In vivo evolution of lactic acid hyper-tolerant Clostridium thermocellum

Hydrogen production pathways in Clostridia and their improvement by metabolic engineering

Biological production of hydrogen has a tremendous potential as an environmentally sustainable technology to generate a clean fuel. Among the different available methods to produce biohydrogen, dark fermentation features the highest productivity and can be used as a means to dispose of organic waste biomass. Within this approach, Clostridia have the highest theoretical H2 production yield. Nonetheless, most strains show actual yields far lower than the theoretical maximum: improving their efficiency becomes necessary for achieving cost-effective fermentation processes. This review aims at providing a survey of the metabolic network involved in H2 generation in Clostridia and strategies used to improve it through metabolic engineering. Together with current achievements, a number of future perspectives to implement these results will be illustrated.

Adaptive Evolution for the Efficient Production of High-Quality d-Lactic Acid Using Engineered Klebsiella pneumoniae

d-Lactic acid serves as a pivotal platform chemical in the production of poly d-lactic acid (PDLA) and other value-added products. This compound can be synthesized by certain bacteria, including Klebsiella pneumoniae. However, industrial-scale lactic acid production in Klebsiella pneumoniae faces challenges due to growth inhibition caused by lactic acid stress, which acts as a bottleneck in commercial microbial fermentation processes. To address this, we employed a combination of evolutionary and genetic engineering approaches to create an improved Klebsiella pneumoniae strain with enhanced lactic acid tolerance and production. In flask fermentation experiments, the engineered strain achieved an impressive accumulation of 19.56 g/L d-lactic acid, representing the highest production yield observed in Klebsiella pneumoniae to date. Consequently, this strain holds significant promise for applications in industrial bioprocessing. Notably, our genome sequencing and experimental analyses revealed a novel correlation between UTP-glucose-1-phosphate uridylyltransferase GalU and lactic acid resistance in Klebsiella pneumoniae. Further research is warranted to explore the potential of targeting GalU for enhancing d-lactic acid production.

Utilization of coffee waste as a sustainable feedstock for high-yield lactic acid production through microbial fermentation

Lactic acid is an important industrial precursor; however, high substrate costs are a major challenge in microbial fermentation-based lactic acid production. Coffee waste is a sustainable feedstock alternative for lactic acid production via microbial fermentation. Herein, the feasibility of coffee waste as a feedstock was explored by employing appropriate pretreatment methods and optimizing enzyme combinations. Coffee waste pretreatment with hydrogen peroxide and acetic acid along with a combination of Viscozyme L, Celluclast 1.5 L, and Pectinex Ultra SP-L achieved the 78.9 % sugar conversion rate at a substrate concentration of 4 % (w/v). Lactiplantibacillus plantarum WiKim0126-induced fermentation with a 4 % solid loading yielded a lactic acid concentration of 22.8 g/L (99.6 % of the theoretical maximum yield) and productivity of 0.95 g/L/h within 24 h. These findings highlight the viability of coffee waste as an eco-friendly resource for sustainable lactic acid production.

Engineering the Metabolic Landscape of Microorganisms for Lignocellulosic Conversion

Bacteria and yeast are being intensively used to produce biofuels and high-added-value products by using plant biomass derivatives as substrates. The number of microorganisms available for industrial processes is increasing thanks to biotechnological improvements to enhance their productivity and yield through microbial metabolic engineering and laboratory evolution. This is allowing the traditional industrial processes for biofuel production, which included multiple steps, to be improved through the consolidation of single-step processes, reducing the time of the global process, and increasing the yield and operational conditions in terms of the desired products. Engineered microorganisms are now capable of using feedstocks that they were unable to process before their modification, opening broader possibilities for establishing new markets in places where biomass is available. This review discusses metabolic engineering approaches that have been used to improve the microbial processing of biomass to convert the plant feedstock into fuels. Metabolically engineered microorganisms (MEMs) such as bacteria, yeasts, and microalgae are described, highlighting their performance and the biotechnological tools that were used to modify them. Finally, some examples of patents related to the MEMs are mentioned in order to contextualize their current industrial use.

Rapid adaptive evolution of Bacillus coagulans to undetoxified corncob hydrolysates for lactic acid production and new insights into its high phenolic degradation

Here, an adapted Bacillus coagulans (Weizmannia coagulans) strain CC17B-1 was developed for lignocellulosic lactic acid production through a short and rapid adaptive laboratory evolution technique. Without any detoxification, two actual corn cob hydrolysates from the factory were effectively fermented to lactic acid within 60 h. Strain CC17B-1 is capable of degrading all nine determined phenolic compounds in the hydrolysate, with the only exception being vanillic acid. Notably, its tolerances for ferulic acid and p-coumaric acid are the highest doses reported in anaerobic microbes. A proposed degradation pathway showed that strain CC17B-1 could convert phenolic aldehydes to phenolic alcohol and then further degrade them completely. This work provides new ideas for the microbe phenolic degradation pathway and paves the way for industrial lactic acid production from lignocellulosic biomass.