Design and development of a “Y-shaped” microbial consortium capable of simultaneously utilizing biomass sugars for efficient production of butanol

Metabolic engineering of Escherichia coli BL21(DE3) cocultured with glucose and xylose for efficient production of 2'-fucosyllactose

2'-Fucosyllactose (2'-FL) is the most abundant human milk oligosaccharide (HMO) and has been approved to be commercially added to infant formula. Microbial synthesis from exogenous lactose via metabolic engineering is currently the major approach to production of 2'-FL. Replacement of lactose with cheaper sugars such as glucose and sucrose has been studied to reduce the production costs. Herein, Escherichia coli BL21(DE3) was engineered to produce 2'-FL by co-culture with glucose and xylose, the main components of lignocellulosic biomass. Firstly, synthetic pathway of lactose from xylose and glucose was constructed by introducing a lactose-forming enzyme, strengthening xylose uptake pathway, and weakening glucose metabolic pathway. Then, a highly-active α1,2-fucosyltransferase BKHT was introduced to produce 2'-FL and GDP-fucose supply was enhanced to increase 2'-FL production. As a result, when cocultured with glucose and xylose, the engineered strain produced 6.53 g/L and 27.53 g/L of 2'-FL by shake-flask and fed-batch cultivation, respectively.

Metabolic engineering of Corynebacterium crenatum for enhanced L-tyrosine production from mannitol and glucose

BACKGROUND

L-Tyrosine (L-Tyr) is a significant aromatic amino acid that is experiencing an increasing demand in the market due to its distinctive characteristics. Traditional production methods exhibit various limitations, prompting researchers to place greater emphasis on microbial synthesis as an alternative approach.

RESULTS

Here, we developed a metabolic engineering-based method for efficient production of L-Tyr from Corynebacterium crenatum, including the elimination of competing pathways, the overexpression of aroB, aroD, and aroE, and the introduction of the mutated E. coli tyrAfbr gene for elevating L-Tyr generation. Moreover, the mtlR gene was knocked out, and the mtlD and pfkB genes were overexpressed, allowing C. crenatum to produce L-Tyr from mannitol. The L-Tyr production achieved 6.42 g/L at a glucose-to-mannitol ratio of 3:1 in a shake flask, which was 16.9% higher than that of glucose alone. Notably, the L-Tyr production of the fed-batch fermentation was elevated to 34.6 g/L, exhibiting the highest titers among those of C. glutamicum previously reported.

CONCLUSION

The importance of this research is underscored by its pioneering application of mannitol as a carbon source for the biosynthesis of L-Tyr, as well as its examination of the influence of mannitol-associated genes in microbial metabolism. A promising platform is provided for the production of target compounds that does not compete with human food source.

Co-utilization of carbon sources in microorganisms for the bioproduction of chemicals

Carbon source is crucial for the cell growth and metabolism in microorganisms, and its utilization significantly affects the synthesis efficiency of target products in microbial cell factories. Compared with a single carbon source, co-utilizing carbon sources provide an alternative approach to optimize the utilization of different carbon sources for efficient biosynthesis of many chemicals with higher titer/yield/productivity. However, the efficiency of bioproduction is significantly limited by the sequential utilization of a preferred carbon source and secondary carbon sources, attributed to carbon catabolite repression (CCR). This review aimed to introduce the mechanisms of CCR and further focus on the summary of the strategies for co-utilization of carbon sources, including alleviation of CCR, engineering of the transport and metabolism of secondary carbon sources, compulsive co-utilization in single culture, co-utilization of carbon sources via co-culture, and evolutionary approaches. The findings of representative studies with a significant improvement in the bioproduction of chemicals via the co-utilization of carbon sources were discussed in this review. It suggested that by combining rational metabolic engineering and irrational evolutionary approaches, co-utilizing carbon sources can significantly contribute to the bioproduction of chemicals.

A molecular toolkit of cross-feeding strains for engineering synthetic yeast communities

Engineered microbial consortia often have enhanced system performance and robustness compared with single-strain biomanufacturing production platforms. However, few tools are available for generating co-cultures of the model and key industrial host Saccharomyces cerevisiae. Here we engineer auxotrophic and overexpression yeast strains that can be used to create co-cultures through exchange of essential metabolites. Using these strains as modules, we engineered two- and three-member consortia using different cross-feeding architectures. Through a combination of ensemble modelling and experimentation, we explored how cellular (for example, metabolite production strength) and environmental (for example, initial population ratio, population density and extracellular supplementation) factors govern population dynamics in these systems. We tested the use of the toolkit in a division of labour biomanufacturing case study and show that it enables enhanced and tuneable antioxidant resveratrol production. We expect this toolkit to become a useful resource for a variety of applications in synthetic ecology and biomanufacturing.

Simultaneous glucose and xylose utilization by an Escherichia coli catabolite repression mutant

As advances are made toward the industrial feasibility of mass-producing biofuels and commodity chemicals with sugar-fermenting microbes, high feedstock costs continue to inhibit commercial application. Hydrolyzed lignocellulosic biomass represents an ideal feedstock for these purposes as it is cheap and prevalent. However, many microbes, including Escherichia coli, struggle to efficiently utilize this mixture of hexose and pentose sugars due to the regulation of the carbon catabolite repression (CCR) system. CCR causes a sequential utilization of sugars, rather than simultaneous utilization, resulting in reduced carbon yield and complex process implications in fed-batch fermentation. A mutant of the gene encoding the cyclic AMP receptor protein, crp*, has been shown to disable CCR and improve the co-utilization of mixed sugar substrates. Here, we present the strain construction and characterization of a site-specific crp* chromosomal mutant in E. coli BL21 star (DE3). The crp* mutant strain demonstrates simultaneous consumption of glucose and xylose, suggesting a deregulated CCR system. The proteomics further showed that glucose was routed to the C5 carbon utilization pathways to support both de novo nucleotide synthesis and energy production in the crp* mutant strain. Metabolite analyses further show that overflow metabolism contributes to the slower growth in the crp* mutant. This highly characterized strain can be particularly beneficial for chemical production by simultaneously utilizing both C5 and C6 substrates from lignocellulosic biomass.IMPORTANCEAs the need for renewable biofuel and biochemical production processes continues to grow, there is an associated need for microbial technology capable of utilizing cheap, widely available, and renewable carbon substrates. This work details the construction and characterization of the first B-lineage Escherichia coli strain with mutated cyclic AMP receptor protein, Crp*, which deregulates the carbon catabolite repression (CCR) system and enables the co-utilization of multiple sugar sources in the growth medium. In this study, we focus our analysis on glucose and xylose utilization as these two sugars are the primary components in lignocellulosic biomass hydrolysate, a promising renewable carbon feedstock for industrial bioprocesses. This strain is valuable to the field as it enables the use of mixed sugar sources in traditional fed-batch based approaches, whereas the wild-type carbon catabolite repression system leads to biphasic growth and possible buildup of non-preferential sugars, reducing process efficiency at scale.

Advances in biosynthesis of higher alcohols in Escherichia coli

In recent years, the development of green energy to replace fossil fuels has been the focus of research. Higher alcohols are important biofuels and chemicals. The production of higher alcohols in microbes has gained attention due to its environmentally friendly character. Higher alcohols have been synthesized in model microorganism Escherichia coli, and the production has reached the gram level through enhancement of metabolic flow, the balance of reducing power and the optimization of fermentation processes. Sustainable bio-higher alcohols production is expected to replace fossil fuels as a green and renewable energy source. Therefore, this review summarizes the latest developments in producing higher alcohols (C3-C6) by E. coli, elucidate the main bottlenecks limiting the biosynthesis of higher alcohols, and proposes potential engineering strategies of improving the production of biological higher alcohols. This review would provide a theoretical basis for further research on higher alcohols production by E. coli.

Metabolic Engineering: Methodologies and Applications

Metabolic engineering aims to improve the production of economically valuable molecules through the genetic manipulation of microbial metabolism. While the discipline is a little over 30 years old, advancements in metabolic engineering have given way to industrial-level molecule production benefitting multiple industries such as chemical, agriculture, food, pharmaceutical, and energy industries. This review describes the design, build, test, and learn steps necessary for leading a successful metabolic engineering campaign. Moreover, we highlight major applications of metabolic engineering, including synthesizing chemicals and fuels, broadening substrate utilization, and improving host robustness with a focus on specific case studies. Finally, we conclude with a discussion on perspectives and future challenges related to metabolic engineering.

Clostridium as microbial cell factory to enable the sustainable utilization of three generations of feedstocks

The sustainable production of chemicals and biofuels from non-fossil carbon sources is considered key to reducing greenhouse gas (GHG) emissions. Clostridium sp. can convert various substrates, including the 1st-generation (biomass crops), the 2nd-generation (lignocellulosic biomass), and the 3rd-generation (C1 gases) feedstocks, into high-value products, which makes Clostridia attractive for biorefinery applications. However, the complexity of lignocellulosic catabolism and C1 gas utilization make it difficult to construct efficient production routes. Accordingly, this review highlights the advances in the development of three generations of feedstocks with Clostridia as cell factories. At the same time, more attention was given to using agro-industrial wastes (lignocelluloses and C1 gases) as the feedstocks, for which metabolic and process engineering efforts were comprehensively analyzed. In addition, the challenges of using agro-industrial wastes are also discussed. Lastly, several new synthetic biology tools and regulatory strategies are emphasized as promising technologies to be developed to address the aforementioned challenges in Clostridia and realize the efficient utilization of agro-industrial wastes.

Production of butanol from lignocellulosic biomass: recent advances, challenges, and prospects

Due to energy and environmental concerns, biobutanol is gaining increasing attention as an alternative renewable fuel owing to its desirable fuel properties. Biobutanol production from lignocellulosic biomass through acetone-butanol-ethanol (ABE) fermentation has gained much interest globally due to its sustainable supply and non-competitiveness with food, but large-scale fermentative production suffers from low product titres and poor selectivity. This review presents recent developments in lignocellulosic butanol production, including pretreatment and hydrolysis of hemicellulose and cellulose during ABE fermentation. Challenges are discussed, including low concentrations of fermentation sugars, inhibitors, detoxification, and carbon catabolite repression. Some key process improvements are also summarised to guide further research and development towards more profitable and commercially viable butanol fermentation.

Engineering E. coli to synthesize butanol

Biobutanol is gaining much attention as a potential biofuel due to its superior properties over ethanol. Butanol has been naturally produced via acetone-butanol-ethanol (ABE) fermentation by many Clostridium species, which are not very user-friendly bacteria. Therefore, to improve butanol titers and yield, various butanol synthesis pathways have been engineered in Escherichia coli, a much more robust and convenient host than Clostridium species. This review mainly focuses on the biosynthesis of n-butanol in engineered E. coli with an emphasis on efficient enzymes for butanol production in E. coli, butanol competing pathways, and genome engineering of E. coli for butanol production. In addition, the use of alternate strategies for butanol biosynthesis/enhancement, alternate substrates for the low cost of butanol production, and genetic improvement for butanol tolerance in E. coli have also been discussed.

Cellulosic ethanol production by consortia of Scheffersomyces stipitis and engineered Zymomonas mobilis

BACKGROUND

As one of the clean and sustainable energies, lignocellulosic ethanol has achieved much attention around the world. The production of lignocellulosic ethanol does not compete with people for food, while the consumption of ethanol could contribute to the carbon dioxide emission reduction. However, the simultaneous transformation of glucose and xylose to ethanol is one of the key technologies for attaining cost-efficient lignocellulosic ethanol production at an industrial scale. Genetic modification of strains and constructing consortia were two approaches to resolve this issue. Compared with strain improvement, the synergistic interaction of consortia in metabolic pathways should be more useful than using each one separately.

RESULTS

In this study, the consortia consisting of suspended Scheffersomyces stipitis CICC1960 and Zymomonas mobilis 8b were cultivated to successfully depress carbon catabolite repression (CCR) in artificially simulated 80G40XRM. With this strategy, a 5.52% more xylose consumption and a 6.52% higher ethanol titer were achieved by the consortium, in which the inoculation ratio between S. stipitis and Z. mobilis was 1:3, compared with the Z. mobilis 8b mono-fermentation. Subsequently, one copy of the xylose metabolic genes was inserted into the Z. mobilis 8b genome to construct Z. mobilis FR2, leading to the xylose final-consumption amount and ethanol titer improvement by 15.36% and 6.81%, respectively. Finally, various corn stover hydrolysates with different sugar concentrations (glucose and xylose 60, 90, 120 g/L), were used to evaluate the fermentation performance of the consortium consisting of S. stipitis CICC1960 and Z. mobilis FR2. Fermentation results showed that a 1.56-4.59% higher ethanol titer was achieved by the consortium compared with the Z. mobilis FR2 mono-fermentation, and a 46.12-102.14% higher ethanol titer was observed in the consortium fermentation when compared with the S. stipitis CICC1960 mono-fermentation. Furthermore, qRT-PCR analysis of xylose/glucose transporter and other genes responsible for CCR explained the reason why the initial ratio inoculation of 1:3 in artificially simulated 80G40XRM had the best fermentation performance in the consortium.

CONCLUSIONS

The fermentation strategy used in this study, i.e., using a genetically modified consortium, had a superior performance in ethanol production, as compared with the S. stipitis CICC1960 mono-fermentation and the Z. mobilis FR2 mono-fermentation alone. This result showed that this strategy has potential for future lignocellulosic ethanol production.

Mutations in adaptively evolved Escherichia coli LGE2 facilitated the cost-effective upgrading of undetoxified bio-oil to bioethanol fuel

Levoglucosan is a promising sugar present in the lignocellulose pyrolysis bio-oil, which is a renewable and environment-friendly source for various value-added productions. Although many microbial catalysts have been engineered to produce biofuels and chemicals from levoglucosan, the demerits that these biocatalysts can only utilize pure levoglucosan while inhibited by the inhibitors co-existing with levoglucosan in the bio-oil have greatly limited the industrial-scale application of these biocatalysts in lignocellulose biorefinery. In this study, the previously engineered Escherichia coli LGE2 was evolved for enhanced inhibitor tolerance using long-term adaptive evolution under the stress of multiple inhibitors and finally, a stable mutant E. coli-H was obtained after ~ 374 generations' evolution. In the bio-oil media with an extremely acidic pH of 3.1, E. coli-H with high inhibitor tolerance exhibited remarkable levoglucosan consumption and ethanol production abilities comparable to the control, while the growth of the non-evolved strain was completely blocked even when the pH was adjusted to 7.0. Finally, 8.4 g/L ethanol was achieved by E. coli-H in the undetoxified bio-oil media with ~ 2.0% (w/v) levoglucosan, reaching 82% of the theoretical yield. Whole-genome re-sequencing to monitor the acquisition of mutations identified 4 new mutations within the globally regulatory genes rssB, yqhA, and basR, and the - 10 box of the putative promoter of yqhD-dgkA operon. Especially, yqhA was the first time to be revealed as a gene responsible for inhibitor tolerance. The mutations were all responsible for improved fitness, while basR mutation greatly contributed to the fitness improvement of E. coli-H. This study, for the first time, generated an inhibitor-tolerant levoglucosan-utilizing strain that could produce cost-effective bioethanol from the toxic bio-oil without detoxification process, and provided important experimental evidence and valuable genetic/proteinic information for the development of other robust microbial platforms involved in lignocellulose biorefining processes.

Metabolic engineering for the utilization of carbohydrate portions of lignocellulosic biomass

The petrochemical industry has grown to meet the need for massive production of energy and commodities along with an explosive population growth; however, serious side effects such as greenhouse gas emissions and global warming have negatively impacted the environment. Lignocellulosic biomass with myriad quantities on Earth is an attractive resource for the production of carbon-neutral fuels and chemicals through environmentally friendly processes of microbial fermentation. This review discusses metabolic engineering efforts to achieve economically feasible industrial production of fuels and chemicals from microbial cell factories using the carbohydrate portion of lignocellulosic biomass as substrates. The combined knowledge of systems biology and metabolic engineering has been applied to construct robust platform microorganisms with maximum conversion of monomeric sugars, such as glucose and xylose, derived from lignocellulosic biomass. By comprehensively revisiting carbon conversion pathways, we provide a rationale for engineering strategies, as well as their features, feasibility, and recent representative studies. In addition, we briefly discuss how tools in systems biology can be applied in the field of metabolic engineering to accelerate the development of microbial cell factories that convert lignocellulosic biomass into carbon-neutral fuels and chemicals with economic feasibility.

Metabolic engineering for the production of butanol, a potential advanced biofuel, from renewable resources

Butanol is an important chemical and potential fuel. For more than 100 years, acetone-butanol-ethanol (ABE) fermentation of Clostridium strains has been the most successful process for biological butanol production. In recent years, other microbes have been engineered to produce butanol as well, among which Escherichia coli was the best one. Considering the crude oil price fluctuation, minimizing the cost of butanol production is of highest priority for its industrial application. Therefore, using cheaper feedstocks instead of pure sugars is an important project. In this review, we summarized butanol production from different renewable resources, such as industrial and food waste, lignocellulosic biomass, syngas and other renewable resources. This review will present the current progress in this field and provide insights for further engineering efforts on renewable butanol production.

Microbial production of butyl butyrate: from single strain to cognate consortium

Butyl butyrate (BB) is an important chemical with versatile applications in beverage, food and cosmetics industries. Since chemical synthesis of BB may cause adverse impacts on the environment, biotechnology is an emerging alternative approach for microbial esters biosynthesis. BB can be synthesized by using a single Clostridium strain natively producing butanol or butyrate, with exogenously supplemented butyrate or butanol, in the presence of lipase. Recently, E. coli strains have been engineered to produce BB, but the titer and yield remained very low. This review highlighted a new trend of developing cognate microbial consortium for BB production and associated challenges, and end up with new prospects for further improvement for microbial BB biosynthesis.

Highly Efficient Production of N-Acetyl-glucosamine in Escherichia coli by Appropriate Catabolic Division of Labor in the Utilization of Mixed Glycerol/Glucose Carbon Sources

Currently, microbial production is becoming a competitive method for N-acetyl-glucosamine production. As the biosynthesis of N-acetyl-glucosamine originating from fructose-6-P directly competes with central carbon metabolism for precursor supply, the consumption of glucose for cell growth and cellular metabolism severely limits the yield of N-acetyl-glucosamine. In this study, appropriate catabolic division of labor in the utilization of mixed carbon sources was achieved by deleting the pfkA gene and enhancing the utilization of glycerol by introducing the glpK mutant. Glycerol thus mainly contributed to cell growth and cellular metabolism, and more glucose was saved for efficient N-acetyl-glucosamine synthesis. By optimizing the ratio of glycerol to glucose, the balancing of cell growth/cellular metabolism and N-acetyl-glucosamine synthesis was achieved. The resulting strain GLALD-7 produced 179.7 g/L N-acetyl-glucosamine using mixed glycerol/glucose (1:8, m/m) carbon sources in a 5 L bioreactor, with a yield of 0.458 g/g total carbon sources (0.529 g/g glucose) and a productivity of 2.57 g/L/h. Coherent high titer/yield/productivity was obtained, with the highest values ever reported, suggesting that an appropriate catabolic division of labor using mixed glycerol/glucose carbon sources is a useful strategy for facilitating the microbial production of chemicals originating from glucose or metabolites upstream of glycolysis.

One-pot production of butyl butyrate from glucose using a cognate "diamond-shaped" E. coli consortium

Esters are widely used in plastics, textile fibers, and general petrochemicals. Usually, esters are produced via chemical synthesis or enzymatic processes from the corresponding alcohols and acids. However, the fermentative production of esters from alcohols and/or acids has recently also become feasible. Here we report a cognate microbial consortium capable of producing butyl butyrate. This microbial consortium consists of two engineered butyrate- and butanol-producing E. coli strains with nearly identical genetic background. The pathways for the synthesis of butyrate and butanol from butyryl-CoA in the respective E. coli strains, together with a lipase-catalyzed esterification reaction, created a "diamond-shaped" consortium. The concentration of butyrate and butanol in the fermentation vessel could be altered by adjusting the inoculation ratios of each E. coli strain in the consortium. After optimization, the consortium produced 7.2 g/L butyl butyrate with a yield of 0.12 g/g glucose without the exogenous addition of butanol or butyrate. To our best knowledge, this is the highest titer and yield of butyl butyrate produced by E. coli reported to date. This study thus provides a new way for the biotechnological production of esters.

Microbial co-culturing strategies for the production high value compounds, a reliable framework towards sustainable biorefinery implementation - an overview

The microbial co-cultures or consortia are a natural set of microorganisms formed from different species or the same species but different strains, in which members can interact with each other. The co-culture systems have wide variety of technological applications such as the production of foods, treatment of wastewater, removal of toxic substances, environmental recovery, and all these without the need to work in sterile conditions. Therefore, the need of understanding communication mechanisms between cell-to-cell within co-culture will allow to construct and to program their biological behavior from the use of complex substrates to produce biocompounds. The technology of co-culture systems enables the development of biorefinery platforms to obtain biofuels, and high value compounds through biomass transformation by sustainable process. This review focuses on understanding the roles of consortia microbial to design and built co-culture systems to produce high value compounds in terms a sustainable biorefinery.