Random mutagenesis of Clostridium butyricum strain and optimization of biosynthesis process for enhanced production of 1,3-propanediol

Effects of Cold Plasma Pretreatment on the Synthesis of Polysaccharide from Pleurotus ostreatus

Fungal polysaccharides have attracted wide attention because of their medical pharmaceutical and health care value. So far, many efforts have been made in strain improvement to produce polysaccharides on a large scale at low cost. Here, a novel cold plasma-induced strain improvement technology was employed to pretreat Pleurotus ostreatus CGMCC 5.374 by radio-frequency (RF) low-vacuum cold plasma (LVCP) for the purpose of obtaining a high-yield polysaccharide strain. The optimum pretreatment conditions including discharge power, treatment time, and working pressure were determined by single factor and orthogonal experiment in succession. Furthermore, transcriptome analysis was conducted to study the effects of RF-LVCP on cell metabolism and proliferation. Results showed that under the optimal condition of discharge power of 130 W, treatment time of 25 s and working pressure of 140 Pa, polysaccharide content in mycelium was increased by 3.16% after 6 days in comparison to the original strain. Transcriptome analysis showed that RF-LVCP is helpful for specific gene transcription profiles, Gene Ontology (GO) and KEGG pathways, of which the differentially expressed genes (DEGs) were mainly involve with the up-regulation of polysaccharide transport, physiology, synthesis and metabolism, as well as the down-regulation of polysaccharide hydrolysis and macromolecular degradation.

UV-ARTP compound mutagenesis breeding improves macrolactins production of Bacillus siamensis and reveals metabolism changes by proteomic

Macrolactins are a type of compound with complex macrolide structure which mainly be obtained through microbiological fermentation now. They have excellent antifungal, antibacterial and antitumor activity. In order to improve macrolactins production, Bacillus siamensis YB304 was used as the research object, and a mutant Mut-K53 with stable genetic characters was selected by UV-ARTP compound mutagenesis. The yield of macrolactins was 156.46 mg/L, 3.95 times higher than original strain. The metabolic pathway changes and regulatory mechanism of macrolactins were analyzed by quantitative proteomics combined with parallel reaction monitoring. This study revealed that 1794 proteins were extracted from strain YB304 and strain Mut-K53, most of them were related to metabolism. After UV-ARTP compound mutagenesis treatment, the expression of 628 proteins were significantly changed, of which 299 proteins were significantly up-regulated. KEGG pathway analysis showed that differentially expression proteins mainly distributed in biological process, cellular component, and molecular function processing pathways. Such as utilization of carbon sources, glycolysis pathway, and amino acid metabolism pathway. Furthermore, key precursor substances such as acyl-CoA and amino acids of macrolactin biosynthesis are mostly up-regulated, which are one of the main reasons for increased production of macrolactin.This study will provide a new way to increase the yield of macrolactins through mutagenesis breeding and proteomics.

Assessment of DNA mutagenicity induced by He-Ne laser using Salmonella typhimurium strains

Helium-neon (He-Ne) laser mutagenesis is widely used in microbiology and plant breeding. In this study, two frameshift mutant representative strains of Salmonella typhimurium TA97a and TA98 and two base pair substitution types TA100 and TA102 were employed as model microorganisms to assess DNA mutagenicity induced by He-Ne laser (3 J·cm^-2·s^-1, 632.8 nm) for 10, 20, and 30 min. The results revealed that the optimal laser application was 6 h in the mid-logarithmic growth stage. Low-power He-Ne laser for short treatment inhibited cell growth, and continued treatment stimulated the metabolism. The effects of the laser on TA98 and TA100 were the most prominent. Sequencing results from 1500 TA98 revertants showed that there were 88 insertion and deletion (InDel) types in the hisD3052 gene, of which the InDels unique to laser were 21 more than that of the control. Sequencing results from 760 TA100 revertants indicated that laser treatment created Pro (CCC) in the product of the hisG46 gene more likely to be replaced by His (CAC) or Ser (TCC) than by Leu (CTC). Two unique non-classical base substitutions, CCC → TAC and CCC → CAA, also appeared in the laser group. These findings will provide a theoretical basis for further exploration of laser mutagenesis breeding. KEY POINTS: • Salmonella typhimurium served as model organism for laser mutagenesis study. • Laser promoted the occurrence of InDels in the hisD3052 gene of TA98. • Laser promoted the occurrence of base substitution in the hisG46 gene of TA100.

Strategies and tools for the biotechnological valorization of glycerol to 1, 3-propanediol: Challenges, recent advancements and future outlook

Global efforts towards decarbonization, environmental sustainability, and a growing impetus for exploiting renewable resources such as biomass have spurred the growth and usage of bio-based chemicals and fuels. In light of such developments, the biodiesel industry will likely flourish, as the transport sector is taking several initiatives to attain carbon-neutral mobility. However, this industry would inevitably generate glycerol as an abundant waste by-product. Despite being a renewable organic carbon source and assimilated by several prokaryotes, presently realizing glycerol-based biorefinery is a distant reality. Among several platform chemicals such as ethanol, lactic acid, succinic acid, 2, 3-butanediol etc. 1, 3-propanediol (1, 3-PDO) is the only chemical naturally produced by fermentation with glycerol as a native substrate. The recent commercialization of glycerol-based 1, 3-PDO by Metabolic Explorer, France, has revived research interests in developing alternate cost-competitive, scalable and marketable bioprocesses. The current review outlines natural glycerol assimilating and 1, 3-PDO-producing microbes, their metabolic pathways, and associated genes. Later, technical barriers are carefully examined, such as the direct use of industrial glycerol as input material and genetic and metabolic issues related to microbes alleviating their industrial use. Biotechnological interventions exploited in the past five years, which can substantially circumvent these challenges, such as microbial bioprospecting, mutagenesis, metabolic, evolutionary and bioprocess engineering, including their combinations, are discussed in detail. The concluding section sheds light on some of the emerging and most promising breakthroughs which have resulted in evolving new, efficient, and robust microbial cell factories and/or bioprocesses for glycerol-based 1, 3-PDO production.

Genome-guided approaches and evaluation of the strategies to influence bioprocessing assisted morphological engineering of Streptomyces cell factories

Streptomyces genera serve as adaptable cell factories for secondary metabolites with various and distinctive chemical structures that are relevant to the pharmaceutical industry. Streptomyces' complex life cycle necessitated a variety of tactics to enhance metabolite production. Identification of metabolic pathways, secondary metabolite clusters, and their controls have all been accomplished using genomic methods. Besides this, bioprocess parameters were also optimized for the regulation of morphology. Kinase families were identified as key checkpoints in the metabolic manipulation (DivIVA, Scy, FilP, matAB, and AfsK) and morphology engineering of Streptomyces. This review illustrates the role of different physiological variables during fermentation in the bioeconomy coupled with genome-based molecular characterization of biomolecules responsible for secondary metabolite production at different developmental stages of the Streptomyces life cycle.

Engineering Microorganisms to Produce Bio-Based Monomers: Progress and Challenges

Bioplastics are polymers made from sustainable bio-based feedstocks. While the potential of producing bio-based monomers in microbes has been investigated for decades, their economic feasibility is still unsatisfactory compared with petroleum-derived methods. To improve the overall synthetic efficiency of microbial cell factories, three main strategies were summarized in this review: firstly, implementing approaches to improve the microbial utilization ability of cheap and abundant substrates; secondly, developing methods at enzymes, pathway, and cellular levels to enhance microbial production performance; thirdly, building technologies to enhance microbial pH, osmotic, and metabolites stress tolerance. Moreover, the challenges of, and some perspectives on, exploiting microorganisms as efficient cell factories for producing bio-based monomers are also discussed.

Adaptive evolutionary strategy coupled with an optimized biosynthesis process for the efficient production of pyrroloquinoline quinone from methanol

BACKGROUND

Pyrroloquinoline quinone (PQQ), a cofactor for bacterial dehydrogenases, is associated with biological processes such as mitochondriogenesis, reproduction, growth, and aging. Due to the extremely high cost of chemical synthesis and low yield of microbial synthesis, the election of effective strains and the development of dynamic fermentation strategies for enhancing PQQ production are meaningful movements to meet the large-scale industrial requirements.

RESULTS

A high-titer PQQ-producing mutant strain, Hyphomicrobium denitrificans FJNU-A26, was obtained by integrating ARTP (atmospheric and room‑temperature plasma) mutagenesis, adaptive laboratory evolution and high-throughput screening strategies. Afterward, the systematic optimization of the fermentation medium was conducted using a one-factor-at-a-time strategy and response surface methodology to increase the PQQ concentration from 1.02 to 1.37 g/L. The transcriptional analysis using qRT-PCR revealed that the expression of genes involved in PQQ biosynthesis were significantly upregulated when the ARTP-ALE-derived mutant was applied. Furthermore, a novel two-stage pH control strategy was introduced to address the inconsistent effects of the pH value on cell growth and PQQ production. These combined strategies led to a 148% increase in the PQQ concentration compared with that of the initial strain FJNU-6, reaching 1.52 g/L with a yield of 40.3 mg/g DCW after 144 h of fed-batch fermentation in a 5-L fermenter.

CONCLUSION

The characteristics above suggest that FJNU-A26 represents an effective candidate as an industrial PQQ producer, and the integrated strategies can be readily extended to other microorganisms for the large-scale production of PQQ.

Biocatalytic gateway to convert glycerol into 3-hydroxypropionic acid in waste-based biorefineries: Fundamentals, limitations, and potential research strategies

Microbial conversion of bioenergy-derived waste glycerol into value-added chemicals has emerged as an important bioprocessing technology due to its eco-friendliness, feasible technoeconomics, and potential to provide sustainability in biodiesel and bioethanol production. Glycerol is an abundant liquid waste from bioenergy plants with a projected volume of 6 million tons by 2025, accounting for about 10% of biodiesel and 2.5% of bioethanol yields. 3-Hydroxypropionic acid (3-HP) is a major product of glycerol bioconversion, which is the third largest biobased platform compound with expected market size and value of 3.6 million tons/year and USD 10 billion/year, respectively. Despite these biorefinery values, 3-HP biosynthesis from glycerol is still at an immature stage of commercial exploitation. The main challenges behind this immaturity are the toxic effects of 3-HPA on cells, the distribution of carbon flux to undesirable pathways, low tolerance of cells to glycerol and 3-HP, co-factor dependence of enzymes, low enzyme activity and stability, and the problems of substrate inhibition and specificity of enzymes. To address these challenges, it is necessary to understand the fundamentals of glycerol bioconversion and 3-HP production in terms of metabolic pathways, related enzymes, cell factories, midstream process configurations, and downstream 3-HP recovery, as discussed in this review critically and comprehensively. It is equally important to know the current challenges and limitations in 3-HP production, which are discussed in detail along with recent research efforts and remaining gaps. Finally, possible research strategies are outlined considering the recent technological advances in microbial biosynthesis, aiming to attract further research efforts to achieve a sustainable and industrially exploitable 3-HP production technology. By discussing the use of advanced tools and strategies to overcome the existing challenges in 3-HP biosynthesis, this review will attract researchers from many other similar biosynthesis technologies and provide a common gateway for their further development.

Enhancing biomass and lipid productivity of a green microalga Parachlorella kessleri for biodiesel production using rapid mutation of atmospheric and room temperature plasma

BACKGROUND

Microalgae, with their high adaptability to various stress conditions and rapid growth, are considered excellent biomass resources for lipid production and biodiesel feedstocks. However, lipid yield and productivity of the natural strains are common bottlenecks in their large-scale use for lipid production, which can be overcome by evolving new strains using conventional and advanced mutagenic techniques. It is challenging to generate microalgae strains capable of high lipid synthesis through natural selection. As a result, random mutagenesis is currently considered a viable option in many scenarios. The objective of this study was to explore atmospheric and room temperature plasma (ARTP) as a random mutagenesis technique to obtain high lipid-accumulating mutants of a green microalga for improved biodiesel production.

RESULTS

A green microalgal species was isolated from the Chinese Yellow Sea and identified as Parachlorella kessleri (OM758328). The isolated microalga was subsequently mutated by ARTP to obtain high lipid-accumulating mutants. Based on the growth rate and lipid content, 5 mutants (named M1, M2, M4, M5, and M8) were selected from 15 pre-selected mutants. These five mutants varied in their growth rate from 0.33 to 0.68 day^-1, with the lipid content varying between 0.25 g/L in M2 to 0.30 g/L in M8 at 10th day of cultivation. Among the mutants, M8 showed the maximum biomass productivity (0.046 g/L/day) and lipid productivity (20.19 mg/L/day), which were 75% and 44% higher than the wild strain, respectively. The triglyceride (TAG) content of M8 was found to be 0.56 g/L at 16th day of cultivation, which was 1.77-fold higher than that of the wild strain. Furthermore, M8 had the highest saturated fatty acids (C16-18) with the lowermost polyunsaturated fatty acid content, which are favorable properties of a biodiesel feedstock according to international standards.

CONCLUSION

The mutant strain of P. kessleri developed by the ARTP technique exhibited significant improvements in biomass productivity, lipid content, and biodiesel quality. Therefore, the biomass of this mutant microalga could be a potential feedstock for biodiesel production.

Efficient production of inositol from glucose via a tri-enzymatic cascade pathway

Inositol is an essential intermediate in cosmetics, food, medicine and other industries. However, developing an efficient biotransformation system for large-scale production of inositol remains challenging. Herein, a tri-enzymatic cascade route with three novel enzymes including polyphosphate glucokinase (PPGK) from Thermobifida fusca, inositol 3-phosphate synthase (IPS) from Archaeoglobus profundus DSM 5631 and inositol monophosphatase (IMP) from Thermotoga petrophila RKU-1 was designed and reconstructed for the production of inositol from glucose. The problem of poor cooperativity of the cascade reactions was addressed by ribosome binding site (RBS) optimization of PPGK and replication of IPS. Under the optimum biotransformation conditions, the engineered whole-cell immobilized with colloidal chitin transformed 120 g/L glucose to 110.8 g/L inositol with 92.3% conversion in four cycles of reuse, representing the highest titer of inositol to date. Furthermore, this is the first study for inositol production using a three-enzyme coordinated immobilized single-cell.

ARTP and NTG compound mutations improved Cry protein production and virulence of Bacillus thuringiensis X023

A high production mutated strain Bacillus thuringiensis X023PN (BtX023PN) was screened from the wild strain Bacillus thuringiensis X023 (BtX023) after atmospheric and room temperature plasma (ARTP) and nitrosoguanidine (NTG) mutation. BtX023PN grows faster than the wild strain, and its lysis of mother cell was 6 h ahead BtX023, but the ability of sporulation was significantly reduced. Bioassay indicated that compared with the wild type strain, the virulence of BtX023PN against Plutella xylostella (P. xylostella) and Mythimna seperata (M. seperata) increased to 2.33-fold and 2.13-fold respectively. qRT-PCR and SDS-PAGE demonstrated that the production of Cry1Ac increased by 61%. Resequence indicated that the mutated sites enriched on the key carbohydrate metabolism and amino acid metabolism. This study provides a new strain resource for the development of Bt insecticides and a feasible technical strategy for the breeding of Bt. KEY POINTS: • Atmospheric and room temperature plasma used in breeding of Bacillus thuringiensis. • Less stationary phase time with more ICP production. • Semi-lethal concentration against Plutella xylostella reduced by about 57.

Fermentative reforming of crude glycerol to 1,3-propanediol using Clostridium butyricum strain L4

Repurposed used cooking oil is a sustainable alternative to other feedstocks for biodiesel production offering enviro-economic benefits. Residual crude glycerol (RCG) from such biodiesel production plants is difficult to utilize due to presence of numerous toxic impurities with various inhibitory effects on biological fermentative reforming process. However, it is a new industrial feedstock for bio-based production of 1,3-propanediol. In this work, a new Clostridium butyricum strain L4 was isolated from biogas reactor leachate after rigorous adaption and 35 subcultures under increasing stress conditions and studied for green production of 1,3-propanediol (PDO) from RCG and further process development. Evaluation of fermentative reforming kinetics was performed and the optimal reaction conditions are pH 7.0, temperature 30 °C, 2 g yeast extract/L and 15 g ammonium sulphate/L. Glycerol-glucose co-fermentation (10:1) enhanced cell growth and thus, PDO output by 11.6 g/L. In comparison to batch fermentation (24.8 g PDO/L; 0.58 mol PDO/mol glycerol) there was 2.8-fold improvement with fed-batch process resulting in accumulation of 70.1 g PDO/L (Yield = 0.65 mol PDO/mol glycerol) using the studied biocatalyst in 150 h. In order to predict yields under different operational conditions a multiple linear regression model was developed (r² = 0.783) with six independent variables (p < 0.05), where biomass (g/L) and temperature (^oC) were forecasted as top contributors to PDO yield. Finally, this biocatalyst appears as a potential candidate for industrial use due to its non-pathogenic nature, ability to grow in wide pH and temperature conditions, tolerance to high substrate and product concentration, insignificant generation of by-products and Coenzyme B12 independent biotransformation. The study can add value to bio-utilization of RCG to produce green 1,3-propanediol.

Functional and Structural Roles of the Dimer Interface in the Activity and Stability of Clostridium butyricum 1,3-Propanediol Oxidoreductase

Biosynthesis of 1,3-propanediol (1,3-PD) by 1,3-propanediol oxidoreductase (PDOR) is often limited by the stability issues. To address this issue, the goal of the present study was to engineer the Clostridium butyricum PDOR dimeric interface. The interface exists between the chains and plays a role in the synthesis of 1,3-PD, which is hindered by the increased temperature and pH. Herein, we engineered PDOR by HotSpot Wizard 3.0 and molecular dynamics simulations, improving its thermal stability, pH tolerance, and catalytic properties with respect to the wild-type PDOR activity at 37 °C. Compared to the activity of the wild-type PDOR, the N298C mutant showed 0.5-fold greater activity at pH 8.0, while the P299E mutant showed significantly increased activity of over five fold at pH 4.0. Further structural comparisons between the wild-type and P299E mutant revealed that the extraordinary stability of the P299E mutant could be due to the formation of additional hydrogen bonds and salt bridges. The N298C mutant also exhibits thermal stability at a broad range of temperature at pH 8 with respect to wild-type PDOR and other mutants. The molecular dynamics simulations revealed that stability profiles of P299E mutants at pH 4.0 are attributed to identical root mean square deviation values and stable conformations in the motif region present in the dimer interface of the enzyme. These findings suggest that the dimer interface motifs are essential for the compactness and stability of the PDOR enzyme; therefore, engineering the PDOR using a structure-guided approach could aid in improving its activity and stability under various physiological conditions (pH and temperature).

Harnessing the Keratinolytic Activity of Bacillus licheniformis Through Random Mutagenesis Using Ultraviolet and Laser Irradiations

Keratinase is one of the important proteases, which is widely used for converting keratin of the keratinaceous materials into various value-added products. In this study, a popular keratinase producer, Bacillus licheniformis PWD-1, was exposed to ultraviolet (UV) and He-Ne laser irradiations to develop high keratinase-producing mutants. Laser irradiation showed a higher lethality of cells (94%) than UV treatment (92%), whereas laser treatment required a longer time (75 min) than UV treatment (20 min). A total of 58 mutants were selected from 176 isolates to study protein and keratinase production capability of the mutants. The highest keratin-to-casein (K:C) ratio (1.43) was exhibited by LU11 mutant, which was obtained from the combined laser and UV irradiations. The purified keratinase (65 kDa) of LU11 showed 40% yield 1.7-fold purity, while the respective value for wild enzyme was 29% and 1.3-fold. Both enzymes showed optimal activity at 55 ℃ and pH 8, with a Z value of 15.78 ℃ for LU11 and 19.72 ℃ for wild strain. The V_max and specific constant (K_cat/K_m) of the mutant enzyme were 357.17 U/ml and 33.11 min^-1 mM^-1, respectively, which were significantly higher than the respective values of wild enzyme (102.04 U/ml and 28.36 min^-1 mM^-1).

ARTP Mutagenesis of Schizochytrium sp. PKU#Mn4 and Clethodim-Based Mutant Screening for Enhanced Docosahexaenoic Acid Accumulation

Schizochytrium species are one of the best oleaginous thraustochytrids for high-yield production of docosahexaenoic acid (DHA, 22:6). However, the DHA yields from most wild-type (WT) strains of Schizochytrium are unsatisfactory for large-scale production. In this study, we applied the atmospheric and room-temperature plasma (ARTP) tool to obtain the mutant library of a previously isolated strain of Schizochytrium (i.e., PKU#Mn4). Two rounds of ARTP mutagenesis coupled with the acetyl-CoA carboxylase (ACCase) inhibitor (clethodim)-based screening yielded the mutant A78 that not only displayed better growth, glucose uptake and ACCase activity, but also increased (54.1%) DHA content than that of the WT strain. Subsequent optimization of medium components and supplementation improved the DHA content by 75.5 and 37.2%, respectively, compared with that of mutant A78 cultivated in the unoptimized medium. Interestingly, the ACCase activity of mutant A78 in a medium supplemented with biotin, citric acid or sodium citrate was significantly greater than that in a medium without supplementation. This study provides an effective bioengineering approach for improving the DHA accumulation in oleaginous microbes.

Efficient production of ε-poly-l-lysine using cassava starch and fish meal by Streptomyces albulus FQC-24

ε-Poly-l-lysine (ε-PL) is used as a natural food preservative which consists of l-lysine units connected. However, due to the expensive culture medium, the production cost of ε-PL remains high. In this study, cheap raw materials cassava starch (CS) and fish meal (FM) were employed by S. albulus FQC-24 for ε-PL production. In the single factor experiment, the maximum ε-PL production reached 0.97 g/L at 60 g/L CS and 15 g/L FM. The results of screening experiments by Plackett-Burman design showed that three main components affecting ε-PL production were CS, FM, and (NH₄)₂SO₄. And the standardized effects of CS, FM, and (NH₄)₂SO₄ were 0.13, -0.22, and -0.2, respectively. The optimum fermentation medium developed by response surface methodology for ε-PL production contained (g/L) CS, 67.56; FM, 14.70 and (NH₄)₂SO₄, 5.41. Under the optimum conditions, the ε-PL production was achieved 1.30 g/L, with 34.02% higher than that before optimization. Moreover, ε-PL productions of batch and fed-batch fermentation in a 7-L fermentor were improved to 2.13 and 17.17 g/L respectively, which increased by 0.64 and 12.2 times compared with the shake flask culture. The results indicated that FM and CS are promising substrates for the efficient production of ε-PL.

Emerging technologies for genetic modification of solventogenic clostridia: From tool to strategy development

Solventogenic clostridia has been considered as one of the most potential microbial cell factories for biofuel production in the biorefinery industry. However, the inherent shortcomings of clostridia strains such as low productivity, by-products formation and toxic tolerance still strongly restrict the large-scale application. Therefore, concerns regarding the genetic modification of solventogenic clostridia have spurred interests into the development of modern gene-editing tools. In this review, we summarize the latest advances of genetic tools involved in modifying solventogenic clostridia. Following a systematic comparison on their respective characteristics, we then review the corresponding strategies for overcoming the obstacles to the enhanced production. Discussing the progress of other microbial cell factories for solventogenesis, we finally describe the key challenges and trends with valuable recommendations for future large-scale biosolvent industrial application.

Isolation and characterization of a newly identified Clostridium butyricum strain SCUT343-4 for 1,3-propanediol production

A novel 1,3-propanediol (1,3-PDO) producing strain was isolated and identified as Clostridium butyricum with respect to its morphological and physiological characteristics, as well as 16S rDNA. The results of substrates test and stress tolerance indicated that C. butyricum SCUT343-4 could produce 1,3-PDO efficiently from glycerol. The optimal fermentation conditions were determined to be 5 g/L yeast extract at 37 °C and pH 6.5. To fully evaluate its 1,3-PDO production capacity, different cultivation strategies have been implemented. The highest 1,3-PDO concentration obtained for batch and fed-batch fermentation were 51.64 and 61.30 g/L, respectively. Immobilized cell fermentation in fibrous-bed bioreactor was also performed, and the concentration of 1,3-PDO further increased to 86 g/L with a yield of 0.52 g/g. In addition, the 1,3-PDO productivity reached 4.20 g/L h, which is the highest level reported for C. butyricum, demonstrating the potential of C. butyricum SCUT343-4 for 1,3-PDO production from glycerol.

Efficient single whole-cell biotransformation for L-2-aminobutyric acid production through engineering of leucine dehydrogenase combined with expression regulation

Leucine dehydrogenase (LDH) is widely used in the preparation of L-2-aminobutyric acid (L-2-ABA), however its wide application is limited by 2-ketobutyric acid (2-OBA) inhibition. Firstly, a novel high-throughput screening method of LDH was established, specific enzyme activity and 2-OBA tolerance of Lys72Ala mutant were 33.3% higher than those of the wild type. Subsequently, we constructed a single cell comprised of ivlA, Esldh_K72A, fdh and optimized expression through fine-tuning RBS intensity, so that the yield of E. coli BL21/pET28a-R3ivlA-Esldh_K72A-fdh was 2.6 times higher than that of the original strain. As a result, 150 g L-threonine was transformed to 121 g L-2-ABA in 5 L fermenter with 95% molar conversion rate, and a productivity of 5.04 g·L^-1·h^-1, which is the highest productivity of L-2-ABA currently reported by single-cell biotransformation. In summary, our research provided a green synthesis for L-2-ABA, which has potential for industrial production of drug precursors.

Efficient Production of 1,3-Propanediol from Diverse Carbohydrates via a Non-natural Pathway Using 3-Hydroxypropionic Acid as an Intermediate

1,3-Propanediol (1,3-PDO) is a promising platform chemical used to manufacture various polyesters, polyethers, and polyurethanes. Microbial production of 1,3-PDO using non-natural producers often requires adding expensive cofactors such as vitamin B₁₂, which increases the whole production cost. In this study, we proposed and engineered a non-natural 1,3-PDO synthetic pathway derived from acetyl-CoA, enabling efficient accumulation of 1,3-PDO in Escherichia coli without adding expensive cofactors. This functional pathway was established by introducing the malonyl-CoA-dependent 3-hydroxypropionic acid (3-HP) module and screening the key enzymes to convert 3-HP to 1,3-PDO. The best engineered strain can produce 2.93 g/L 1,3-PDO with a yield of 0.35 mol/mol glucose in shake flask cultivation (and 7.98 g/L in fed-batch fermentation), which is significantly higher than previous reports based on homoserine- or malate-derived non-natural pathways. We also demonstrated for the first time the feasibility of producing 1,3-PDO from diverse carbohydrates including xylose, glycerol, and acetate based on the same pathway. Thus, this study provides an alternative route for 1,3-PDO production.

Screening of Beauveria bassiana with high biocontrol potential based on ARTP mutagenesis and high-throughput FACS

Beauveria bassiana is a promising biocontrol agent due to its entomopathogenic activities and residue-free characteristics. However, its susceptibility to abiotic stresses and naturally low virulence limit the effective application of this fungus. To effectively obtain fungal strains with high biocontrol potential, fluorescence-activated cell sorting (FACS) was used to screen mutant libraries generated by atmospheric and room temperature plasma (ARTP). Among about 8000 mutants obtained by ARTP mutagenesis, six candidate mutants were selected according to the forward scatter (FSC) signal readings of FACS. B6, with a 37.4% higher FSC reading than wild-type (WT), showed a 32.6% increase in virulence. It also presented a 13.5% decrease in median germinating time (GT₅₀) and a 12.1% increase in blastospore production. Comparative analysis between insect transcriptional responses to B6 and WT infection showed that the immune response coupled with protein digestion and absorption progress was highly activated in B6-infected Galleria mellonella larvae, while fatty acid synthesis was suppressed after 3 days of infection. Our results confirmed the feasibility of sorting B. bassiana with high biocontrol potential via the combination of ARTP and FACS and facilitated the understanding of insect-pathogen interactions, highlighting a new strategy for modifying entomopathogenic fungi to improve the efficiency of biological control.

Co-fermentation of glycerol and glucose by a co-culture system of engineered Escherichia coli strains for 1,3-propanediol production without vitamin B12 supplementation

The necessity of costly co-enzyme B₁₂ for the activity of glycerol dehydratase (GDHt) is considered as a major bottleneck in sustainable bioproduction of 1,3-propanediol (1,3-PD) from glycerol. Here, an E. coil Rosetta-dhaB1-dhaB2 strain was constructed by overexpressing a B₁₂-independent GDHt (dhaB1) and its activating factor (dhaB2) from Clostridium butyricum. Subsequently, it was used in designing a co-culture with E. coli BL21-dhaT that overexpressed 1,3-PD oxidoreductase (dhaT), to produce 1,3-PD during co-fermentation of glycerol and glucose. The optimum initial ratio of BL21-dhaT to Rosetta-dhaB1-dhaB2 strains in the co-culture was 1.5. Compared to the fermentation of glycerol alone, co-fermentation approach provided 1.3-folds higher 1,3-PD. Finally, co-fermentation was done in a 10 L bioreactor that produced 41.65 g/L 1,3-PD, which corresponded to 0.69 g/L/h productivity and 0.67 mol/mol yield of 1,3-PD. Hence, the developed co-culture could produce 1,3-PD cost-effectively without requiring vitamin B₁₂.

1,3-Propanediol production from glycerol in polyurethane foam containing anaerobic reactors: performance and biomass cultivation and retention

The use of batch and upflow anaerobic reactors filled with polyurethane foam for pure glycerol fermentation was evaluated. The best reactor operational conditions to obtain high yield and productivity of 1,3-propanediol (1,3-PDO) as the main product and the role of the polyurethane foam in the growth and retention of suspended and attached biomass in the reactors were investigated. In the experiment at 30 °C with a batch reactor (700 mL), biomass growth was mostly as immobilized attached cells, and the achieved 1,3-PDO yield was up to 0.58 mol mol-gly^-1. In the experiment (30 °C) with an upflow anaerobic reactor (717 mL), glycerol loading rates (gly-LR) ranging from 6.94 to 15.47 g gly L^-1 day^-1 were applied during a 102-day period. During the operation, average 1,3-PDO yield was 0.47 mol mol-gly^-1, reaching a maximum of 0.51 mol mol-gly^-1 at gly-LR of 13.57 g gly L^-1 day^-1. High 1,3-PDO productivity (5.35 to 5.44 g L^-1 day^-1) was obtained when gly-LR was 13.57 to 15.47 g gly L^-1 day^-1. Comparing the close yield values in both batch and continuous reactors and based on microbial evaluation, it is concluded that most of the 1,3-PDO generated in the continuous reactor was due to the suspended biomass retained by the foam cubes. The Clostridium genus was the predominant 1,3-PDO producer. Good yields and productivities with packed reactors were attributed to polyurethane foam used for mixed culture growth and retention. Consequently, they are worth considering for 1,3-PDO production from pure glycerol.

Sequential fed-batch fermentation of 1,3-propanediol from glycerol by Clostridium butyricum DL07

The demand for 1,3-propanediol (1,3-PDO) has increased sharply due to its role as a monomer for the synthesis of polytrimethylene terephthalate (PTT). Although Clostridium butyricum is considered to be one of the most promising bioproducers for 1,3-PDO, its low productivity hinders its application on industrial scale because of the longer time needed for anaerobic cultivation. In this study, an excellent C. butyricum (DL07) strain was obtained with high-level titer and productivity of 1,3-PDO, i.e., 104.8 g/L and 3.38 g/(L•h) vs. 94.2 g/L and 3.04 g/(L•h) using pure or crude glycerol as substrate in fed-batch fermentation, respectively. Furthermore, a novel sequential fed-batch fermentation was investigated, in which the next bioreactor was inoculated by C. butyricum DL07 cells growing at exponential phase in the prior bioreactor. It could run steadily for at least eight cycles. The average concentration of 1,3-PDO in eight cycles was 85 g/L with the average productivity of 3.1 g/(L•h). The sequential fed-batch fermentation could achieve semi-continuous production of 1,3-PDO with higher productivity than repeated fed-batch fermentation and would greatly contribute to the industrial production of 1,3-PDO by C. butyricum. KEY POINTS: • A novel C. butyricum strain was screened to produce 104.8 g/L 1,3-PDO from glycerol. • Corn steep liquor powder was used as a cheap nitrogen source for 1,3-PDO production. • A sequential fed-batch fermentation process was established for 1,3-PDO production. • An automatic glycerol feeding strategy was applied in the production of 1,3-PDO.

The Draft Genome Sequence of Clostridium butyricum QXYZ514, a Potent Bacterium for Converting Glycerol into Fuels and Bioproducts in the Waste-Based Biorefinery

An anaerobic bacterium Clostridium buturicum QXYZ514 was isolated from a pond soil sample located neighboring to a biodiesel factory. This bacterium possesses excellent metabolic features for converting biodiesel-derived glycerol into various bioproducts, including 1,3-propanediol, butyrate, lactate, and acetate, and fuels, like ethanol and butanol. To further improve the yield of the target products and minimize the production of the by-products, the whole genome sequence of this multipurpose strain might provide necessary genetic information, and hence, the complete genome of QXYZ514 strain was sequenced using the PacBio RS II sequencing method. According to the complete genome sequence, the genome of QXYZ514 consisted of two circular chromosomes with a total of 4,636,461 bp, where GC content was found to be 28.76%. Major predicted features of the genome included a total of 4220 coding sequences (CDS), 87 tRNAs genes, and 36 rRNAs genes, which were annotated with the help of different databases for a better understanding of this strain. Six possible CRISPR components were also predicted in the genome. The exploration of the complete genome sequence of the QXYZ514 strain would have the potential to enrich the diversity of this species, and to recognize some significant hydrolytic enzymes, which could provide the references for overcoming the bottlenecks in the biorefinery usage of this bacterium in the valorization of biodiesel-derived glycerol.

A robust soft sensor to monitor 1,3-propanediol fermentation process by Clostridium butyricum based on artificial neural network

With the aggravation of environmental pollution and energy crisis, the sustainable microbial fermentation process of converting glycerol to 1,3-propanediol (1,3-PDO) has become an attractive alternative. However, the difficulty in the online measurement of glycerol and 1,3-PDO creates a barrier to the fermentation process and then leads to the residual glycerol and therefore, its wastage. Thus, in the present study, the four-input artificial neural network (ANN) model was developed successfully to predict the concentration of glycerol, 1,3-PDO, and biomass with high accuracy. Moreover, an ANN model combined with a kinetic model was also successfully developed to simulate the fed-batch fermentation process accurately. Hence, a soft sensor from the ANN model based on NaOH-related parameters has been successfully developed which cannot only be applied in software to solve the difficulty of glycerol and 1,3-PDO online measurement during the industrialization process, but also offer insight and reference for similar fermentation processes.

Enhancement of Acid Protease Activity of Aspergillus oryzae Using Atmospheric and Room Temperature Plasma

Atmospheric and room temperature plasma (ARTP) system is a novel and efficient mutagenesis protocol for microbial breeding. In this study, ARTP was employed to treat spores of Aspergillus oryzae strain 3.042 for selection of high acid protease producers. With an irradiation time of 150 s at the lethal rate of 90%, 19 mutants with higher acid protease activity were initially selected based on different mutant colony morphology and ratio of the clarification halo of protease activity to the colony diameter. Measurements of the acid protease activity revealed that mutant strain B-2 is characterized by a steady hereditary stability with increased acid protease, neutral protease and total protease activities of 54.7, 17.3, and 8.5%, respectively, and decreased alkaline protease activity of 8.1%. In summary, the identified mutant strain B-2 exhibits great potential for the enhancement of the insufficient acid protease activity during the middle and later stages of soy sauce fermentation.

Designing an Artificial Pathway for the Biosynthesis of a Novel Phenazine N-Oxide in Pseudomonas chlororaphis HT66

Aromatic N-oxides are valuable due to their versatile chemical, pharmaceutical, and agricultural applications. Natural phenazine N-oxides possess potent biological activities and can be applied in many ways; however, few N-oxides have been identified. Herein, we developed a microbial system to synthesize phenazine N-oxides via an artificial pathway. First, the N-monooxygenase NaphzNO1 was predicted and screened in Nocardiopsis sp. 13-12-13 through a product comparison and gene sequencing. Subsequently, according to similarities in the chemical structures of substrates, an artificial pathway for the synthesis of a phenazine N-oxide in Pseudomonas chlororaphis HT66 was designed and established using three heterologous enzymes, a monooxygenase (PhzS) from P. aeruginosa PAO1, a monooxygenase (PhzO) from P. chlororaphis GP72, and the N-monooxygenase NaphzNO1. A novel phenazine derivative, 1-hydroxyphenazine N'10-oxide, was obtained in an engineered strain, P. chlororaphis HT66-SN. The phenazine N-monooxygenase NaphzNO1 was identified by metabolically engineering the phenazine-producing platform P. chlororaphis HT66. Moreover, the function of NaphzNO1, which can catalyze the conversion of 1-hydroxyphenazine but not that of 2-hydroxyphenazine, was confirmed in vitro. Additionally, 1-hydroxyphenazine N'10-oxide demonstrated substantial cytotoxic activity against two human cancer cell lines, MCF-7 and HT-29. Furthermore, the highest microbial production of 1-hydroxyphenazine N'10-oxide to date was achieved at 143.4 mg/L in the metabolically engineered strain P3-SN. These findings demonstrate that P. chlororaphis HT66 has the potential to be engineered as a platform for phenazine-modifying gene identification and derivative production. The present study also provides a promising alternative for the sustainable synthesis of aromatic N-oxides with unique chemical structures by N-monooxygenase.