Microbial production of valuable chemicals by modular co-culture strategy

Bioconversion of CO2 into valuable bioproducts via synthetic modular co-culture of engineered Chlamydomonas reinhardtii and Escherichia coli

With increasing concern over environmental problems and energy crises, interest in the biological conversion of CO2 into bioproducts is growing. Although microalgae efficiently utilize CO2, their metabolic engineering remains challenging. In contrast, while synthetic biology tools are advanced for many heterotrophic bacteria, these organisms cannot directly utilize CO2. As such, a modular co-culture system with a glycolate dehydrogenase 1 (GYD1) deficient Chlamydomonas reinhardtii mutant and Escherichia coli was developed. The GYD1 mutant secretes glycolic acid via photorespiration, which E. coli metabolizes via the glyoxylate cycle. E. coli growth was improved by implementing two-stage continuous systems to 2.0 mg L-1 h-1 on CO2. The production of green fluorescent protein (0.52 ng L-1 h-1) and lycopene (6.3 μg L-1 h-1) was also demonstrated. This study represents a successful case of a synthetic modular co-culture with a microalga and a heterotrophic bacterium, potentially contributing to sustainable industrial processes and reducing environmental impact.

Efficient biosynthesis of gallic acid by a syntrophic Escherichia coli co-culture system

Gallic acid (GA), a natural phenolic acid antioxidant, has significant therapeutic and industrial applications. However, its traditional manufacturing approach, based on plant extraction, has been associated with risks of environmental pollution as well as a limited range of applications. Consequently, microbial-based production of GA, being more environmental-friendly, is viewed as a potential alternative. This study reports the efficient biosynthesis of GA from renewable glucose via a syntrophic Escherichia coli co-culture system. An effective GA biosynthesis pathway was first analyzed and determined. Then the rate-limiting step involving the hydroxylation of protocatechuic acid (PCA) to GA was removed by integrating multiple copies of the key gene pobAT294A/Y385F into the chromosome of a PCA-overproducing strain. The resulting strain GA10 produced 41.88 g/L GA with a yield of 0.185 mol/mol, but up to 9.54 g/L of the intermediate PCA accumulated in the fermentation broth. To overcome this issue, a catalytic strain COT03 was constructed by coupling the metabolism of excess intracellular NADPH supply with the NADPH-consuming reaction catalyzed by PobAT294A/Y385F. This yielded a syntrophic E. coli co-culture system that consisted of a GA-overproducing strain (GA10) and a growth-coupled biocatalytic strain (COT03). Following optimization of the culture conditions, the co-culture system produced 57.66 g/L GA from glucose within 75 h, with a yield of 0.233 mol/mol and an average productivity of 0.769 g/L/h. This study lays the foundation for the potential industrial biomanufacturing of GA from glucose.

Relieving metabolic burden to improve robustness and bioproduction by industrial microorganisms

Metabolic burden is defined by the influence of genetic manipulation and environmental perturbations on the distribution of cellular resources. The rewiring of microbial metabolism for bio-based chemical production often leads to a metabolic burden, followed by adverse physiological effects, such as impaired cell growth and low product yields. Alleviating the burden imposed by undesirable metabolic changes has become an increasingly attractive approach for constructing robust microbial cell factories. In this review, we provide a brief overview of metabolic burden engineering, focusing specifically on recent developments and strategies for diminishing the burden while improving robustness and yield. A variety of examples are presented to showcase the promise of metabolic burden engineering in facilitating the design and construction of robust microbial cell factories. Finally, challenges and limitations encountered in metabolic burden engineering are discussed.

Development of a co-culture system for green production of caffeic acid from sugarcane bagasse hydrolysate

Caffeic acid (CA) is a phenolic acid compound widely used in pharmaceutical and food applications. However, the efficient synthesis of CA is usually limited by the resources of individual microbial platforms. Here, a cross-kingdom microbial consortium was developed to synthesize CA from sugarcane bagasse hydrolysate using Escherichia coli and Candida glycerinogenes as chassis. In the upstream E. coli module, shikimate accumulation was improved by intensifying the shikimate synthesis pathway and blocking shikimate metabolism to provide precursors for the downstream CA synthesis module. In the downstream C. glycerinogenes module, conversion of p-coumaric acid to CA was improved by increasing the supply of the cytoplasmic cofactor FAD(H2). Further, overexpression of ABC transporter-related genes promoted efflux of CA and enhanced strain resistance to CA, significantly increasing CA titer from 103.8 mg/L to 346.5 mg/L. Subsequently, optimization of the inoculation ratio of strains SA-Ec4 and CA-Cg27 in this cross-kingdom microbial consortium resulted in an increase in CA titer to 871.9 mg/L, which was 151.6% higher compared to the monoculture strain CA-Cg27. Ultimately, 2311.6 and 1943.2 mg/L of CA were obtained by optimization of the co-culture system in a 5 L bioreactor using mixed sugar and sugarcane bagasse hydrolysate, respectively, with 17.2-fold and 14.6-fold enhancement compared to the starting strain. The cross-kingdom microbial consortium developed in this study provides a reference for the production of other aromatic compounds from inexpensive raw materials.