Enhanced 2-keto-L-gulonic acid production by applying L-sorbose-tolerant helper strain in the co-culture system

Rhodotorula mucilaginosa A8, a potential helper strain in a vitamin C microbial fermentation process

In the vitamin C microbial fermentation system, oxidative stress limits the growth and 2-keto-l-gulonic acid (2-KLG, the precursor of vitamin C) production of Ketogulonicigenium vulgare. Most Bacillus strains, as helper strains, have been reported to release key biomolecules to reduce oxidative stress and promote the growth and 2-KLG production of K. vulgare. To understand the specific mechanism by which the helper strain and K. vulgare interact to reduce oxidative stress, a novel helper strain, Rhodotorula mucilaginosa A8, was used to construct a consortium in the co-culture fermentation system. Based on the activities of the antioxidant enzymes and quantitative polymerase chain reaction (qPCR) analysis, R. mucilaginosa A8 could reduce oxidative stress and increase 2-KLG production in K. vulgare by upregulating antioxidant enzyme activities and related gene-expression levels. In addition, the carotenoids of R. mucilaginosa promoted 2-KLG production in K. vulgare. Coculture of R. mucilaginosa with K. vulgare increased the yield of carotenoids. This study suggested that helper strains with the ability to reduce oxidative stress in K. vulgare would likely act as potential helper strains for facilitating 2-KLG biosynthesis. This work could provide a theoretical basis for the search for potential helper strains for vitamin C microbial fermentation and for the construction of synthetic microbial communities to produce valuable products.

iTRAQ-based proteomics analysis of Bacillus pumilus responses to acid stress and quorum sensing in a vitamin C fermentation system

Microbial consortia play a key role in human health, bioenergy, and food manufacturing due to their strong stability, robustness and versatility. One of the microbial consortia consisting of Ketogulonicigenium vulgare and Bacillus megaterium for the production of the vitamin C precursor, 2-keto-L-gulonic acid (2-KLG), has been widely used for large-scale industrial production. To further investigate the cell-cell communication in microbial consortia, a microbial consortium consisting of Ketogulonicigenium vulgare and Bacillus pumilus was constructed and the differences in protein expression at different fermentation time points (18 h and 40 h) were analyzed by iTRAQ-based proteomics. The results indicated that B. pumilus was subjected to acid shocks in the coculture fermentation system and responded to it. In addition, the quorum sensing system existed in the coculture fermentation system, and B. pumilus could secrete quorum-quenching lactonase (YtnP) to inhibit the signaling pathway of K. vulgare. This study offers valuable guidance for further studies of synthetic microbial consortia.

Novel thermo-alkali-stable cellulase-producing Serratia sp. AXJ-M cooperates with Arthrobacter sp. AXJ-M1 to improve degradation of cellulose in papermaking black liquor

There is an urgent requirement to treat cellulose present in papermaking black liquor since it induces severe economic wastes and causes environmental pollution. We characterized cellulase activity at different temperatures and pH to seek thermo-alkali-stable cellulase-producing bacteria, a natural consortium of Serratia sp. AXJ-M and Arthrobacter sp. AXJ-M1 was used to improve the degradation of cellulose. Notably, the enzyme activities and the degradation rate of cellulose were increased by 30%-70% and 30% after co-culture, respectively. In addition, the addition of cosubstrates increased the degradation rate of cellulose beyond 30%. The thermo-alkali-stable endoglucanase (bcsZ) gene was derived from the strain AXJ-M and was cloned and expressed. The purified bcsZ displayed the maximum activity at 70 °C and pH 9. Mn²⁺, Ca²⁺, Mg²⁺ and Tween-20 had beneficial effects on the enzyme activity. Structurally, bcsZ potentially catalyzed the degradation of cellulose. The co-culture with ligninolytic activities significantly decreased target the parameters (cellulose 45% and COD 95%) while using the immobilized fluidized bed reactors (FBRs). Finally, toxicological tests and antioxidant enzyme activities indicated that the co-culture had a detoxifying effect on black liquor. Our study showed that Serratia sp. AXJ-M acts synergistically with Arthrobacter sp. AXJ-M1 may be potentially useful for bioremediation for black liquor.

Cobalt Phosphate Nanocrystals: A Catalase-Like Nanozyme and In Situ Enzyme-Encapsulating Carrier for Efficient Chemoenzymatic Synthesis of α-Keto Acid

Chemoenzymatic catalysis combining the traits of chemical and enzymatic catalysis provides tremendous possibilities for the design of biosynthetic pathways utilizing inorganic catalysts and enzymes. However, the efficiency of chemoenzymatic catalysis is usually governed by the synergy and compatibility of the two catalysts. Here, we report for the first time the catalase-like activity of cobalt phosphate nanocrystals (CoPs). By a one-pot biomimetic mineralization with CoPs and l-amino acid oxidase (LAAO) under a mild condition, we have fabricated a hybrid nanobiocatalyst, LAAO@CoPs, for the chemoenzymatic synthesis of α-keto acid. The as-fabricated nanobiocatalyst with directly contacted catalytic sites of the enzyme and nanozyme maximizes the substrate channeling effects for in situ chemical decomposition of the oxidative intermediate, H₂O₂, during the enzymatic oxidation of l-tryptophan (l-Trp), thus minimizing the H₂O₂ accumulation and byproduct generation. Benefiting from the superiority of LAAO@CoPs, complete conversion (100.0%) of l-Trp to indole pyruvic acid is achieved, over two times higher than the yield of the free LAAO system (47.6%). Meanwhile, LAAO@CoPs show high stabilities against heat and proteolytic treatments. This work offers a new design approach for constructing a high-performance nanobiocatalyst for cascade reactions, especially for those systems with toxic or reactive intermediates.

A novel thermostable cellulase-producing Bacillus licheniformis A5 acts synergistically with Bacillus subtilis B2 to improve degradation of Chinese distillers' grains

Lack of effective degradation approaches of Chinese distillers' grains (CDGs) produced by Chinese liquor industry results in environmental pollution and economic waste. Cellulase activity was characterized at different temperatures to find thermostable cellulase-producing bacteria, and microbial co-culture method was used to improve the degradation of CDGs. Incubation of endoglucanase produced by Bacillus licheniformis A5 at 80 °C for 120 min showed 82% residual enzyme activity. Notably, enzyme activity increased by 30%-70% after co-culturing Bacillus licheniformis A5 and Bacillus subtilis B2. The two strains increased degradation rate of CDGs by 70% compared with optimized results of Bacillus subtilis B2 culture alone, and increased the reducing sugar content to 16.6 mg/mL. In addition, 2% ethanol increased degradation rate of CDGs by 15% in co-culture. The findings of this study imply that Bacillus licheniformis A5 acts synergistically with Bacillus subtilis B2 to improve degradation of CDGs.

Comparative analysis of the chemical and biochemical synthesis of keto acids

Keto acids are essential organic acids that are widely applied in pharmaceuticals, cosmetics, food, beverages, and feed additives as well as chemical synthesis. Currently, most keto acids on the market are prepared via chemical synthesis. The biochemical synthesis of keto acids has been discovered with the development of metabolic engineering and applied toward the production of specific keto acids from renewable carbohydrates using different metabolic engineering strategies in microbes. In this review, we provide a systematic summary of the types and applications of keto acids, and then summarize and compare the chemical and biochemical synthesis routes used for the production of typical keto acids, including pyruvic acid, oxaloacetic acid, α-oxobutanoic acid, acetoacetic acid, ketoglutaric acid, levulinic acid, 5-aminolevulinic acid, α-ketoisovaleric acid, α-keto-γ-methylthiobutyric acid, α-ketoisocaproic acid, 2-keto-L-gulonic acid, 2-keto-D-gluconic acid, 5-keto-D-gluconic acid, and phenylpyruvic acid. We also describe the current challenges for the industrial-scale production of keto acids and further strategies used to accelerate the green production of keto acids via biochemical routes.

Enhanced 2-keto-L-gulonic acid production by a mixed culture of Ketogulonicigenium vulgare and Bacillus megaterium using three-stage temperature control strategy

As a key precursor of vitamin C, 2-keto-L-gulonic acid (2-KLG) was mainly produced from L-sorbose by mixed fermentation of Ketogulonicigenium vulgare and a helper strain (Bacillus spp.) with a low conversion rate for decades. The aim of this study was to enhance the 2-KLG production by co-culturing K. vulgare and Bacillus megaterium using three-stage temperature control (TSTC) strategy. By investigating the temperature effect on the 2-KLG fermentation, the optimum temperatures for the growths of K. vulgare and B. megaterium were 32 °C and 29 °C, respectively, while the optimum temperature for 2-KLG production was 35 °C. We developed a TSTC process: the temperature was kept at 32 °C during the first 16 h of fermentation, then decreased to 29 °C for the following 14 h, and maintained at 35 °C to the end of fermentation. By using this new process, the productivity and yield of 2-KLG from L-sorbose were obtained at 2.19 ± 0.19 g/L/h and 92.91 ± 1.02 g/L in 20-L fermentors for 5 batches, respectively, which were 22.35% and 6.02% higher than that of the control treatment (the single temperature of 29 °C). The increased cell density of K. vulgare during the exponential phase and the enhanced SDH activity (increased by 25.18% at 36 h, 17.14% at 44 h) in the production stage might be the reasons for enhanced 2-KLG conversion rate and yield. Our results demonstrated the feasibility of the TSTC strategy for 2-KLG production.

Efficient Optimization of Gluconobacter oxydans Based on Protein Scaffold-Trimeric CutA to Enhance the Chemical Structure Stability of Enzymes for the Direct Production of 2-Keto-L-gulonic Acid

2-Keto-L-gulonic acid (2-KLG), the direct precursor of vitamin C, is produced by a two-step fermentation route from D-sorbitol in industry. However, this route is a complicated mix-culture system which involves three bacteria. Thus, replacement of the conventional two-step fermentation process with a one-step process could be revolutionary in vitamin C industry. The one-step fermentation of 2-keto-L-gulonic acid (2-KLG) has been achieved in our previous study; 32.4 g/L of 2-KLG production was obtained by the one-step strain G. oxydans/pGUC-tufB-sdh-GGGGS-sndh after 168 h. In this study, L-sorbose dehydrogenase (SDH) and L-sorbosone dehydrogenase (SNDH) were expressed in G. oxydans after the codon optimization. Furthermore, the trimeric protein CutA was used to improve the chemical structure stability of SDH and SNDH. The recombinant strain G. oxydans/pGUC-tufB-SH3-sdh-GGGGS-sndh-tufB-SH3lig-(GGGGS)2-cutA produced 40.3 g/L of 2-KLG after 168 h. In addition, the expression levels of the cofactor PQQ were enhanced to further improve 2-KLG production. With the stepwise metabolic engineering of G. oxydans, the final 2-KLG production was improved to 42.6 g/L. The efficient one-step production of 2-KLG was achieved, and the final one-step industrial-scale production of 2-KLG is drawing near.

Antioxidant effect of glutathione on promoting 2-keto-l-gulonic acid production in vitamin C fermentation system

AIMS

Oxidative stress limited the growth of cells and 2-keto-l-gulonic acid (2-KGA) production in vitamin C (Vc) fermentation system. The study aims to investigate the antioxidant effect of glutathione on promoting 2-KGA in Vc fermentation system using Ketogulonicigenium vulgare 25B-1 and Bacillus endophyticus ST-1 as the co-culturing microbes.

METHODS AND RESULTS

The activities of antioxidant-related enzymes and qPCR were used to study the antioxidant effect of glutathione addition in Vc fermentation system. The addition of GSH and GSH/GSSG increased 2-KGA production and decreased fermentation time, and the highest 2-KGA production increased by 40·63% and the lowest fermentation time shortened to 60 h when the addition of optimal concentration ratio of GSH/GSSG was 50 : 1. Moreover, the increased production of 2-KGA was accompanied by up-regulated the activities of total antioxidant capacity (T-AOC), total superoxide dismutase (T-SOD), catalase (CAT) and over-expressed oxidative stress-related genes sod, gst, gr, zwf, gp, which resulted in scavenging reactive oxygen species to reduce oxidative stress in Vc fermentation system.

CONCLUSIONS

Glutathione showed a significant effect on increasing 2-KGA production and decreasing fermentation time in Vc fermentation system. GSH/GSSG could maintain a dynamic balance with two forms of glutathione and the optimal concentration ratio of GSH/GSSG was 50 : 1.

SIGNIFICANCE AND IMPACT OF THE STUDY

Glutathione is proved to be effective to relieve oxidative stress. The promotion effects of GSSG and GSH on 2-KGA production could help to further explore the optimization of co-culture fermentation process for Vc industrial production.