Enhanced dipicolinic acid production during the stationary phase in Bacillus subtilis by blocking acetoin synthesis

Engineering Pseudomonas putida KT2440 for Dipicolinate Production via the Entner-Doudoroff Pathway

Dipicolinic acid (DPA), a cyclic diacid, has garnered significant interest due to its potential applications in antimicrobial agents, antioxidants, chelating reagents, and polymer precursors. However, its natural bioproduction is limited since DPA is only accumulated in Bacillus and Clostridium species during sporulation. Thus, heterologous production by engineered strains is of paramount importance for developing a sustainable biological route for DPA production. Pseudomonas putida KT2440 has emerged as a promising host for the production of various chemicals thanks to its robustness, metabolic versatility, and genetic tractability. The dominant Entner-Doudoroff (ED) pathway for glucose metabolism in this strain offers an ideal route for DPA production due to the advantage of NADPH generation and the naturally balanced flux between glyceraldehyde-3-phosphate and pyruvate, which are both precursors for DPA synthesis. In this study, DPA production via the ED pathway was in silico designed in P. putida KT2440. The systematically engineered strain produced dipicolinate with a titer of 11.72 g/L from glucose in a 5 L fermentor. This approach not only provides a sustainable green route for DPA production but also expands our understanding of the metabolic potential of the ED pathway in P. putida KT2440.

Dipicolinic acid enhances kiwifruit resistance to Botrytis cinerea by promoting phenolics accumulation

BACKGROUND

Kiwifruit is highly susceptible to fungal pathogens, such as Botrytis cinerea, which reduce crop production and quality. In this study, dipicolinic acid (DPA), which is one of the main components of Bacillus spores, was evaluated as a new elicitor to enhance kiwifruit resistance to B. cinerea.

RESULTS

DPA enhances antioxidant capacity and induces the accumulation of phenolics in B. cinerea-infected 'Xuxiang' kiwifruit. The contents of the main antifungal phenolics in kiwifruit, including caffeic acid, chlorogenic acid and isoferulic acid, increased after DPA treatment. DPA enhanced H2 O2 levels after 0 and 1 days, which promoted catalase (CAT) and superoxide dismutase (SOD) activities, reducing long-term H2 O2 levels. DPA promoted the up-regulation of several kiwifruit defense genes, including CERK1, MPK3, PR1-1, PR1-2, PR5-1 and PR5-2. Furthermore, DPA at 5 mM inhibited B. cinerea symptoms in kiwifruit (95.1% lesion length inhibition) more effectively than the commercial fungicides carbendazim, difenoconazole, prochloraz and thiram.

CONCLUSIONS

The antioxidant properties of DPA and the main antifungal phenolics of kiwifruit were examined for the first time. This study uncovers new insights regarding the potential mechanisms used by Bacillus species to induce disease resistance. © 2023 Society of Chemical Industry.

Constitutive expression of the global regulator AbrB restores the growth defect of a genome-reduced Bacillus subtilis strain and improves its metabolite production

Partial bacterial genome reduction by genome engineering can improve the productivity of various metabolites, possibly via deletion of non-essential genome regions involved in undesirable metabolic pathways competing with pathways for the desired end products. However, such reduction may cause growth defects. Genome reduction of Bacillus subtilis MGB874 increases the productivity of cellulases and proteases but reduces their growth rate. Here, we show that this growth defect could be restored by silencing redundant or less important genes affecting exponential growth by manipulating the global transcription factor AbrB. Comparative transcriptome analysis revealed that AbrB-regulated genes were upregulated and those involved in central metabolic pathway and synthetic pathways of amino acids and purine/pyrimidine nucleotides were downregulated in MGB874 compared with the wild-type strain, which we speculated were the cause of the growth defects. By constitutively expressing high levels of AbrB, AbrB regulon genes were repressed, while glycolytic flux increased, thereby restoring the growth rate to wild-type levels. This manipulation also enhanced the productivity of metabolites including γ-polyglutamic acid. This study provides the first evidence that undesired features induced by genome reduction can be relieved, at least partly, by manipulating a global transcription regulation system. A similar strategy could be applied to other genome engineering-based challenges aiming toward efficient material production in bacteria.

Effectiveness of the Bacillus subtilis genome-reduced strain as an ethanol production host

We investigated the performance of a genome-reduced strain of Bacillus subtilis MGB874, whose 0.87 Mbp of genomic DNA was cumulatively deleted, as an ethanol production host. A recombinant strain A267_EtOH was constructed by introducing the pdc and adhB genes from Zymomonas mobilis, both of which were expressed from an isopropyl-β-d-1-thiogalactopyranoside-inducible spac promoter, into the A267 strain, a tryptophan prototrophic derivative of the MGB874 with disruption of metabolic pathways for producing lactic acid, acetic acid, and acetoin. Focusing on the stationary phase in fed-batch fermentation, 1.6 g L-1 ethanol was produced by the A267_EtOH strain after 144 h. Moreover, its ethanol production further increased by approximately 3.7-fold (5.9 g L-1) at 80 h through replacing the spac promoter for expressing pdc and adhB genes with the lytR promoter and the yield was about 112%. These results indicate that the MGB874 is an effective host for ethanol production during the stationary phase.

Lysinibacillus sphaericus III(3)7 and Plasmid Vector pMK4: New Challenges in Cloning Platforms

The acquisition and especially the maintenance of a plasmid usually brings a fitness cost that reduces the reproductive rate of the bacterial host; for strains like Lysinibacillus sphaericus III(3)7, which possesses important environmental properties, this alteration along with morphological changes and reduced sporulation rates may exert a negative effect on metabolic studies using plasmids as cloning platforms. The aim of this study is to approach the metabolic behavior of pMK4-bearing cells of L. sphaericus III(3)7 through the use of bioinformatic and in vitro analyses. An incompatibility model between the pMK4 vector and a predicted megaplasmid, pBsph, inside III(3)7 cells was constructed based on an incA region. Additionally, in vitro long-term plasmid stability was not found in plasmid-bearing cells. Alignments between replicons, mobile genetic elements and RNA-RNA interactions were assessed, pairwise alignment visualization, graphic models and morphological changes were evaluated by SEM. Metabolite analysis was done through HPLC coupled to a Q-TOF 6545, and electrospray ionization was used, finally, Aedes aegypti and Culex quinquefasciatus larvae were used for larvicidal activity assessment. Results found, a decreased growth rate, spore formation reduction and morphological changes, which supported the idea of metabolic cost exerted by pMK4. An incompatibility between pMK4 and pBsph appears to take place inside L. sphaericus III(3)7 cells, however, further in vitro studies are needed to confirm it.

Engineering microorganisms for the biosynthesis of dicarboxylic acids

Dicarboxylic acids (DCAs) are important commodity chemicals which have been widely applied in polymer, food and pharmaceutical industries. Biosynthesis of DCAs from renewable carbon sources represents a promising alternative to chemical synthesis. Over the years, the recombinant strains have been constructed to produce an increasing number of DCAs. In this review, recent advances on the microbial synthesis of various DCAs have been summarized and categorized into three groups: the tricarboxylic acid cycle-derived, lysine metabolism-related, and aromatic compounds degradation-derived DCAs. We focused mainly on the metabolic engineering and synthetic biology strategies for improving the production efficiency, including metabolic flux analysis, fine-tuning of gene expression, cofactor balancing, metabolic compartmentalization, dynamic regulation and co-culture to regulate the production at multiple levels. The current challenges and perspectives have also been discussed.

Antifungal Mechanism of Dipicolinic Acid and Its Efficacy for the Biocontrol of Pear Valsa Canker

Valsa pyri is a fatal canker pathogen that causes significant reduction of crop yield in pear orchards. V. pyri invades the trunk phloem, and is difficult to control by chemical treatment. In this work, it was found for the first time that Bacillus subtilis-produced dipicolinic acid (DPA) exhibits antifungal activity against different canker pathogens, including Alteraria alternata, Botryosphaeria dothidea, Rhizoctonia solani, and V. pyri. Growth inhibition of V. pyri was observed at less than 5 mM concentration (pH = 5.6). DPA showed the highest antifungal activity at acidic pH values and in the presence of bivalent metals, such as zinc(II), cobalt(II), and copper(II). Measurement of mRNA expression levels and scanning electron microscope (SEM) observations revealed that DPA causes V. pyri apoptosis via inhibition of chitin biosynthesis and subsequent cell lysis. Interestingly, DPA showed high stability in the pear bark and was able to cross the pear tree bark into the phloem, protecting the internal phases of the pear trunk. In preventive applications, DPA reduced the canker symptoms of V. pyri on Cuigan pear trees by 90%. Taken together, an efficient strategy for the management of V. pyri-caused canker disease was developed using a novel antifungal agent, DPA, with strong antifungal activity and particular diffusion properties.

Metabolic engineering of carbon overflow metabolism of Bacillus subtilis for improved N-acetyl-glucosamine production

Bacillus subtilis is widely used as cell factories for the production of important industrial biochemicals. Although many studies have demonstrated the effects of organic acidic byproducts, such as acetate, on microbial fermentation, little is known about the effects of blocking the neutral byproduct overflow, such as acetoin, on bioproduction. In this study, we focused on the influences of modulating overflow metabolism on the production of N-acetyl-d-glucosamine (GlcNAc) in engineered B. subtilis. We found that acetoin overflow competes with GlcNAc production, and blocking acetoin overflow increased GlcNAc titer and yield by 1.38- and 1.39-fold, reaching 48.9 g/L and 0.32 g GlcNAc/g glucose, respectively. Further blocking acetate overflow inhibited cell growth and GlcNAc production may be induced by inhibiting glucose uptake. Taken together, our results show that blocking acetoin overflow is a promising strategy for enhancing GlcNAc production. The strategies developed in this work may be useful for engineering strains of B. subtilis for producing other important biochemicals.