Facilitating stable gene integration expression and copy number amplification in Bacillus subtilis through a reversible homologous recombination switch

High-efficiency secretion expression of cellobiose 2-epimerase in Escherichia coli and its applications

Cellobiose 2-epimerase (CE) plays a crucial role in catalyzing the conversion of lactose. In this study, the N-terminal 20 amino acids of Lactobacillus amylovorus feruloyl esterase (N20) were employed as a signal peptide and fused with the CE gene from Caldicellulosiruptor bescii for recombinant expression. Following ligation with the pET-22b(+) vector, Escherichia coli BL21 (DE3) was transformed. SDS-PAGE analysis confirmed the extracellular secretion of the CE following fusion with the signal peptide. Following fermentation optimization to maximize extracellular protein secretion, the optimal conditions were identified as a 2 × YT medium, supplemented with 0.8 mM IPTG, 0.1 mM ferrous ion (Fe2+), and 25 mM glycine after a 2.5 h induction, with incubation at 37 °C and 200 rpm for 36 h. The CE was purified using ammonium sulfate precipitation at 60 % saturation, yielding 1529.61 mg of enzyme protein per liter of fermentation broth, with a specific activity of 19.25 U/mg. A lactose substrate at 40 % concentration was employed, with varying enzyme concentrations (0.3825 g/L, 0.765 g/L, 1.1475 g/L, and 1.53 g/L) and reaction times (3 h, 6 h, 9 h, 12 h, and 24 h). After reaction, high-performance liquid chromatography (HPLC) was used for analysis, determining that an enzyme concentration of 1.53 g/L reacting with the lactose substrate for 24 h achieved the highest lactulose conversion rate at 56 %. This constitutes the first study on the direct extracellular secretion of CE, laying the groundwork for its production and application.

Harnessing microbial heterogeneity for improved biosynthesis fueled by synthetic biology

Metabolic engineering-driven microbial cell factories have made great progress in the efficient bioproduction of biochemical and recombinant proteins. However, the low efficiency and robustness of microbial cell factories limit their industrial applications. Harnessing microbial heterogeneity contributes to solving this. In this review, the origins of microbial heterogeneity and its effects on biosynthesis are first summarized. Synthetic biology-driven tools and strategies that can be used to improve biosynthesis by increasing and reducing microbial heterogeneity are then systematically summarized. Next, novel single-cell technologies available for unraveling microbial heterogeneity and facilitating heterogeneity regulation are discussed. Furthermore, a combined workflow of increasing genetic heterogeneity in the strain-building step to help in screening highly productive strains - reducing heterogeneity in the production process to obtain highly robust strains (IHP-RHR) facilitated by single-cell technologies was proposed to obtain highly productive and robust strains by harnessing microbial heterogeneity. Finally, the prospects and future challenges are discussed.

Plasmid-Stabilizing Strains for Antibiotic-Free Chemical Fermentation

Plasmid-mediated antibiotic-free fermentation holds significant industrial potential. However, the requirements for host elements and energy during plasmid inheritance often cause cell burden, leading to plasmid loss and reduced production. The stable maintenance of plasmids is primarily achieved through a complex mechanism, making it challenging to rationally design plasmid-stabilizing strains and characterize the associated genetic factors. In this study, we introduced a fluorescence-based high-throughput method and successfully screened plasmid-stabilizing strains from the genomic fragment-deletion strains of Escherichia coli MG1655 and Bacillus subtilis 168. The application of EcΔ50 in antibiotic-free fermentation increased the alanine titer 2.9 times. The enhanced plasmid stability in EcΔ50 was attributed to the coordinated deletion of genes involved in plasmid segregation and replication control, leading to improved plasmid maintenance and increased plasmid copy number. The increased plasmid stability of BsΔ44 was due to the deletion of the phage SPP1 surface receptor gene yueB, resulting in minimized sporulation, improved plasmid segregational stability and host adaptation. Antibiotic-free fermentation results showed that strain BsΔyueB exhibited a 61.99% higher acetoin titer compared to strain Bs168, reaching 3.96 g/L. When used for the fermentation of the downstream product, 2,3-butanediol, strain BsΔyueB achieved an 80.63% higher titer than Bs168, reaching 14.94 g/L using rich carbon and nitrogen feedstocks. Overall, our work provided a plasmid-stabilizing chassis for E. coli and B. subtilis, highlighting their potential for antibiotic-free fermentation of valuable products and metabolic engineering applications.