics altered the gene expression profiles related to membrane transporters such as ATP-binding cassette (ABC)-transporter and phosphotransferase system (PTS), glycolysis, and heat shock proteins. The lower expression levels of PTS-related genes might result in reduced glucose consumption and finally inhibited solvents synthesis under phenolic aldehydes stress. Some genes encoding histidine kinase (CA_C0323, CA_C0903, and CA_C3319) were also affected by phenolics, which might inhibit sporulation. In conclusion, our results provide valuable guidance for the construction of robust strain to efficiently produce bio-butanol from lignocellulosic biomass.

Organic acids as antimicrobial food agents: applications and microbial productions

Food safety is a global health and socioeconomic concern since many people still suffer from various acute and life-long diseases, which are caused by consumption of unsafe food. Therefore, ensuring safety of the food is one of the most essential issues in the food industry, which needs to be considered during not only food composition formulation but also handling and storage. For safety purpose, various chemical preservatives have been used so far in the foods. Recently, there has been renewed interest in replacing chemically originated food safety compounds with natural ones in the industry, which can also serve as antimicrobial agents. Among these natural compounds, organic acids possess the major portion. Therefore, in this paper, it is aimed to review and compile the applications, effectiveness, and microbial productions of various widely used organic acids as antimicrobial agents in the food industry.

Application of IWA Anaerobic Digestion Model No. 1 to simulate butyric acid, propionic acid, mixed acid, and ethanol type fermentative systems using a variable acidogenic stoichiometric approach

IWA Anaerobic Digestion Model No. 1 (ADM1) is the most widely recognised and popular mathematical model for anaerobic digestion processes. However, the application of ADM1 to acidogenic fermentation is limited by its use of constant stoichiometry to describe the formation of products via carbohydrate fermentation. This study presents a modification of ADM1 using a variable acidogenic stoichiometric approach in which the hydrogen partial pressure (p_H2) and pH are used to predict and regulate the acidogenic process. The fermentation of ethanol and its kinetics were introduced into the model structure. Experimental data from mixed acid-type fermentation in a 28.4 L anaerobic baffled reactor (ABR) fed with a sucrose solution with a chemical oxygen demand of 4000 mg L^-1 were used to calibrate the model parameters. Two case studies involving continuous ethanol-type fermentation in an ABR and a continuous stirring tank reactor (CSTR) were used to validate the approach. The modified model achieved good predictions of the experimental data collected from butyric acid, propionic acid, mixed acid, and ethanol-type fermentation in the ABR and CSTR using the standard ADM1 parameter values without any parameter fitting beyond implementation of the variable acidogenic stoichiometry. The p_H2 and pH thresholds in butyric acid, propionic acid, mixed acid, and ethanol-type fermentation could be predicted using this model, which was shown to be a valid mathematical tool for the regulation of fermentation type.

Co-production of 1,2,4-butantriol and ethanol from lignocellulose hydrolysates

The aim of this work was to realize 1,2,4-butantriol (BT) production from sugarcane bagasse hydrolysates by microbial fermentation, and obtain co-production of BT and ethanol. Candida glycerinogenes UG21 was utilized to reduce the effect of osmolality resulting from high glucose concentration and furfural in hydrolysates on cell growth of BT-producing strains, and produced 54.1 g/L ethanol from glucose. After ethanol recovering, xylose containing stillage was obtained and used for BT production. 1.3 g/L BT was generated by a BT-producing strain. By the deletion of the crr gene and process optimization, BT titer reached 4.9 g/L. Meanwhile, the efficient utilization of sugarcane bagasse was achieved by a two-stage fermentation for co-production of BT and ethanol. This study provided a novel strategy for BT production from sugarcane bagasse, and demonstrated the potential for making full use of sugarcane bagasse hydrolysates to co-production value-added products.

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.

Metabolic engineering of Clostridium tyrobutyricum for enhanced butyric acid production from undetoxified corncob acid hydrolysate

Resistance to furan derivatives and phenolic compounds plays an important role in the use of lignocellulosic biomass for biological production of chemicals and fuels. This study confirmed that expression of short-chain dehydrogenase/reductase (SDR) from Clostridium beijerinckii NCIMB 8052 significantly improved the tolerance of C. tyrobutyricum to furfural due to the enhanced activity for furfural reduction. And on this basis, co-expression of SDR and heat shock chaperones GroESL could simultaneously enhance the tolerance of C. tyrobutyricum to furan derivatives and phenolic compounds, which were the main inhibitors presented in dilute-acid lignocellulosic hydrolysates. Consequently, the recombinant strain ATCC 25755/sdr+groESL exhibited good performance in butyric acid production with corncob acid hydrolysate as the substrate. Batch fermentation in bioreactor showed that the butyrate produced by ATCC 25755/sdr+groESL was 32.8 g/L, increased by 28.1% as compared with the wild-type strain. Meanwhile, the butyrate productivity increased from 0.19 g/L·h to 0.29 g/L·h.

The significance of proline on lignocellulose-derived inhibitors tolerance in Clostridium acetobutylicum ATCC 824

When lignocellulosic biomass was used for acetone-butanol-ethanol (ABE) fermentation, several lignocellulose-derived inhibitors, which are toxic to Clostridium acetobutylicum, were generated during acid hydrolysis process and seriously hindered the industrialization of lignocellulosic butanol. In this study, an engineered strain 824(proABC) with significantly improved tolerance to multiple lignocellulose-derived inhibitors (formic acid and phenolic compounds) was constructed by strengthening the proline biosynthesis. The engineered strain exhibited more effective synthesis ability of proline and scavenging ability of reactive oxygen species (ROS). Consequently, the butanol produced by 824(proABC) was 1-, 2.4- or 3.4-fold higher than that of the wild type strain when using the undetoxified hydrolysate of soybean straw, rice straw or corn straw as the substrate, respectively. Therefore, enhancing the proline biosynthesis can be used as an effective strategy to improve the tolerance of C. acetobutylicum to multiple lignocellulose-derived inhibitors, and 824(proABC) has great potential to produce butanol from undetoxified lignocellulosic hydrolysates.

New coculture system of Clostridium spp. and Megasphaera hexanoica using submerged hollow-fiber membrane bioreactors for caproic acid production

In this study, a coculture bioprocess was developed with Clostridium strains producing butyric acid and Megasphaera hexanoica producing caproic acid from the butyric acid. The two bacterial strains were each cultivated in two submerged hollow-fiber membrane bioreactors (s-HF/MBRs), separately. Each fermentation broth was filtered through the membrane modules, and the filtered broth was either interchanged on another reactor or obtained sequentially through. Using s-HF/MBRs, the caproic acid concentration increased to 10.08 g L^-1, with the fastest productivity of 0.69 g L^-1 h^-1, which higher than that previously reported.

Pretreatment with γ-Valerolactone/[Mmim]DMP and Enzymatic Hydrolysis on Corncob and Its Application in Immobilized Butyric Acid Fermentation

Corncob is a widely available raw material with high carbohydrate and low lignin content. To improve corncob conversion to the fermentable sugars, a novel method encompassing pretreatment using the γ-valerolactone (GVL)/1-methyl-3-methylimidazolium dimethylphosphite ([Mmim]DMP) system integrated with cellulase hydrolysis was developed and optimized. It is confirmed that lignin was extracted efficiently after combined pretreatment and that the subsequent enzymatic saccharification efficiency could be significantly enhanced, resulting in the yield of 94.9% glucose from cellulose and 53.3% xylose from xylan, respectively. Furthermore, the above fermentable sugars were used as carbon source for Clostridium tyrobutyricum immobilized in macroporous Ca-alginate-lignin beads with the extracted lignin as the active ingredient to evaluate the fermentability of butyric acid. The results showed that high butyrate productivity of 0.47 g/L/h and yield of 0.45 g/g were obtained after 10 repeated batches of fermentation, demonstrating an effective process for the production of butyric acid from abundant corncob waste-biomass.

Butyric acid: Applications and recent advances in its bioproduction

Butyric acid is an important C4 organic acid with broad applications. It is currently produced by chemosynthesis from petroleum-based feedstocks. However, the fermentative production of butyric acid from renewable feedstocks has received growing attention because of consumer demand for green products and natural ingredients in foods, pharmaceuticals, animal feed supplements, and cosmetics. In this review, strategies for improving microbial butyric acid production, including strain engineering and novel fermentation process development are discussed and compared regarding product yield, titer, purity and productivity. Future perspectives on strain and process improvements for butyric acid production are also discussed.