Optimization of Bacillus subtilis natto growth parameters in glycerol-based medium for vitamin K (Menaquinone-7) production in biofilm reactors

Enhancing menaquinone-7 biosynthesis by optimized fermentation strategies and fed-batch cultivation of food-derived Bacillus subtilis

BACKGROUND

Menaquinone-7 (MK-7), a type of vitamin K2 (VK2), is considered to be a health-promoting functional ingredient in the current recommended dietary supply. However, it is a challenge to isolate high-yield MK-7-producing strains with reasonable safety from natural food, which further limits their use as starter culture.

RESULTS

In this study, an MK-7 production strain derived from commercial natto with a yield of 2.33 ± 0.06 mg L-1, Bacillus subtilis 4-b4, was screened using a selective medium containing rotenone combined with a VK2 enzyme-linked immunosorbent assay kit, and high-performance liquid chromatography. This strain exhibited excellent safety with no hemolysis, excellent antioxidant activity, and sensitivity to antibiotics. Furthermore, its fermentation strategies (fermentation condition and the supply of key precursor substances) were optimized to improve MK-7 biosynthesis. As a result, the yield increased 3.31-fold, reaching 7.70 ± 0.24 mg L-1 at shake flask fermentation level. After scaling up through fed-batch fermentation, the yield further increased to 23.43 ± 0.41 mg L-1 in a 5 L bioreactor.

CONCLUSION

The current study developed a strain with good safety and antibiotic sensitivity for producing MK-7, and improved its biosynthesis through optimized fermentation strategies and scaling up the process, laying the foundation for the availability of biologically active MK-7 fermented food in the food industry. © 2025 Society of Chemical Industry.

Optimal fermentation of Pseudomonas synxantha M1 and metabolomics analysis

The microbial agents based on plant growth promoting rhizobacteria (PGPR) have become a hot topic in agricultural research, while the optimization of fermentation conditions for PGPR-based microbial agents still lack systematic research. The single-factor and orthogonal experiments were conducted to determine the optimal fermentation conditions of Pseudomonas synxantha M1. The results indicated that the glycerol and shaker speed was the most significant factors that influence the number of bacteria of P. synxantha M1 fermentation liquid. The viable bacteria count of microbial agent reached 7.1 × 1012 cfu/mL at 36 h, which OD600 value increased by 116.40% compared to before optimization, and promote the growth of highland barley. Significant differences of metabolites of fermentation liquid was observed in different fermentation times, including organic acids, lipids, and organoheterocyclic compounds using liquid chromatography tandem mass spectrometry (LC-MS/MS). In addition, the fermentation liquid was found to contain indoleacetic acid, glutathione and xanthine at the end of fermentation, which might contribute for the growth of plants as bioactive substances.

Characterization of heat, salt, acid, alkaline, and antibiotic stress response in soil isolate Bacillus subtilis strain PSK.A2

Microbes play an essential role in soil fertility by replenishing the nutrients; they encounter various biotic and abiotic stresses disrupting their cellular homeostasis, which expedites activating a conserved signaling pathway for transient over-expression of heat shock proteins (HSPs). In the present study, a versatile soil bacterium Bacillus subtilis strain PSK.A2 was isolated and characterized. Further, the isolated bacterium was exposed with several stresses, viz., heat, salt, acid, alkaline, and antibiotics. Stress-attributed cellular morphological modifications such as swelling, shrinkage, and clump formation were observed under the scanning electron microscope. The comparative protein expression pattern was studied by SDS-PAGE, relative protein stabilization was assessed by protein aggregation assay, and relative survival was mapped by single spot dilution and colony-counting method under control, stressed, lethal, and stressed lethal conditions of the isolate. The findings demonstrated that bacterial stress tolerance was maintained via the activation of various HSPs of molecular weight ranging from 17 to 115 kD to respective stimuli. The treatment of subinhibitory dose of antibiotics not interfering protein synthesis (amoxicillin and ciprofloxacin) resulted in the expression of eight HSPs of molecular weight ranging from 18 to 71 kD. The pre-treatment of short stress dosage showed endured overall tolerance of bacterium to lethal conditions, as evidenced by moderately enhanced total soluble intracellular protein content, better protein stabilization, comparatively over-expressed HSPs, and relatively enhanced cell survival. These findings hold an opportunity for developing novel approaches towards enhancing microbial resilience in a variety of conditions, including industrial bioprocessing, environmental remediation, and infectious disease management.

Formation of a Novel Antagonistic Bacterial Combination to Enhance Biocontrol for Cucumber Fusarium Wilt

Paenibacillus polymyxa strain PJH16, isolated and tested by our team, suppresses cucumber Fusarium wilt as an efficient biocontrol agent. For further investigation, the strain has been combined with two other Bacillus strains (Bacillus velezensis VJH504 and Bacillus subtilis JNF2) to enhance biocontrol ability, which formed high-efficiency microbial agents in the current study. The methodological target taken is based on achieving the optimal growth conditions of the combined microbial agents; hence, the medium composition and culture conditions were optimized through a single-factor test, orthogonal test and response surface methodology. Following this, the effectiveness of the microbial combination was assessed through pot experiments, which provided a theoretical foundation for the synthesis of microbial flora to significantly control cucumber Fusarium wilt. The results showed excellent compatibility, proving suitable for the proliferation and growth of Paenibacillus polymyxa PJH16, Bacillus velezensis VJH504, and Bacillus subtilis JNF2 strains together, specifically, when the inoculation amounts were adjusted to 4% of each. Using the single-factor test and orthogonal test analysis, the optimum composition of culture medium for the composite strain was identified as 3% glucose as the optimal carbon source, 2% yeast extract powder as the preferred nitrogen source, and 1% dipotassium hydrogen phosphate as the most suitable inorganic salt. Furthermore, the optical density (OD600) of the composite strain solution reached its highest level at 3.16 under the following culture conditions: inoculation volume of 200 µL, 171 rpm culture speed, 21.6 h culture time, 30 °C cultural temperature, and an initial pH of 7.0. The pot experiment demonstrated that the mixed bacterial solution achieved a relative control efficacy of 93.4% against cucumber Fusarium wilt, which was significantly superior to that of single- strain or pesticide treatment, and also promoted cucumber growth. In summary, the microbial flora synthesized by the three Bacillus strains displayed a high bacterial concentration, following the optimization of culture conditions, and exerted remarkable control and growth-promoting effects on cucumber Fusarium wilt. This finding holds great significance for future developments of composite microbial agents.

Simultaneous Production of MK-7 and Iturin A by Bacillus velezensis ND

Bacillus velezensis can produce various secondary metabolites, such as the antibacterial compound iturin A and the coagulation-promoting menaquinone-7 (MK-7). To enhance the economic feasibility of the fermentation process, a co-production strategy, involving the simultaneous production of MK-7 and iturin A by Bacillus velezensis ND, was investigated in this study. Firstly, the effects of cultivation temperature and initial pH on the synthesis of MK-7 and iturin A were investigated. Considering the co-production of iturin A and MK-7, the optimal temperature and pH were determined as 32 °C and 7, respectively. Subsequently, important nutrients for the co-production process were investigated. It was observed that glycerol, soybean meal, yeast extract, and L-glutamate had a significant effect on the co-produce process. An optimal medium composed of glycerol (72.19 mL L-1), L-glutamate (1.4 g L-1), yeast extract (16.88 g L-1), and soybean meal (130.95 g L-1) was obtained by response surface methodology (RSM). This co-produce process was further scaled up in a biofilm reactor, and the maximum concentration of MK-7 and iturin A reached 46.88 mg L-1 and 5.58 g L-1, respectively. Finally, we established an effective method for separately extracting the two metabolites from the fermentation broth. The superiority of this co-production fermentation strategy demonstrates its significant potential for industrial production.

The efficient green bio-manufacturing of Vitamin K2: design, production and applications

Vitamin K2, also known as methylnaphthoquinone, is a crucial fat-soluble nutrient necessary for the human body. The biological production of Vitamin K2 has received widespread attention due to its environmental friendliness and maneuverability in recent years. This review provides insights into the modular metabolic pathways of Vitamin K2, lays the foundation for microbial metabolic flow balancing, cofactor engineering and dynamic regulation, and realizes the production of Vitamin K2 by synthesizing artificial cells from scratch. With the intensive development of modern fermentation technology, methods for the preparation of Vitamin K2 using the fermentation strategies of co-culturing and biofilm reactors have emerged. In prokaryotes, the introduction of heptenyl pyrophosphate synthase (HepPPS) and mevalonate acid (MVA) pathway solved the problem of insufficient precursors for Vitamin K2 production but still did not meet the market demand. Therefore, enhancing expression through multi-combinatorial metabolic regulation and innovative membrane reactors is an entry point for future research. Due to the light-induced decomposition and water-insoluble nature of Vitamin K2, the secretion regulation and purification processing also need to be considered in the actual production. Also, it summarizes the research progress of Vitamin K2 in the food and pharmaceutical fields. Additionally, the future development trend and application prospect of Vitamin K2 are also discussed to provide guidance for Vitamin K2 biosynthesis and application.

Sustainable menaquinone-7 production through continuous fermentation in biofilm bioreactors

Menaquinone-7 (MK-7), a vital vitamin with numerous health benefits, is synthesized and secreted extracellularly by the formation of biofilm, dominantly in Bacillus strains. Our team developed an innovative biofilm reactor utilizing Bacillus subtilis natto cells to foster biofilm growth on plastic composite supports to produce MK-7. Continuous fermentation in biofilm reactors offers a promising strategy for achieving sustainable and efficient production of Menaquinone-7 (MK-7). Unlike conventional batch fermentation, continuous biofilm reactors maintain a steady state of operation, which reduces resource consumption and waste generation, contributing to sustainability. By optimizing fermentation conditions, MK-7 production was significantly enhanced in this study, demonstrating the potential for sustainable industrial-scale production. To determine the optimal operational parameters, various dilution rates were tested. These rates were selected based on their potential to enhance nutrient supply and biofilm stability, thereby improving MK-7 production. By carefully considering the fermentation conditions and systematically varying the dilution rates, MK-7 production was significantly enhanced during continuous fermentation. The MK-7 productivity was found to increase from 0.12 mg/L/h to 0.33 mg/L/h with a dilution rate increment from 0.007 to 0.042 h-1). This range was chosen to explore the impact of various nutrient supply rates on MK-7 production and to identify the optimal conditions for maximizing productivity. However, a further increase in the dilution rate to 0.084 h-1 led to reduced productivity at approximately 0.16 mg/L/h, likely due to insufficient retention time for effective biofilm formation. Consequently, a dilution rate of 0.042 h-1 exhibited the highest productivity of 0.33 mg/L/h, outperforming all investigated dilution rates and demonstrating the critical balance between nutrient supply and retention time in continuous fermentation. These findings validate the feasibility of operating continuous fermentation at a 0.084 h-1 dilution rate, corresponding to a 48 h retention time, to achieve the highest MK-7 productivity compared to conventional batch fermentation. The significant advancements achieved in enhancing Menaquinone-7 (MK-7) productivity through continuous fermentation at optimal dilution rates in the present work indicate promising prospects for even greater efficiency and sustainability in MK-7 production through future developments.

Characterization of K-binding factor involved in water-soluble complex of menaquinone-7 produced by Bacillus subtilis natto

Vitamin Ks are expected to contribute bone and cardiovascular health. Especially, menaquinone-7 has a higher bioavailability and a longer half-life than other vitamin Ks in the human body. However, their low water-solubility limits their application. On the other hand, Bacillus subtilis natto produces a water-soluble complex, which comprises menaquinone-7 and peptides. The peptide named K-binding factor (KBF) has been reported as the main component of the complex. In the present, the structural characteristics of KBF were studied. Mass spectrometry showed significant peaks at m/z = 1050, while the previous PAGE suggested that molecular weight of KBF was ~ 3k. Amino acid analysis revealed that the 1k peptides were the various combinations of nine amino acids, among which Asx, Glx, Val, Leu and Met were found to be the most abundant. The peptides could serve as detergent properties. The 1k peptides could be isolated by reverse-phase high performance liquid chromatography. The bundle of three 1k detergent-like peptides would participate to the micelle structure containing menqauinone-7 inside. In conclusion, a basic unit of KBF would be the ~ 1k peptides, and the three basic unit assemble to the ~ 3k bundle, then the bundle form a water-soluble micelle including menqauinone-7 inside.

Multiomics reveals the mechanism of B. longum in promoting the formation of mixed-species biofilms

It has been found previously that Bifidobacterium longum, Bacteroides ovatus, Enterococcus faecalis, and Lactobacillus gasseri can form a biofilm better when co-cultured in vitro and B. longum is the core biofilm-formation-promoting strain in this community. B. longum is part of the core microbiota in the gut and is widely recognized as a probiotic. Therefore, it is necessary to explore its role in mixed-species biofilms through transcriptomics and metabolomics. Metabolomics showed that the increase in amino acid and purine content could promote biofilm formation. In transcriptomic analysis, many genes related to carbohydrate metabolism, amino acid metabolism, and environmental tolerance of B. longum were up-regulated. Combined with the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis and Gene Ontology (GO) analysis, the differentially expressed genes (DEGs) of B. longum in mixed-species biofilms were mainly correlated to "quorum sensing (QS)", "ABC transporters", "biosynthesis of amino acids", "microbial metabolism in different environments", "carbohydrate metabolism" and "two-component system". In addition, the rpl and rps gene families, which function in the metabolism of organic substances and the biosynthesis of amino acids, were the core DEGs according to the analysis of the protein-protein interaction (PPI) network. Finally, by combining metabolomics and quorum sensing mechanisms, it was found that the metabolism of autoinducer peptides (proliylglycine and glycylleucine), N-acyl homoserine lactone (N-(3-oxo hydroxy) homoserine lactone), and AI-2 can promote the formation of biofilms, both mono- and mixed-species biofilms composed of B. longum. Our research enabled us to understand the critical role of B. longum in mixed-species biofilms and the interactions between biofilm metabolism and gut health. In addition, the generated knowledge will be of great significance for us to develop biofilm products with beneficial functions in future.

Enhanced vitamin K2 production by engineered Bacillus subtilis during leakage fermentation

Menaquinone-7 (MK-7), a valuable member of the vitamin K2 series, is an essential nutrient for humans. It is used for treating coagulation disorders, and osteoporosis, promoting liver function recovery, and preventing cardiovascular diseases. In this study, to further improve the metabolic synthesis of MK-7 by the mutant strain, the effect of surfactants on the metabolic synthesis of MK-7 by the mutant strain Bacillus subtilis 168 KO-SinR (BS168 KO-SinR) was analyzed. The scanning electron microscopy and flow cytometry results showed that the addition of surfactants changed the permeability of the cell membrane of the mutant strain and the structural components of the biofilm. When 0.7% Tween-80 was added into the medium, the extracellular and intracellular synthesis of MK-7 reached 28.8 mg/L and 59.2 mg/L, respectively, increasing the total synthesis of MK-7 by 80.3%. Quantitative real-time PCR showed that the addition of surfactant significantly increased the expression level of MK-7 synthesis-related genes, and the electron microscopy results showed that the addition of surfactant changed the permeability of the cell membrane. The research results of this paper can serve as a reference for the industrial development of MK-7 prepared by fermentation.

Nutritional Health Perspective of Natto: A Critical Review

Natto, a traditional soy food fermented by Bacillus subtilis, is made by steaming or cooking soaked soybean seeds, inoculating them with the bacteria, and then letting them sit for an incubation period. Natto soya has grown popular because of its nutritional importance and health advantages. As a result, farmers have more opportunities, thanks to the natto soybean market. For the natto soybean market to remain stable and grow, improved soybean cultivars with enhanced natto quality traits are essential. Natto's high-quality attributes are influenced by the bacteria strain, processing parameters, and soybean variety. Natto has a specific flavor and aroma with a slimy, sticky consistency. Natto possesses various therapeutic potentials and contains a range of essential nutrients and bioactive compounds, i.e., nattokinase, soybean isoflavone, γ-polyglutamic acid, vitamin K2, and biogenic amines. Bacterial species, processing conditions, and cultivars of soybean determine the quality characteristics of natto. Natto food is higher in menaquinone-7 and contains 100 times more menaquinone-7 than most cheeses. The present review highlights the production technology, microbiology, nutritional composition, and therapeutic potentials of natto.

Optimisation of the fermentation media to enhance the production of the bioactive isomer of vitamin menaquinone-7

Menaquinone-7 (MK-7) offers significant health benefits; however, only the all-trans form is biologically active. MK-7 produced through fermentation can occur as all-trans and cis isomers, and the therapeutic value of the resulting MK-7 is exclusively determined by the quantity of the all-trans isomer. Therefore, this study aimed to investigate the effect of the media composition on the isomer profile obtained from fermentation and determine the optimum media combination to increase the concentration of the all-trans isomer and diminish the production of cis MK-7. For this purpose, design of experiments (DOE) was used to screen the most effective nutrients, and a central composite face-centred design (CCF) was employed to optimise the media components. The optimum media consisted of 1% (w/v) glucose, 2% (w/v) yeast extract, 2% (w/v) soy peptone, 2% (w/v) tryptone, and 0.1% (w/v) CaCl2. This composition resulted in an average all-trans and cis isomer concentration of 36.366 mg/L and 1.225 mg/L, respectively. In addition, the optimised media enabled an all-trans isomer concentration 12.2-fold greater and a cis isomer concentration 2.9-fold less than the unoptimised media. This study was the first to consider the development of an optimised fermentation media to enhance the production of the bioactive isomer of MK-7 and minimise the concentration of the inactive isomer. Furthermore, this media is commercially promising, as it will improve the process productivity and reduce the costs associated with the industrial fermentation of the vitamin.

Molecular Pathways and Roles for Vitamin K2-7 as a Health-Beneficial Nutraceutical: Challenges and Opportunities

Vitamin K2-7, also known as menaquinone-7 (MK-7) is a form of vitamin K that has health-beneficial effects in osteoporosis, cardiovascular disease, inflammation, cancer, Alzheimer's disease, diabetes and peripheral neuropathy. Compared to vitamin K1 (phylloquinone), K2-7 is absorbed more readily and is more bioavailable. Clinical studies have unequivocally demonstrated the utility of vitamin K2-7 supplementation in ameliorating peripheral neuropathy, reducing bone fracture risk and improving cardiovascular health. We examine how undercarboxylated osteocalcin (ucOC) and matrix Gla protein (ucMGP) are converted to carboxylated forms (cOC and cMGP respectively) by K2-7 acting as a cofactor, thus facilitating the deposition of calcium in bones and preventing vascular calcification. K2-7 is beneficial in managing bone loss because it upregulates osteoprotegerin which is a decoy receptor for RANK ligand (RANKL) thus inhibiting bone resorption. We also review the evidence for the health-beneficial outcomes of K2-7 in diabetes, peripheral neuropathy and Alzheimer's disease. In addition, we discuss the K2-7-mediated suppression of growth in cancer cells via cell-cycle arrest, autophagy and apoptosis. The mechanistic basis for the disease-modulating effects of K2-7 is mediated through various signal transduction pathways such as PI3K/AKT, MAP Kinase, JAK/STAT, NF-κB, etc. Interestingly, K2-7 is also responsible for suppression of proinflammatory mediators such as IL-1α, IL-1β and TNF-α. We elucidate various genes modulated by K2-7 as well as the clinical pharmacometrics of vitamin K2-7 including K2-7-mediated pharmacokinetics/pharmacodynamics (PK/PD). Further, we discuss the current status of clinical trials on K2-7 that shed light on dosing strategies for maximum health benefits. Taken together, this is a synthetic review that delineates the health-beneficial effects of K2-7 in a clinical setting, highlights the molecular basis for these effects, elucidates the clinical pharmacokinetics of K2-7, and underscores the need for K2-7 supplementation in the global diet.

Population balance modelling captures host cell protein dynamics in CHO cell cultures

Monoclonal antibodies (mAbs) have been extensively studied for their wide therapeutic and research applications. Increases in mAb titre has been achieved mainly by cell culture media/feed improvement and cell line engineering to increase cell density and specific mAb productivity. However, this improvement has shifted the bottleneck to downstream purification steps. The higher accumulation of the main cell-derived impurities, host cell proteins (HCPs), in the supernatant can negatively affect product integrity and immunogenicity in addition to increasing the cost of capture and polishing steps. Mathematical modelling of bioprocess dynamics is a valuable tool to improve industrial production at fast rate and low cost. Herein, a single stage volume-based population balance model (PBM) has been built to capture Chinese hamster ovary (CHO) cell behaviour in fed-batch bioreactors. Using cell volume as the internal variable, the model captures the dynamics of mAb and HCP accumulation extracellularly under physiological and mild hypothermic culture conditions. Model-based analysis and orthogonal measurements of lactate dehydrogenase activity and double-stranded DNA concentration in the supernatant show that a significant proportion of HCPs found in the extracellular matrix is secreted by viable cells. The PBM then served as a platform for generating operating strategies that optimise antibody titre and increase cost-efficiency while minimising impurity levels.

Production of Vitamin K by Wild-Type and Engineered Microorganisms

Vitamin K is a fat-soluble vitamin that mainly exists as phylloquinone or menaquinone in nature. Vitamin K plays an important role in blood clotting and bone health in humans. For use as a nutraceutical, vitamin K is produced by natural extraction, chemical synthesis, and microbial fermentation. Natural extraction and chemical synthesis methods for vitamin K production have limitations, such as low yield of products and environmental concerns. Microbial fermentation is a more sustainable process for industrial production of natural vitamin K than two other methods. Recent advanced genetic technology facilitates industrial production of vitamin K by increasing the yield and productivity of microbial host strains. This review covers (i) general information about vitamin K and microbial host, (ii) current titers of vitamin K produced by wild-type microorganisms, and (iii) vitamin K production by engineered microorganisms, including the details of strain engineering strategies. Finally, current limitations and future directions for microbial production of vitamin K are also discussed.

Enhancing in vitro oocyte maturation competence and embryo development in farm animals: roles of vitamin-based antioxidants – a review

Oocyte/embryo in vitro culture is one of the most important assisted reproductive technologies used as a tool for maintaining genetic resources biodiversity and the inheritance of valuable genetic resources through generations. The success of such processes affects the final goal of the in vitro culture, getting viable and healthy offspring. In common in vitro oocyte maturation and/or embryo development techniques, the development of oocytes/embryos is carried out at 5% carbon dioxide and roughly 20% atmosphere-borne oxygen ratios in cell culture incubators due to their reduced cost in comparison with low atmospheric oxygen-tension incubators. These conditions are usually accompanying by the emergence of reactive oxygen species (ROS), which can extremely damage cell membrane integrity and other vital cellular organelles, as well as genetic material. The present review mainly focuses on the antioxidant roles of different vitamins on in vitro oocyte maturation competence and embryo development in farm animals. Because, the conditions of in vitro embryo production (IVEP) are usually accompanying by the emergence of reactive oxygen species (ROS), which can extremely damage cell membrane integrity and other vital cellular organelles as well as genetic material. The use of antioxidant agents may prevent the extreme augmentation of ROS generation and enhance in vitro matured oocyte competence and embryo development. Therefore, this review aimed to provide an updated outline of the impact of antioxidant vitamin (Vit) supplementations during in vitro maturation (IVM) and in vitro fertilization (IVF) on oocyte maturation and consequent embryo development, in various domestic animal species. Thus, the enrichment of the culture media with antioxidant agents may prevent and neutralize the extreme augmentation of ROS generation and enhance the in vitro embryo production (IVEP) outcomes.

Gastrointestinal biofilms in health and disease

Microorganisms colonize various ecological niches in the human habitat, as they do in nature. Predominant forms of multicellular communities called biofilms colonize human tissue surfaces. The gastrointestinal tract is home to a profusion of microorganisms with intertwined, but not identical, lifestyles: as isolated planktonic cells, as biofilms and in biofilm-dispersed form. It is therefore of major importance in understanding homeostatic and altered host-microorganism interactions to consider not only the planktonic lifestyle, but also biofilms and biofilm-dispersed forms. In this Review, we discuss the natural organization of microorganisms at gastrointestinal surfaces, stratification of microbiota taxonomy, biogeographical localization and trans-kingdom interactions occurring within the biofilm habitat. We also discuss existing models used to study biofilms. We assess the contribution of the host-mucosa biofilm relationship to gut homeostasis and to diseases. In addition, we describe how host factors can shape the organization, structure and composition of mucosal biofilms, and how biofilms themselves are implicated in a variety of homeostatic and pathological processes in the gut. Future studies characterizing biofilm nature, physical properties, composition and intrinsic communication could shed new light on gut physiology and lead to potential novel therapeutic options for gastrointestinal diseases.

New aspects of microbial vitamin K2 production by expanding the product spectrum

Vitamin K2 (menaquinone, MK) is an essential lipid-soluble vitamin with critical roles in blood coagulation and bone metabolism. Chemically, the term vitamin K2 encompasses a group of small molecules that contain a common naphthoquinone head group and a polyisoprenyl side chain of variable length. Among them, menaquinone-7 (MK-7) is the most potent form. Here, the biosynthetic pathways of vitamin K2 and different types of MK produced by microorganisms are briefly introduced. Further, we provide a new aspect of MK-7 production, which shares a common naphthoquinone ring and polyisoprene biosynthesis pathway, by analyzing strategies for expanding the product spectrum. We review the findings of metabolic engineering strategies targeting the shikimate pathway, polyisoprene pathway, and menaquinone pathway, as well as membrane engineering, which provide comprehensive insights for enhancing the yield of MK-7. Finally, the current limitations and perspectives of microbial menaquinone production are also discussed. This article provides in-depth information on metabolic engineering strategies for vitamin K2 production by expanding the product spectrum.

Combinatorial Methylerythritol Phosphate Pathway Engineering and Process Optimization for Increased Menaquinone-7 Synthesis in Bacillus subtilis

Vitamin K2 (menaquinone) is an essential vitamin existing in the daily diet, and menaquinone-7 (MK-7) is an important form of it. In a recent work, we engineered the synthesis modules of MK-7 in Bacillus subtilis, and the strain BS20 could produce 360 mg/l MK-7 in shake flasks, while the methylerythritol phosphate (MEP) pathway, which provides the precursor isopentenyl diphosphate for MK-7 synthesis, was not engineered. In this study, we overexpressed five genes of the MEP pathway in BS20 and finally obtained a strain (BS20DFHG) with MK-7 titer of 415 mg/l in shake flasks. Next, we optimized the fermentation process parameters (initial pH, temperature and aeration) in an 8-unit parallel bioreactor system consisting of 300-ml glass vessels. Based on this, we scaled up the MK-7 production by the strain BS20DFHG in a 50-l bioreactor, and the highest MK-7 titer reached 242 mg/l. Here, we show that the engineered strain BS20DFHG may be used for the industrial production of MK-7 in the future.

The effect of aeration and mixing in developing a dairy-based functional food rich in menaquinone-7

Vitamin menaquinone-7 (MK-7) supplementation improves bone health and reduces the incidence of osteoporosis. Despite the recent developments in MK-7 fermentation using Bacillus subtilis natto, low fermentation yields, as well as complicated downstream processing steps, are still the main reasons for the expensive final product. To overcome these issues, developing a fermented dairy-based product rich in MK-7 by avoiding costly downstream steps and optimising the fermentation operating conditions to enhance the MK-7 concentration would be an alternative approach. The present study, therefore, aims to evaluate the role of agitation and aeration as the key operating conditions on MK-7 production by Bacillus subtilis natto using a milk media. The agitation and aeration rates of 525 RPM and 5 VVM were found to be the optimum levels leading to the production of 3.54 mg/L of MK-7. Further, the sensory evaluation was performed to compare the sensory properties of the freeze-dried fermented samples with non-fermented milk samples. The results illustrated that the fermented samples had a significant saltiness with intense aroma resulting in the less acceptability of them by the panellists.

A chemical screening method for menaquinone-producing strains based on HPLC-UV technology

Despite menaquinones (MKs)-4 and - 7 being known to have extensive biological activities and applications, less attention has been paid to the other MKs. Thus, to obtain a range of MKs to further explore their pharmacological activities, structure-activity relationships, and applications, a chemical screening method for MK-producing strains was established based on high-performance liquid chromatography-ultraviolet (HPLC-UV) technology. Considering that Bacillus strains have proven to be an important MK-producing bioresource, twenty-nine putative Bacillus isolates previously sought from a fermented soybean sample were used for the validation of the chemical screening method, which ultimately led to the discovery of sixteen MK-producing strains. Among them, Bacillus subtilis DC-1 presented excellent ability to produce MKs. Another, a purchased strain of B. amyloliquefaciens was discovered to be an MK-producing strain. These results indicated this screening method was simple and highly efficient for the discovery of MK-producing strains, especially those producing a range of MK structures.

Cis and trans isomers of the vitamin menaquinone-7: which one is biologically significant?

Recently, several studies have indicated that an adequate intake of menaquinone-7 (MK-7) offers numerous health benefits. However, the low availability of MK-7 in the diet necessitates the development of dietary supplements or functional food products to complement natural food sources and meet the daily intake requirements. Like most biological molecules, MK-7 can exist as geometric isomers that can occur in the cis, trans, and cis/trans forms; however, only the all-trans form is biologically significant. MK-7 is traditionally produced through bacterial fermentation, but various synthetic preparations have lately become available. The isomer composition in the final product is influenced by numerous factors, including the methods of production and purification, as well as particular environmental and storage conditions. The MK-7 profile obtained from the various production methods has not yet been elucidated, and the ideal method for the synthesis of the all-trans form of the vitamin is also debatable. Consequently, the quantification of the MK-7 profile of various products is necessary to develop an understanding of the factors that influence the proportion of isomers that are obtained in different preparations. Several possible methods exist for the quantification of MK-7 isomers, and of these, liquid chromatography in conjunction with mass spectrometry techniques appears to be the most promising. Evaluation of the isomer composition is an important consideration, as only the all-trans form sustains biological activity. Furthermore, knowledge of the prominent factors that influence the MK-7 composition may also enable their manipulation to obtain a more favorable MK-7 profile in the final product.

Microbial production of vitamin K2: current status and future prospects

Vitamin K2, also called menaquinone, is an essential lipid-soluble vitamin that plays a critical role in blood clotting and prevention of osteoporosis. It has become a focus of research in recent years and has been widely used in the food and pharmaceutical industries. This review will briefly introduce the functions and applications of vitamin K2 first, after which the biosynthesis pathways and enzymes will be analyzed in-depth to highlight the bottlenecks facing the microbial vitamin K2 production on the industrial scale. Then, various strategies, including strain mutagenesis and genetic modification, different cultivation modes, fermentation and separation processes, will be summarized and discussed. The future prospects and perspectives of microbial menaquinone production will also be discussed finally.

Improvement of menaquinone-7 production by Bacillus subtilis natto in a novel residue-free medium by increasing the redox potential

Bacillus subtilis natto is a GRAS bacterium. Nattokinase, with fibrinolytic and antithrombotic activities, is one of the major products of this organism. It is being gradually recognized that B. subtilis natto can also be used as a biosynthetic strain for vitamin K2, which has phenomenal benefits, such as effects in the prevention of cardiovascular diseases and osteoporosis along with antitumor effects. Knocking out of the aprN gene by homologous recombination could improve the redox potential and slightly increase the concentration of MK-7. By detecting the change in redox potential during the growth of B. subtilis natto, a good oxygen supply and state of the cell membrane were found to be beneficial to vitamin K2 synthesis. A two-step RSM was used to optimize the operation parameters and substrate concentration in the new residue-free fermentation culture. The optimal conditions for the residue-free medium and control were determined. The optimum concentrations of soybean flour, corn flour, and peptone were 78.9, 72.4, and 24.8 g/L, respectively. The optimum rotational speed and volume of the culture medium using a shaking flask were 117 rpm and 10%, respectively. The state and composition of the cell membranes were more stable when engineered bacteria were cultured in this residue-free fermentation medium. Finally, the concentration of MK-7 increased by 37% to 18.9 mg/L, and the fermentation time was shortened by 24 h.

Biofilm reactors as a promising method for vitamin K (menaquinone-7) production

Menaquinone-7 (MK-7) is the most potent subtype of vitamin K with extraordinarily high half-life in the circulatory system. Therefore, MK-7 plays a critical role in promoting human wellbeing today. Studies on MK-7 every year show more and more magnificent benefits of it in preventing cardiovascular diseases and osteoporosis to battling cancer cells, Alzheimer's and Parkinson's diseases. Thus, it needs to be supplemented to daily diet for accumulative and long-term benefits. Chemical synthesis of MK-7 produces a significant cis-isomer form of it, which has no biological activity. Fortunately, due to its key role in electron transfer in bacteria, trans-MK-7 is biosynthesized by especially Gram-positive strains mainly Bacillus genus. Concordantly, MK-7 could be produced via solid or liquid state fermentation strategies. In either regime, when static fermentation is applied in the absence of agitation and aeration, operational issues arise such as heat and mass transfer inefficiencies. Thus, scaling up the process becomes a challenge. On the other hand, studies have indicated that biofilm and pellicle formation that occur in static fermentations are key characteristics for extracellular MK-7 secretion. Therefore, this review covers the most recent discoveries of the therapeutic properties of MK-7 and optimization attempts at increasing its biosynthesis in different media compositions and effective growth parameters as well as the cutting-edge use of biofilm reactors where B. subtilis cells have the infrastructures to form mature biofilm formations on plastic composite supports. Biofilm reactors therefore can provide robust extracellular MK-7 secretion while simultaneously enduring high agitation and aeration rates, which then address the scale-up and operational issues associated with static fermentation strategies.

Effects of medium components in a glycerol-based medium on vitamin K (menaquinone-7) production by Bacillus subtilis natto in biofilm reactors

Menaquinone-7 (MK-7) as the most important form of Vitamin K has been reported to have miraculous benefits such as preventing cardiovascular diseases and osteoporosis along with antitumor effects. Therefore, there have been numerous studies in the past decades to improve MK-7 production via microbial fermentation. Unfortunately, both solid and liquid state fermentation strategies that are utilized for MK-7 production, face fundamental operational and scale-up issues as well as intense heat and mass transfer problems during fermentation. In this regard, biofilm reactors seem to be a practical solution to overcome these issues and enhance the production in agitated liquid fermentation. Therefore, this study was undertaken to utilize biofilm reactors in investigating and optimizing different media components in a glycerol-based medium. Using response surface methodology, the effects of glycerol, yeast extract, and soytone were studied in the fermentation medium on MK-7 production in biofilm reactor. With a composition of 48.2 g/L of glycerol, 8.1 g/L of yeast extracts, 13.6 g/L of soytone and 0.06 g/L of K₂HPO₄, MK-7 concentrations could reach 14.7 ± 1.4 mg/L in biofilm reactors, which was 57% higher compared to the MK-7 concentration achieved in suspended-cell reactors under similar conditions, while glycerol was depleted by the end of the fifth day in biofilm reactors, but glycerol was never depleted in suspended-cell reactors. Evidently, biofilm reactors present a reliable strategy to address the operational issues that occur during MK-7 biosynthesis on an industrial scale production.

Implementation of fed-batch strategies for vitamin K (menaquinone-7) production by Bacillus subtilis natto in biofilm reactors

Recent studies show the essential health benefits associated with vitamin K, especially menaquinone-7 (MK-7). These benefits include reducing risks of cardiovascular diseases, osteoporosis, and even cancer. However, MK-7 production on an industrial level is only possible through bacterial fermentation and also current static fermentation strategies are not potent enough with difficulties to scale up. Biofilm reactors, however, may be a practical alternative. Biofilm reactors provide a controlled environment for the microorganisms to form mature and robust biofilms that enable them to produce value-added products with enhanced efficiencies. In this study, fed-batch addition of glucose and glycerol were investigated to the base media in biofilm reactors, as carbon source addition seemed crucial in batch fermentations. Results indicated that fed-batch strategies can be significantly effective in glucose-based medium, increasing the end-product concentrations to 28.7 ± 0.3 mg/L of MK-7 which was 2.3 fold higher than the level produced in suspended-cell bioreactors and renders the biofilm reactors as a potential replacement for static fermentation strategies. Moreover, morphological changes of B. subtilis were tracked during the 12-day long fermentation runs and finally, SEM investigations confirmed significant biofilm and extracellular matrices formed on the plastic composite support (PCS) in the biofilm reactors. In conclusion, biofilm reactors especially with fed-batch fermentation regimes seem to be an effective tool for MK-7 production at industrial scales.

Vitamin K: Redox-modulation, prevention of mitochondrial dysfunction and anticancer effect

This review is directed to the redox-modulating properties and anticancer effect of vitamin K. The concept is focused on two aspects: (i) redox-cycle of vitamin K and its effect on the calcium homeostasis, “oncogenic” and “onco-suppressive” reactive oxygen species and the specific induction of oxidative stress in cancer; (ii) vitamin K plus C as a powerful redox-system, which forms a bypass between mitochondrial complexes II and III and thus prevents mitochondrial dysfunction, restores oxidative phosphorylation and aerobic glycolysis, modulates the redox-state of endogenous redox-pairs, eliminates the hypoxic environment of cancer cells and induces cell death. The analyzed data suggest that vitamin C&K can sensitize cancer cells to conventional chemotherapy, which allows achievement of a lower effective dose of the drug and minimizing the harmful side-effects. The review is intended for a wide audience of readers - from students to specialists in the field.