Folate requirements of the 2-keto-L-gulonic acid-producing strain Ketogulonigenium vulgare LMP P-20356 in L-sorbose/CSL medium

Recovery strategies and mechanisms of anammox reaction following inhibition by environmental factors: A review

Anaerobic ammonium oxidation (anammox) is a promising biological method for treating nitrogen-rich, low-carbon wastewater. However, the application of anammox technology in actual engineering is easily limited by environmental factors. Considerable progress has been investigated in recent years in anammox restoration strategies, significantly addressing the challenge of poor reaction performance following inhibition. This review systematically outlines the strategies employed to recover anammox performance following inhibition by conventional environmental factors and emerging pollutants. Additionally, comprehensive summaries of strategies aimed at promoting anammox activity and enhancing nitrogen removal performance provide valuable insights into the current research landscape in this field. The review contributes to a comprehensive understanding of restoration strategies of anammox-based technologies.

Recent Advances in 2-Keto-l-gulonic Acid Production Using Mixed-Culture Fermentation and Future Prospects

Vitamin C, also known as ascorbic acid, is an essential vitamin that cannot be synthesized by the human body and must be acquired through our diet. At present, the precursor of vitamin C, 2-keto-l-gulonic acid (2-KGA), is typically produced via a two-step fermentation process utilizing three bacterial strains. The second step of this traditional two-step fermentation method involves mixed-culture fermentation employing 2-KGA-producing bacteria (Ketogulonicigenium vulgare) along with associated bacteria. Because K. vulgare has defects in various metabolic pathways, associated bacteria are needed to provide key substances to promote K. vulgare growth and 2-KGA production. Unlike previous reviews where the main focus was the interaction between associated bacteria and K. vulgare, this Review presents the latest scientific research from the perspective of the metabolic pathways associated with 2-KGA production by K. vulgare and the mechanism underlying the interaction between K. vulgare and the associated bacteria. In addition, the dehydrogenases that are responsible for 2-KGA production, the 2-KGA synthesis pathway, strategies for simplifying 2-KGA production via a one-step fermentation route, and, finally, future prospects and research goals in vitamin C production are also presented.

The industrial versatility of Gluconobacter oxydans: current applications and future perspectives

Gluconobacter oxydans is a well-known acetic acid bacterium that has long been applied in the biotechnological industry. Its extraordinary capacity to oxidize a variety of sugars, polyols, and alcohols into acids, aldehydes, and ketones is advantageous for the production of valuable compounds. Relevant G. oxydans industrial applications are in the manufacture of L-ascorbic acid (vitamin C), miglitol, gluconic acid and its derivatives, and dihydroxyacetone. Increasing efforts on improving these processes have been made in the last few years, especially by applying metabolic engineering. Thereby, a series of genes have been targeted to construct powerful recombinant strains to be used in optimized fermentation. Furthermore, low-cost feedstocks, mostly agro-industrial wastes or byproducts, have been investigated, to reduce processing costs and improve the sustainability of G. oxydans bioprocess. Nonetheless, further research is required mainly to make these raw materials feasible at the industrial scale. The current shortage of suitable genetic tools for metabolic engineering modifications in G. oxydans is another challenge to be overcome. This paper aims to give an overview of the most relevant industrial G. oxydans processes and the current strategies developed for their improvement.

New Insight Into the Interspecies Shift of Anammox Bacteria Ca. "Brocadia" and Ca. "Jettenia" in Reactors Fed With Formate and Folate

The sensitivity of anaerobic ammonium-oxidizing (anammox) bacteria to environmental fluctuations is a frequent cause of reactor malfunctions. It was hypothesized that the addition of formate and folate would have a stimulating effect on anammox bacteria, which in turn would lead to the stability of the anammox process under conditions of a sharp increase in ammonium load, i.e., it helps overcome a stress factor. The effect of formate and folate was investigated using a setup consisting of three parallel sequencing batch reactors equipped with a carrier. Two runs of the reactors were performed. The composition of the microbial community was studied by the 16S rRNA gene profiling and metagenomic analysis. Among anammox bacteria, Ca. "Brocadia" spp. dominated during the first run. A stimulatory effect of folate on the daily nitrogen removal rate (dN) was identified. The addition of formate led to progress in dissimilatory nitrate reduction and stimulated the growth of Ca. "Jettenia" spp. The spatial separation of two anammox species was observed in the formate reactor: Ca. "Brocadia" occupied the carrier and Ca. "Jettenia"-the walls of the reactors. Biomass storage at low temperature without feeding led to an interspecies shift in anammox bacteria in favor of Ca. "Jettenia." During the second run, a domination of Ca. "Jettenia" spp. was recorded along with a stimulating effect of formate, and there was no effect of folate on dN. A comparative genome analysis revealed the patterns suggesting different strategies used by Ca. "Brocadia" and Ca. "Jettenia" spp. to cope with environmental changes.

Construction of synthetic microbial consortia for 2-keto-L-gulonic acid biosynthesis

Currently, the establishment of synthetic microbial consortia with rational strategies has gained extensive attention, becoming one of the important frontiers of synthetic biology. Systems biology can offer insights into the design and construction of synthetic microbial consortia. Taking the high-efficiency production of 2-keto-l-gulonic acid (2-KLG) as an example, we constructed a synthetic microbial consortium “Saccharomyces cerevisiae-Ketogulonigenium vulgare” based on systems biology analysis. In the consortium, K. vulgare was the 2-KLG producing strain, and S. cerevisiae acted as the helper strain. Comparative transcriptomic analysis was performed on an engineered S. cerevisiae (VTC2) and a wild-type S. cerevisiae BY4741. The results showed that the up-regulated genes in VTC2, compared with BY4741, were mainly involved in glycolysis, TCA cycle, purine metabolism, and biosynthesis of amino acids, B vitamins, and antioxidant proteases, all of which play important roles in promoting the growth of K. vulgare. Furthermore, Vitamin C produced by VTC2 could further relieve the oxidative stress in the environment to increase the production of 2-KLG. Therefore, VTC2 would be of great advantage in working with K. vulgare. Thus, the synthetic microbial consortium "VTC2-K. vulgare" was constructed based on transcriptomics analyses, and the accumulation of 2-KLG was increased by 1.49-fold compared with that of mono-cultured K. vulgare, reaching 13.2 ± 0.52 g/L. In addition, the increased production of 2-KLG was accompanied by the up-regulated activities of superoxide dismutase and catalase in the medium and the up-regulated oxidative stress-related genes (sod, cat and gpd) in K. vulgare. The results indicated that the oxidative stress in the synthetic microbial consortium was efficiently reduced. Thus, systems analysis confirmed a favorable symbiotic relationship between microorganisms, providing guidance for further engineering synthetic consortia.

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.

Reconstruction of amino acid biosynthetic pathways increases the productivity of 2-keto-l-gulonic acid in Ketogulonicigenium vulgare-Bacillus endophyticus consortium via genes screening

Defect in the amino acid biosynthetic pathways of Ketogulonicigenium vulgare, the producing strain for 2-keto-l-gulonic acid (2-KGA), is the key reason for its poor growth and low productivity. In this study, five different strains were firstly reconstructed by expressing absent genes in threonine, proline and histidine biosynthetic pathways for better 2-KGA productivity. When mono-cultured in the shake flasks, the strain SyBE_Kv02080002 expressing hsk from Gluconobacter oxydans in threonine biosynthetic pathway achieved the highest biomass and the titer increased by 25.13%. When co-cultured with Bacillus endophyticus, the fermentation cycle decreased by 28.57% than that of the original consortium in 5-L fermenter. Furthermore, reconstruction of threonine biosynthetic pathway resulted in up-regulation of genes encoding sorbosone dehydrogenase and idonate-dehydrogenase, which increased the 2-KGA productivity in SyBE_Kv02080002. This study shows that reconstruction of absent biosynthetic pathways in bacteria is an effective way to enhance the productivity of target products.

Comparative genomics and metabolomics analyses of the adaptation mechanism in Ketogulonicigenium vulgare-Bacillus thuringiensis consortium

Adaptive evolution by serial subcultivation of co-cultured Bacillus thuringiensis and Ketogulonicigenium vulgare significantly enhanced the productivity of 2-keto-L-gulonic acid in two-step vitamin C production. The adaptation mechanism in K. vulgare-B. thuringiensis consortium was investigated in this study based on comparative genomics and metabolomics studies. It was found that the growth, anti-oxidation, transcription and regulation were significantly enhanced in the adapted consortium. The mutation of the genes, which encode amidohydrolase in adapted K. vulgare (K150) and amino acid permease in adapted B. thuringiensis (B150), resulted in the increase of some amino acids levels in each species, and further enhanced the metabolic exchange and growth ability of the two species. Besides, the mutation of the gene encoding spore germination protein enhanced the metabolic levels of tricarboxylic acid cycle, and decreased the sporulation in B150, which induced its growth. The mutation of the genes, which encode NADPH nitroreductase in K150 and NADPH-dependent FMN reductase in B150, may enhance the ability of anti-oxidation. Overall, the long-term adaptation of K. vulgare and B. thuringiensis influenced the global regulation and made them more inseparable in metabolite exchange. Our work will provide ideas for the molecular design and optimization in microbial consortium.

Comparative analysis of L-sorbose dehydrogenase by docking strategy for 2-keto-L-gulonic acid production in Ketogulonicigenium vulgare and Bacillus endophyticus consortium

Improving the yield of 2-keto-L-gulonic acid (2-KGA), the direct precursor of vitamin C, draws more and more attention in industrial production. In this study, we try to increase the 2-KGA productivity by computer-aided selection of genes encoding L-sorbose dehydrogenases (SDH) of Ketogulonicigenium vulgare. First, six SDHs were modeled by docking strategy to predict the binding mode with co-factor PQQ. The binding energy between SSDA1-H/SSDA1-L and PQQ was the highest, followed by SSDA3/SSDA2. The binding energy between SSDA1-P/SSDB and PQQ was the lowest. Then, these genes were overexpressed, respectively, in an industrial strain K. vulgare HKv604. Overexpression of ssda1-l and ssda1-h enhanced the 2-KGA production by 7.89 and 12.56 % in mono-cultured K. vulgare, and by 13.21 and 16.86 % when K. vulgare was co-cultured with Bacillus endophyticus. When the engineered K. vulgare SyBE_Kv000116013 (overexpression of ssda1-p) or SyBE_Kv000116016 (overexpression of ssdb) was co-cultured with B. endophyticus, the 2-KGA production decreased significantly. The docking results were in accordance with the experimental data, which indicated that computer-aided modeling is an efficient strategy for screening more efficient enzymes.

Symbiotic relationship analysis of predominant bacteria in a lab-scale anammox UASB bioreactor

In order to provide the comprehensive insight into the key microbial groups in anaerobic ammonium oxidation (anammox) process, high-throughput sequencing analysis has been used for the investigation of the bacterial communities of a lab-scale upflow anaerobic sludge bed (UASB) anammox bioreactor. Results revealed that 109 operational taxonomic units (OTUs; out of 14,820 reads) were identified and a domination of anammox bacteria of Candidatus Kuenenia stuttgartiensis (OTU474, 35.42 %), along with heterotrophs of Limnobacter sp. MED105 (OTU951, 14.98 %), Anerolinea thermophila UNI-1 (OTU465 and OTU833, 6.60 and 3.93 %), Azoarcus sp. B72 (OTU26, 9.47 %), and Ignavibacterium sp. JCM 16511 (OTU459, 8.33 %) were detected. Metabolic pathway analysis showed that Candidatus K. stuttgartiensis encountered gene defect in synthesizing a series of metabolic cofactors for growth, implying that K. stuttgartiensis is auxotrophic. Coincidentally, the other dominant species severally showed complete metabolic pathways with full set gene encoding to corresponding cofactors presented in the surrounding environment. Furthermore, it was likely that the survival of heterotrophs in the autotrophic system indicates the existence of a symbiotic and mutual relationship in anammox system.

Insights into mutualism mechanism and versatile metabolism of Ketogulonicigenium vulgare Hbe602 based on comparative genomics and metabolomics studies

Ketogulonicigenium vulgare has been widely used in vitamin C two steps fermentation and requires companion strain for optimal growth. However, the understanding of K. vulgare as well as its companion strain is still preliminary. Here, the complete genome of K. vulgare Hbe602 was deciphered to provide insight into the symbiosis mechanism and the versatile metabolism. K. vulgare contains the LuxR family proteins, chemokine proteins, flagellar structure proteins, peptides and transporters for symbiosis consortium. Besides, the growth state and metabolite variation of K. vulgare were observed when five carbohydrates (D-sorbitol, L-sorbose, D-glucose, D-fructose and D-mannitol) were used as carbon source. The growth increased by 40.72% and 62.97% respectively when K. vulgare was cultured on D-mannitol/D-sorbitol than on L-sorbose. The insufficient metabolism of carbohydrates, amino acids and vitamins is the main reason for the slow growth of K. vulgare. The combined analysis of genomics and metabolomics indicated that TCA cycle, amino acid and nucleotide metabolism were significantly up-regulated when K. vulgare was cultured on the D-mannitol/D-sorbitol, which facilitated the better growth. The present study would be helpful to further understand its metabolic structure and guide the engineering transformation.

Reorganization of a synthetic microbial consortium for one-step vitamin C fermentation

Background

In the industry, the conventional two-step fermentation method was used to produce 2-keto-l-gulonic acid (2-KGA), the precursor of vitamin C, by three strains, namely, Gluconobacter oxydans, Bacillus spp. and Ketogulonicigenium vulgare. Despite its high production efficiency, the long incubation period and an additional second sterilization process inhibit the further development. Therefore, we aimed to reorganize a synthetic consortium of G. oxydans and K. vulgare for one-step fermentation of 2-KGA and enhance the symbiotic interaction between microorganisms to perform better.

Results

During the fermentation, competition for sorbose of G. oxydans arose when co-cultured with K. vulgare. In this study, the competition between the two microbes was alleviated and their mutualism was enhanced by deleting genes involved in sorbose metabolism of G. oxydans. In the engineered synthetic consortium (H₆ + Kv), the yield of 2-KGA (mol/mol) against d-sorbitol reached 89.7 % within 36 h, increased by 29.6 %. Furthermore, metabolomic analysis was used to verify the enhancement of the symbiotic relationship and to provide us potential strategies for improving the synthetic consortium. Additionally, a significant redistribution of metabolism occurred by co-culturing the K. vulgare with the engineered G. oxydans, mainly reflected in the increased TCA cycle, purine, and fatty acid metabolism.

Conclusions

We reorganized and optimized a synthetic consortium of G. oxydans and K. vulgare to produce 2-KGA directly from d-sorbitol. The yield of 2-KGA was comparable to that of the conventional two-step fermentation. The metabolic interaction between the strains was further investigated by metabolomics, which verified the enhancement of the mutualism between the microbes and gave us a better understanding of the synthetic consortium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0418-6) contains supplementary material, which is available to authorized users.

l-sorbose is not only a substrate for 2-keto-l-gulonic acid production in the artificial microbial ecosystem of two strains mixed fermentation

The co-culture system of the fermentation process of vitamin C can be regarded as an artificial microbial ecosystem (AME). To extend our understanding of this AME, an investigation of the relationship between strains, substrate and product was carried out in this study. The results showed that both Ketogulonicigenium vulgare and 2-keto-l-gulonic acid (2-KLG, the precursor of vitamin C) can inhibit the growth of the helper strain, while the helper strain promoted the growth of K. vulgare and 2-KLG production. Moreover, l-sorbose is not only a substrate for 2-KLG production in the AME, but also a promoter of K. vulgare and an inhibitor of the helper strain. In the earlier stage of fermentation, the inhibition of l-sorbose on the helper strain’s growth is a key factor for ensuring an efficient fermentation. In the condition of adding the extra helper strain (OD: 0.57, ratio of inoculation: 2 %), the yields of 2-KLG is increased by 9 % in the 14 % l-sorbose medium. To the best of our knowledge, this is the first report about the inhibition of substrate in the AME of 2-KLG production.

Metabolic model reconstruction and analysis of an artificial microbial ecosystem for vitamin C production

An artificial microbial ecosystem (AME) consisting of Ketogulonicigenium vulgare and Bacillus megaterium is currently used in a two-step fermentation process for vitamin C production. In order to obtain a comprehensive understanding of the metabolic interactions between the two bacteria, a two-species stoichiometric metabolic model (iWZ-KV-663-BM-1055) consisting of 1718 genes, 1573 metabolites, and 1891 reactions (excluding exchange reactions) was constructed based on separate genome-scale metabolic models (GSMMs) of K. vulgare and B. megaterium. These two compartments (k and b) of iWZ-KV-663-BM-1055 shared 453 reactions and 548 metabolites. Compartment b was richer in subsystems than compartment k. In minimal media with glucose (MG), metabolite exchange between compartments was assessed by constraint-based analysis. Compartment b secreted essential amino acids, nucleic acids, vitamins and cofactors important for K. vulgare growth and biosynthesis of 2-keto-l-gulonic acid (2-KLG). Further research showed that when co-cultured with B. megaterium in l-sorbose-CSLP medium, the growth rate of K. vulgare and 2-KLG production were increased by 111.9% and 29.42%, respectively, under the constraints employed. Our study demonstrated that GSMMs and constraint-based methods can be used to decode the physiological features and inter-species interactions of AMEs used in industrial biotechnology, which will be of benefit for improving regulation and refinement in future industrial processes.

Metabolomic analysis of cooperative adaptation between co-cultured Bacillus cereus and Ketogulonicigenium vulgare

The cooperative adaptation of subcultivated Bacillus cereus and Ketogulonicigenium vulgare significantly increased the productivity of 2-keto-L-gulonic acid, the precursor of vitamin C. The mechanism of cooperative adaptation of the serial subcultivated B. cereus and K. vulgare was investigated in this study by culturing the two strains orthogonally on agar plates. It was found that the swarming distance of B. cereus along the trace of K. vulgare on the plate decreased after 150 days' subcultivation. Metabolomic analysis on these co-cultured B. cereus and K. vulgare strains showed that their cooperative adaptation was accomplished by three key events: (i) the ability of nutrients (e.g., amino acids and purines) searching and intaking, and proteins biosynthesis is increased in the evolved B. cereus; (ii) the capability of protein degradation and amino acids transportation is enhanced in evolved K. vulgare; (iii) the evolved B. cereus was found to provide more nutrients (mostly amino acids and purines) to K. vulgare, thus strengthening the oxidation and energy generation of K. vulgare. Our results provided novel insights into the systems-level understanding of the cooperative adaptation between strains in synergistic consortium.

Metabolomic analysis of the positive effects on Ketogulonigenium vulgare growth and 2-keto-L-gulonic acid production by reduced glutathione

Ketogulonigenium vulgare has long been used in industry to produce 2-keto-L-gulonic acid (2KGA), the precursor of vitamin C. This fermentation process involves co-culture of K. vulgare and a Bacillus species. Early studies demonstrated that the presence of the Bacillus strain can enhance the cellular growth and 2KGA production of K. vulgare. However, the molecular mechanism behind how Bacillus affects the growth of K. vulgare and 2KGA production remains unclear. In addition, the inclusion of Bacillus in the fermentation process presents difficulties for the post-separation and purification of 2KGA. To address these issues, efforts have been made to replace the Bacillus strain with chemical compounds. In this study, we found that adding thiol compounds such as reduced glutathione (GSH) and dithiothreitol (DTT) to the K. vulgare mono-culture system can increase the growth of K. vulgare about twofold, and increase 2KGA production by about fivefold. The effects of thiols on the concentrations of some cellular metabolites were determined using gas chromatography coupled to time-of-flight mass spectrometry. The results showed that the levels of intracellular amino acids and intermediates in the pentose phosphate pathway increased significantly after thiol addition. Interestingly, when GSH was added, the levels of key intracellular metabolites in primary metabolic pathways and the cell biomass both reached their maximum in the first 36 h, and then decreased when the thiol was exhausted. These findings indicate that cell growth needs the assistance of a high concentration of thiols. This study is the first report that chemically defined compounds were used to enhance the growth of K. vulgare and 2KGA production. Furthermore, it also provides new insights into the possible cellular interaction between Bacillus species and K. vulgare.

Proteomic analysis of Ketogulonicigenium vulgare under glutathione reveals high demand for thiamin transport and antioxidant protection

Ketogulonicigenium vulgare, though grows poorly when mono-cultured, has been widely used in the industrial production of the precursor of vitamin C with the coculture of Bacillus megaterium. Various efforts have been made to clarify the synergic pattern of this artificial microbial community and to improve the growth and production ability of K. vulgare, but there is still no sound explanation. In previous research, we found that the addition of reduced glutathione into K. vulgare monoculture could significantly improve its growth and productivity. By performing SEM and TEM, we observed that after adding GSH into K. vulgare monoculture, cells became about 4–6 folds elongated, and formed intracytoplasmic membranes (ICM). To explore the molecular mechanism and provide insights into the investigation of the synergic pattern of the co-culture system, we conducted a comparative iTRAQ-2-D-LC-MS/MS-based proteomic analysis of K. vulgare grown under reduced glutathione. Principal component analysis of proteomic data showed that after the addition of glutathione, proteins for thiamin/thiamin pyrophosphate (TPP) transport, glutathione transport and the maintenance of membrane integrity, together with several membrane-bound dehydrogenases had significant up-regulation. Besides, several proteins participating in the pentose phosphate pathway and tricarboxylic acid cycle were also up-regulated. Additionally, proteins combating intracellular reactive oxygen species were also up-regulated, which similarly occurred in K. vulgare when the co-cultured B. megaterium cells lysed from our former research results. This study reveals the demand for transmembrane transport of substrates, especially thiamin, and the demand for antioxidant protection of K. vulgare.

Metabolic engineering of microorganisms for vitamin C production

Vitamin C, an important organic acid, is widely used in the industries of pharmaceuticals, cosmetics, food, beverage and feed additives. Compared with the Reichstein method, biotechnological production of vitamin C is an attractive approach due to the low cost and high product quality. In this chapter, biosynthesis of vitamin C, including one-step fermentation processes and two-step fermentation processes are discussed and compared. Furthermore, the prospects of the biotechnological production of vitamin C are also presented.

Genetic engineering of Ketogulonigenium vulgare for enhanced production of 2-keto-L-gulonic acid

Folate derivatives are crucial growth factors for Ketogulonigenium vulgare which is used in mixed culture with Bacillus megaterium for the industrial production of 2-keto-L-gulonic acid (2-KGA), the precursor of L-ascorbic acid (L-AA) or vitamin C (Vc). To improve the growth and 2-KGA production, five genes involved in folate biosynthesis identified in a folate gene cluster from Lactococcus lactis MG1363, including folB, folKE, folP, folQ and folC, were over-expressed in K. vulgare. Intracellular folate concentration in the recombinant strain harboring folate biosynthesis genes cluster under the control of P(sdh) (sorbose dehydrogenase gene sdh promoter from K. vulgare) was 8 times higher than that of the wildtype K. vulgare DSM 4025 (P<0.001). In shake flask studies, the cell density and 2-KGA production of the recombinant K. vulgare Rif (pMCS2PsdhfolBC) were increased by 18% (P<0.001) and 14% (P<0.001), respectively, under a relatively stable pH 7 condition. In fermentor studies, enhancements around 25% cell density (P<0.001) and approximately 35% 2-KGA productivity (P<0.001) were observed in comparison with the controls without over-expressing the folate biosynthesis genes. This was the first successful study of metabolic engineering on K. vulgare for enhanced 2-KGA production.

Integrated proteomic and metabolomic analysis of an artificial microbial community for two-step production of vitamin C

An artificial microbial community consisted of Ketogulonicigenium vulgare and Bacillus megaterium has been used in industry to produce 2-keto-gulonic acid (2-KGA), the precursor of vitamin C. During the mix culture fermentation process, sporulation and cell lysis of B. megaterium can be observed. In order to investigate how these phenomena correlate with 2-KGA production, and to explore how two species interact with each other during the fermentation process, an integrated time-series proteomic and metabolomic analysis was applied to the system. The study quantitatively identified approximate 100 metabolites and 258 proteins. Principal Component Analysis of all the metabolites identified showed that glutamic acid, 5-oxo-proline, L-sorbose, 2-KGA, 2, 6-dipicolinic acid and tyrosine were potential biomarkers to distinguish the different time-series samples. Interestingly, most of these metabolites were closely correlated with the sporulation process of B. megaterium. Together with several sporulation-relevant proteins identified, the results pointed to the possibility that Bacillus sporulation process might be important part of the microbial interaction. After sporulation, cell lysis of B. megaterium was observed in the co-culture system. The proteomic results showed that proteins combating against intracellular reactive oxygen stress (ROS), and proteins involved in pentose phosphate pathway, L-sorbose pathway, tricarboxylic acid cycle and amino acids metabolism were up-regulated when the cell lysis of B. megaterium occurred. The cell lysis might supply purine substrates needed for K. vulgare growth. These discoveries showed B. megaterium provided key elements necessary for K. vulgare to grow better and produce more 2-KGA. The study represents the first attempt to decipher 2-KGA-producing microbial communities using quantitative systems biology analysis.

Metabolome profiling reveals metabolic cooperation between Bacillus megaterium and Ketogulonicigenium vulgare during induced swarm motility

The metabolic cooperation in the ecosystem of Bacillus megaterium and Ketogulonicigenium vulgare was investigated by cultivating them spatially on a soft agar plate. We found that B. megaterium swarmed in a direction along the trace of K. vulgare on the agar plate. Metabolomics based on gas chromatography coupled with time-of-flight mass spectrometry (GC-TOF-MS) was employed to analyze the interaction mechanism between the two microorganisms. We found that the microorganisms interact by exchanging a number of metabolites. Both intracellular metabolism and cell-cell communication via metabolic cooperation were essential in determining the population dynamics of the ecosystem. The contents of amino acids and other nutritional compounds in K. vulgare were rather low in comparison to those in B. megaterium, but the levels of these compounds in the medium surrounding K. vulgare were fairly high, even higher than in fresh medium. Erythrose, erythritol, guanine, and inositol accumulated around B. megaterium were consumed by K. vulgare upon its migration. The oxidization products of K. vulgare, including 2-keto-gulonic acids (2KGA), were sharply increased. Upon coculturing of B. megaterium and K. vulgare, 2,6-dipicolinic acid (the biomarker of sporulation of B. megaterium), was remarkably increased compared with those in the monocultures. Therefore, the interactions between B. megaterium and K. vulgare were a synergistic combination of mutualism and antagonism. This paper is the first to systematically identify a symbiotic interaction mechanism via metabolites in the ecosystem established by two isolated colonies of B. megaterium and K. vulgare.

Development of chemically defined media supporting high cell density growth of Ketogulonicigenium vulgare and Bacillus megaterium

The immediate precursor of L-ascorbic acid, or vitamin C, is 2-keto-L-gulonic acid (2-KLG). This is commonly produced commercially by Ketogulonicigenium vulgare and Bacillus megaterium, using corn steep liquor powder (CSLP) as an organic nitrogen source. In this study, the effects of the individual CSLP components (amino acids, vitamins, and metal elements) on 2-KLG production were evaluated, with the aim of developing a complete, chemically defined medium for 2-KLG production. Forty components of CSLP were analyzed, and key components were correlated to biomass, 2-KLG productivity, and consumption rate of L-sorbose. Glycine had the greatest effect, followed by serine, biotin, proline, nicotinic acid, and threonine. The combination of 0.28 g L(-1) serine, 0.36 g L(-1) glycine, 0.18 g L(-1) threonine, 0.28 g L(-1) proline, 0.19 g L(-1) nicotinic acid, and 0.62 mg L(-1)biotin in a chemically defined medium produced the highest maximum biomass concentration (4.2 × 10(9) cfu mL(-1)), 2-KLG concentration (58 g L(-1)), and yield (0.76 g g(-1)) after culturing for 28 h.