Production of optically pure 2,3-butanediol from Miscanthus floridulus hydrolysate using engineered Bacillus licheniformis strains

Bacillales: From Taxonomy to Biotechnological and Industrial Perspectives

For a long time, the genus Bacillus has been known and considered among the most applicable genera in several fields. Recent taxonomical developments resulted in the identification of more species in Bacillus-related genera, particularly in the order Bacillales (earlier heterotypic synonym: Caryophanales), with potential application for biotechnological and industrial purposes such as biofuels, bioactive agents, biopolymers, and enzymes. Therefore, a thorough understanding of the taxonomy, growth requirements and physiology, genomics, and metabolic pathways in the highly diverse bacterial order, Bacillales, will facilitate a more robust designing and sustainable production of strain lines relevant to a circular economy. This paper is focused principally on less-known genera and their potential in the order Bacillales for promising applications in the industry and addresses the taxonomical complexities of this order. Moreover, it emphasizes the biotechnological usage of some engineered strains of the order Bacillales. The elucidation of novel taxa, their metabolic pathways, and growth conditions would make it possible to drive industrial processes toward an upgraded functionality based on the microbial nature.

Current Advances in Microbial Production of Acetoin and 2,3-Butanediol by Bacillus spp

The growing need for industrial production of bio-based acetoin and 2,3-butanediol (2,3-BD) is due to both environmental concerns, and their widespread use in the food, pharmaceutical, and chemical industries. Acetoin is a common spice added to many foods, but also a valuable reagent in many chemical syntheses. Similarly, 2,3-BD is an indispensable chemical on the platform in the production of synthetic rubber, printing inks, perfumes, antifreeze, and fuel additives. This state-of-the-art review focuses on representatives of the genus Bacillus as prospective producers of acetoin and 2,3-BD. They have the following important advantages: non-pathogenic nature, unpretentiousness to growing conditions, and the ability to utilize a huge number of substrates (glucose, sucrose, starch, cellulose, and inulin hydrolysates), sugars from the composition of lignocellulose (cellobiose, mannose, galactose, xylose, and arabinose), as well as waste glycerol. In addition, these strains can be improved by genetic engineering, and are amenable to process optimization. Bacillus spp. are among the best acetoin producers. They also synthesize 2,3-BD in titer and yield comparable to those of the pathogenic producers. However, Bacillus spp. show relatively lower productivity, which can be increased in the course of challenging future research.

Acetic acid acting as a signaling molecule in the quorum sensing system increases 2,3-butanediol production in Saccharomyces cerevisiae

2,3-Butanediol (2,3-BD) has been extensively used in chemical syntheses. This study aimed to explore acetic acid as a signaling molecule that activates a quorum sensing (QS) system to promote the production of 2,3-BD. The yield of 2,3-BD is proportional to the cell density. Saccharomyces cerevisiae W141 does not produce 2,3-BD when the cell density is lower than the threshold concentration (OD_600 nm = 10 or cell density 4.4 × 10⁸ CFU/mL). When 1.5 g/L acetic acid is added, the yield of 2,3-BD is 3.01 ± 0.04 g/L. Subsequently, S. cerevisiae W141 was cocultured with Acetobacter pasteurianus Huniang 1.01 under the optimal conditions, the acetic acid production was increased by 76.7% and 30.6% compared with the original strain and the strain cultivated with 1.5 g/L acetic acid, and the yield of 2,3-BD was increased by 81.9% and 3.3%, respectively. This difference is due to the activity of acetyl lactic acid synthase (ILV2) and 2,3-BD dehydrogenase (BDH1), as the relative expression of the ilv2 and bdh1 genes is increased. The results showed that the biosynthesis of 2,3-BD was regulated by acetic acid as a signaling molecule. S. cerevisiae is a promising host for producing 2,3-BD for industrial applications.

C4 Bacterial Volatiles Improve Plant Health

Plant growth-promoting rhizobacteria (PGPR) associated with plant roots can trigger plant growth promotion and induced systemic resistance. Several bacterial determinants including cell-wall components and secreted compounds have been identified to date. Here, we review a group of low-molecular-weight volatile compounds released by PGPR, which improve plant health, mostly by protecting plants against pathogen attack under greenhouse and field conditions. We particularly focus on C4 bacterial volatile compounds (BVCs), such as 2,3-butanediol and acetoin, which have been shown to activate the plant immune response and to promote plant growth at the molecular level as well as in large-scale field applications. We also disc/ uss the potential applications, metabolic engineering, and large-scale fermentation of C4 BVCs. The C4 bacterial volatiles act as airborne signals and therefore represent a new type of biocontrol agent. Further advances in the encapsulation procedure, together with the development of standards and guidelines, will promote the application of C4 volatiles in the field.

Disruption of the lactate dehydrogenase and acetate kinase genes in Klebsiella pneumoniae HD79 to enhance 2,3-butanediol production, and related transcriptomics analysis

OBJECTIVES

2,3-Butanediol (2,3-BD) is widely used in several chemical syntheses as well as the manufacture of plastics, solvents, and antifreeze formulations, and can be manufactured by microbial glucose fermentation. Conventional (2,3-BD) fermentation typically has low productivity, yield, and purity, and is expensive for commercial applications. We aimed to delete the lactate dehydrogenase and acetate kinase (ldhA and ack) genes in Klebsiella pneumoniae HD79 by using λRed homologous recombination technology, to eliminate by-products and thereby improve (2,3-BD) production. We also analyzed the resulting gene changes by using transcriptomics.

RESULTS

The yield of (2,3-BD) from the mutant Klebsiella strain was 46.21 g/L, the conversion rate was 0.47 g/g, and the productivity was 0.64 g/L·h, which represented increases of 54.9%, 20.5%, and 106.5% respectively, compared to (WT) strains. Lactate and acetate decreased by 48.2% and 62.8%, respectively. Transcriptomics analysis showed that 4628 genes were differentially expressed (404 significantly up-regulated and 162 significantly down-regulated). Moreover, the (2,3-BD) operon genes were differentially expressed.

CONCLUSION

Our data showed that the biosynthesis of (2,3-BD) was regulated by inducers (lactate and acetate), a regulator (BudR), and carbon flux. Elimination of acidic by-products by ldhA and ack knockdown significantly improved (2,3-BD) production. Our results provide a deeper understanding of the mechanisms underlying (2,3-BD) production, and form a molecular basis for the improvement this process by genetic modification in the future.

The current strategies and parameters for the enhanced microbial production of 2,3-butanediol

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2,3-Butanediol (2,3-BD) is a propitious compound with many industrial uses.

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2,3-BD production has always been hampered by low fermentation yields and high production costs.

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2,3-BD production may be enhanced by optimization of culture conditions and use of high-producing strains.

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TMetabolic engineering tools are currently used to generate high-yielding strains.

2,3-Butanediol (2,3-BD) is a propitious compound with many industrial uses ranging from rubber, fuels, and cosmetics to food additives. Its microbial production has especially attracted as an alternative way to the petroleum-based production. However, 2,3-BD production has always been hampered by low yields and high production costs. The enhanced production of 2,3-butanediol requires screening of the best strains and a systematic optimization of fermentation conditions. Moreover, the metabolic pathway engineering is essential to achieve the best results and minimize the production costs by rendering the strains to use efficiently low cost substrates. This review is to provide up-to-date information on the current strategies and parameters for the enhanced microbial production of 2,3-BD.