Expression of Lactate Dehydrogenase in Aspergillus niger for L-Lactic Acid Production

Converting a D-/L lactic acid bacteria to its d-type counterpart via a combined chemical mutagenesis and biosensor screening method, and its application in lignocellulosic biorefinery

Optically pure lactic acid is essential for poly(lactic acid) manufacturing, nonetheless, most wild-type lactic acid bacteria inherently produce D-/L- mixed lactic acid. In this study, an elaborate system was designed for developing d-lactic acid producing mutants from a wild-type D-/L- lactic acid producingLactobacillus by combining chemical mutagenesis with ethyl methane sulfonate as the mutagen and high throughput screening with a l-lactate dehydrogenase based biosensor. Mechanistic analysis revealed that the loss of l-lactate dehydrogenase activity via C → T transitions in the corresponding gene is responsible for generation of d-lactic acid producing mutants. The obtained d-lactic acid producing mutant exhibited excellent performance in different types of lignocellulosic hydrolysates and produced 128.3 g/L d-lactic acid from corn stover hydrolysate in 3-L bioreactor fermentation, with optical purity higher than 98 %. In conclusion, this study provided a practical framework for obtaining optically pure lactic acid producers without genetic engineering operations, advancing the sustainable production of cellulosic bioplastics.

Progress on production of malic acid and succinic acid by industrially-important engineered microorganisms

Organic acids are widely used as additives in the food, pharmaceutical, chemical, and plastic industries. Currently, the industrial production methods of organic acids mainly include plant extraction and chemical synthesis. The latter mainly uses petroleum-based compounds as raw materials to synthesize organic acids through a series of chemical reactions. All of these methods have problems such as environmental pollution, high cost, and unsustainability. By contrast, microbial fermentation can effectively utilize a variety of carbon sources. Due to its low production cost, environmental friendliness, and high product purity, microbial fermentation has received increasing attention in recent years. However, the low yield and long fermentation cycle of microbial fermentation limits its industrial application. With the development of genomics, transcriptomics, and other omics technologies, the metabolic pathways of various strains producing organic acids have gradually been elucidated. Based on this, new technologies such as synthetic biology and high-throughput screening have also been extensively studied. This review summarizes the latest research progress in improving organic acid biosynthesis through metabolic engineering, focusing on L-malic acid (L-MA) and succinic acid (SA). Finally, we also discuss the challenges and future prospects of this field. This review has important reference value in the fields of food, pharmaceuticals, and chemicals, providing a theoretical basis for the study of organic acid biosynthesis.

Implications of carbon catabolite repression for Aspergillus-based cell factories: A review

Carbon catabolite repression (CCR) is a global regulatory mechanism that allows organisms to preferentially utilize a preferred carbon source (usually glucose) by suppressing the expression of genes associated with the utilization of nonpreferred carbon sources. Aspergillus is a large genus of filamentous fungi, some species of which have been used as microbial cell factories for the production of organic acids, industrial enzymes, pharmaceuticals, and other fermented products due to their safety, substrate convenience, and well-established post-translational modifications. Many recent studies have verified that CCR-related genetic alterations can boost the yield of various carbohydrate-active enzymes (CAZymes), even under CCR conditions. Based on these findings, we emphasize that appropriate regulation of the CCR pathway, especially the expression of the key transcription factor CreA gene, has great potential for further expanding the application of Aspergillus cell factories to develop strains for industrial CAZymes production. Further, the genetically modified CCR strains (chassis hosts) can also be used for the production of other useful natural products and recombinant proteins, among others. We here review the regulatory mechanisms of CCR in Aspergillus and its direct application in enzyme production, as well as its potential application in organic acid and pharmaceutical production to illustrate the effects of CCR on Aspergillus cell factories.

Recent trends in lactic acid-producing microorganisms through microbial fermentation for the synthesis of polylactic acid

Polylactic acid (PLA) is a range of unique bioplastics that are bio-based and biodegradable. PLA is currently driving market expansion for lactic acid (LA) due to its high demand as a building block in production. One of the most practical and environmentally benign techniques for synthesising PLA is through enzymatic polymerisation of microbial LA monomers. However, microbial LA fermentation does have some limitations. Firstly, it requires the use of a nutritionally rich medium. Secondly, LA production can be disrupted by bacteriophage infection or other microorganisms. Lastly, the yield can be low due to the formation of by-products through heterofermentative pathway. Considering the potential use of PLA as a replacement for conventional petrochemical-based polymers in industrial applications, researchers are focused on exploring the diversity of LA-producing microorganisms from various niches. Their goal is to study the functional properties of these microorganisms and their ability to produce industrially valuable metabolites. This review highlights the advantages and disadvantages of lactic acid-producing microorganisms used in microbial fermentation for PLA synthesis.

Cloning and screening of the putative hexokinase genes from Rhizopus oryzae and their heterologous expression in Saccharomyces cerevisiae

BACKGROUND

A filamentous fungus, Rhizopus oryzae (R. oryzae) is one of the ideal candidates for ethanol and lactic acid production due to its ability to grow on renewable carbon sources.

METHODS AND RESULTS

In this study, the nucleotide sequence of hexokinases and glucokinase from S. cerevisiae was found on the NCBI site ( http://www.ncbi.nlm.nih.gov/blast/Blast.cgi ) were used. With these nucleotide sequences, a blast search was done on the R. oryzae genome database ( http://www.broad.mit.edu/annotation/genome/rhizopus_oryzae/Home.html ) and ten probable genes were obtained. cDNA was synthesized from the total RNA and PCR products of the seven of these putative genes were determined using the primers designed for them.

CONCLUSION

The results of the sequences and the complementation studies revealed that three of these seven putative genes were expressed in R. oryzae and the growth was observed on selective media.

Nutrition Component Adjustment of Distilled Dried Grain with Solubles via Aspergillus niger and Its Change about Dynamic Physiological Metabolism

The low fiber digestibility and unbalanced amino acids restricted the use of DDGS in swine diets. Key nutrition components dynamic monitoring and key regulatory pathways analysis were performed to find the rules of nutrition changes for DDGS fermented by Aspergillus niger. Cellulose and hemicellulose were reduced to 15.3% and 15.2%. 1,4-D-Xylobiose was decreased from 16.8 μg/mL to 0.2 μg/mL. Lys, Arg, and Thr were increased to 3.00%, 2.89%, and 4.40%, and met the requirements of pigs. The whole fermentation process was divided into three stages. Cellulose degradation and Lys and Arg synthesis occurred in the early stage, while Asp synthesis occurred in the last stage. α-Ketoglutarate was the key factor for Aspergillus niger degrading cellulose to synthesize Lys and Arg. The key active metabolic pathways that respond to the changes in nutrition were identified which preliminarily revealed the rules of nutrition adjustment of DDGS during fermentation with Aspergillus niger.

Designing and Constructing a Novel Artificial Pathway for Malonic Acid Production Biologically

Malonic acid is used as a common component of many products and processes in the pharmaceutical and cosmetic industries. Here, we designed a novel artificial synthetic pathway of malonic acid, in which oxaloacetate, an intermediate of cytoplasmic reductive tricarboxylic acid (rTCA) pathway, is converted to malonic semialdehyde and then to malonic acid, sequentially catalyzed by a-keto decarboxylase and malonic semialdehyde dehydrogenase. After the systematic screening, we discovered the enzyme oxaloacetate decarboxylase Mdc, catalyzing the first step of the artificially designed pathway in vitro. Then, this synthetic pathway was functionally constructed in cellulolytic thermophilic fungus Myceliophthora thermophila. After enhancement of glucose uptake, the titer of malonic acid achieved 42.5 mg/L. This study presents a novel biological pathway for producing malonic acid from renewable resources in the future.

Rapid and marker-free gene replacement in citric acid-producing Aspergillus tubingensis (A. niger) WU-2223L by the CRISPR/Cas9 system-based genome editing technique using DNA fragments encoding sgRNAs

Strains belonging to Aspergillus section Nigri, including Aspergillus niger, are used for industrial production of citric acid from carbohydrates such as molasses and starch. The objective of this study was to construct the genome editing system that could enable rapid and efficient gene replacement in citric acid-producing fungi for genetic breeding. Using the citric acid-hyperproducer A. tubingensis (formerly A. niger) WU-2223L as a model strain, we developed a CRISPR/Cas9 system-based genome editing technique involving co-transformation of Cas9 and the DNA fragment encoding single guide RNA (sgRNA). Using this system, ATP-sulfurylase gene (sC) knock-out strain derived from WU-2223L was generated; the knock-out efficiency was 29 transformants when 5 μg Cas9 was added to 5 × 105 protoplasts. In the gene replacement method based on this system, a DNA fragment encoding sgRNAs that target both the gene of interest and marker gene was used, and replacement of nitrate reductase gene (niaD) using sC gene as a marker gene was attempted. More than 90% of the sC-knock-out transformants exhibited replaced niaD, indicating efficient gene replacement. Moreover, one-step marker rescue of the sC marker gene was accomplished by excising the knock-in donor via intramolecular homologous recombination, enabling marker-free genome editing and drastically shortening the gene replacement period by circumventing the transformation procedure to recover the sC gene. Thus, we succeeded in constructing a CRISPR/Cas9 system-based rapid and marker-free gene replacement system for the citric acid-hyperproducer strain WU-2223L.

Metabolic engineering of Aspergillus niger via ribonucleoprotein-based CRISPR-Cas9 system for succinic acid production from renewable biomass

BACKGROUND

Succinic acid has great potential to be a new bio-based building block for deriving a number of value-added chemicals in industry. Bio-based succinic acid production from renewable biomass can provide a feasible approach to partially alleviate the dependence of global manufacturing on petroleum refinery. To improve the economics of biological processes, we attempted to explore possible solutions with a fungal cell platform. In this study, Aspergillus niger, a well-known industrial production organism for bio-based organic acids, was exploited for its potential for succinic acid production.

RESULTS

With a ribonucleoprotein (RNP)-based CRISPR-Cas9 system, consecutive genetic manipulations were realized in engineering of the citric acid-producing strain A. niger ATCC 1015. Two genes involved in production of two byproducts, gluconic acid and oxalic acid, were disrupted. In addition, an efficient C₄-dicarboxylate transporter and a soluble NADH-dependent fumarate reductase were overexpressed. The resulting strain SAP-3 produced 17 g/L succinic acid while there was no succinic acid detected at a measurable level in the wild-type strain using a synthetic substrate. Furthermore, two cultivation parameters, temperature and pH, were investigated for their effects on succinic acid production. The highest amount of succinic acid was obtained at 35 °C after 3 days, and low culture pH had inhibitory effects on succinic acid production. Two types of renewable biomass were explored as substrates for succinic acid production. After 6 days, the SAP-3 strain was capable of producing 23 g/L and 9 g/L succinic acid from sugar beet molasses and wheat straw hydrolysate, respectively.

CONCLUSIONS

In this study, we have successfully applied the RNP-based CRISPR-Cas9 system in genetic engineering of A. niger and significantly improved the succinic acid production in the engineered strain. The studies on cultivation parameters revealed the impacts of pH and temperature on succinic acid production and the future challenges in strain development. The feasibility of using renewable biomass for succinic acid production by A. niger has been demonstrated with molasses and wheat straw hydrolysate.

Synthesis and initial screening of lactate dehydrogenase inhibitor activity of 1,3-benzodioxole derivatives

Cancer is one of the main causes of mortality in the world. Many cancer cells produce ATP through high-level lactic acid fermentation catalyzed by lactate dehydrogenase (LDH), which converts pyruvic acid to lactic acid. LDH plays a dominant role in the Warburg effect, wherein aerobic glycolysis is favored over oxidative phosphorylation. Due to the high lactic acid production level in cancer cells, LDH-targeting could be a potential cancer treatment strategy. A few approaches, such as drug treatment, reportedly inhibited LDH activity. In this study, we describe new 1,3-benzodioxole derivatives that might be potential small molecule candidates for LDHA inhibition. The synthesis was carried out by trans-esterification between aryl ester and alcohol groups from piperonyl alcohol. Compounds 2 and 10 exhibited a selective LDHA IC₅₀ value of 13.63 µM and 47.2 µM, respectively. Whereas only compound 10 showed significant cytotoxicity in several lines of cancer cells, especially in human pancreatic cancer PANC-1 cells. These synthesized compounds possess 2 aromatic rings and –CF₃ moiety, which expectedly contributes to LDHA inhibition. The presented products have the potential to become a promising LDHA inhibitor drug candidate.

Lactic acid production - producing microorganisms and substrates sources-state of art

Lactic acid is an organic compound produced via fermentation by different microorganisms that are able to use different carbohydrate sources. Lactic acid bacteria are the main bacteria used to produce lactic acid and among these, Lactobacillus spp. have been showing interesting fermentation capacities. The use of Bacillus spp. revealed good possibilities to reduce the fermentative costs. Interestingly, lactic acid high productivity was achieved by Corynebacterium glutamicum and E. coli, mainly after engineering genetic modification. Fungi, like Rhizopus spp. can metabolize different renewable carbon resources, with advantageously amylolytic properties to produce lactic acid. Additionally, yeasts can tolerate environmental restrictions (for example acidic conditions), being the wild-type low lactic acid producers that have been improved by genetic manipulation. Microalgae and cyanobacteria, as photosynthetic microorganisms can be an alternative lactic acid producer without carbohydrate feed costs. For lactic acid production, it is necessary to have substrates in the fermentation medium. Different carbohydrate sources can be used, from plant waste as molasses, starchy, lignocellulosic materials as agricultural and forestry residues. Dairy waste also can be used by the addition of supplementary components with a nitrogen source.

Engineering Thermoanaerobacterium aotearoense SCUT27 with argR knockout for enhanced ethanol production from lignocellulosic hydrolysates

Although Thermoanaerobacterium aotearoense SCUT27 (SCUT27) could co-utilize glucose and xylose, the presence of glucose still repressed xylose catabolism. Arginine repressors (ArgRs) were involved in several key metabolic pathways and might be the global regulator. In SCUT27, three genes (V518_0585; V518_1870; V518_1864) were annotated as argR and only the deficiency of argR₁₈₆₄ could greatly improve the co-utilization of glucose and xylose, due to the enhanced activity of xylose isomerase, xylulokinase and the higher energy level. The metabolic flux of SCUT27/ΔargR₁₈₆₄ indicated that new carbon distribution had been re-established and the ethanol yield had increased by 82.95%, strains growth and acetate yield improved by ~35.91% without detectable lactate for the poor activity of lactate dehydrogenase. The improved concentration of ATP and NAD(H) in SCUT27/ΔargR₁₈₆₄ provided more energy to respond the stress, which enabled the mutant the better cell viability to utilize lignocellulosic hydrolysates for enhanced ethanol formation.

Ca2+ ionophore A23187 inhibits ATP generation reducing mouse sperm motility and PKA-dependent phosphorylation

Male infertility is a global problem in modern society of which capacitating defects are a major cause. Previous studies have demonstrated that Ca²⁺ ionophore A23187 can make mouse sperm capable of fertilizing in vitro, which may aid in clinical treatment of capacitating defects. However, the detailed role and mechanism of Ca²⁺ in the capacitating process are still unclear especially how A23187 quickly renders sperm immotile and inhibits cAMP/PKA-mediated phosphorylation. We report that A23187 induces a Ca²⁺ flux in the mitochondria enriched sperm tail and excess Ca²⁺ inhibits key metabolic enzymes involved in acetyl-CoA biosynthesis, TCA cycle and electron transport chain pathways resulting in reduced ATP and overall energy production, however this flux does not destroy the structure of the sperm tail. Due to the decrease in ATP production, which is the main phosphate group donator and the power of sperm, the sperm is rendered immobile and PKA-mediated phosphorylation is inhibited. Our study proposed a possible mechanism through which A23187 reduces sperm motility and PKA-mediated phosphorylation from ATP generation, thus providing basic data for exploring the functional roles of Ca²⁺ in sperm in the future.

In silico evolution of Aspergillus niger organic acid production suggests strategies for switching acid output

BACKGROUND

The fungus Aspergillus niger is an important industrial organism for citric acid fermentation; one of the most efficient biotechnological processes. Previously we introduced a dynamic model that captures this process in the industrially relevant batch fermentation setting, providing a more accurate predictive platform to guide targeted engineering. In this article we exploit this dynamic modelling framework, coupled with a robust genetic algorithm for the in silico evolution of A. niger organic acid production, to provide solutions to complex evolutionary goals involving a multiplicity of targets and beyond the reach of simple Boolean gene deletions. We base this work on the latest metabolic models of the parent citric acid producing strain ATCC1015 dedicated to organic acid production with the required exhaustive genomic coverage needed to perform exploratory in silico evolution.

RESULTS

With the use of our informed evolutionary framework, we demonstrate targeted changes that induce a complete switch of acid output from citric to numerous different commercially valuable target organic acids including succinic acid. We highlight the key changes in flux patterns that occur in each case, suggesting potentially valuable targets for engineering. We also show that optimum acid productivity is achieved through a balance of organic acid and biomass production, requiring finely tuned flux constraints that give a growth rate optimal for productivity.

CONCLUSIONS

This study shows how a genome-scale metabolic model can be integrated with dynamic modelling and metaheuristic algorithms to provide solutions to complex metabolic engineering goals of industrial importance. This framework for in silico guided engineering, based on the dynamic batch growth relevant to industrial processes, offers considerable potential for future endeavours focused on the engineering of organisms to produce valuable products.

Autophagy is important to the acidogenic metabolism of Aspergillus niger

Significant phenotypic overlaps exist between autophagy and acidogenesis in Aspergillus niger. The possible role of autophagy in the acidogenic growth and metabolism of this fungus was therefore examined and the movement of cytosolic EGFP to vacuoles served to monitor this phenomenon. An autophagy response to typical as well as a metabolic inhibitor-induced nitrogen starvation was observed in A. niger mycelia. The vacuolar re-localization of cytosolic EGFP was not observed upon nitrogen starvation in the A. niger Δatg1 strain. The acidogenic growth of the fungus consisted of a brief log phase followed by an extended autophagy-like state throughout the idiophase of fermentation. Mycelia in the idiophase were highly vacuolated and EGFP was localized to the vacuoles but no autolysis was observed. Both autophagy and acidogenesis are compromised in Δatg1 and Δatg8 strains of A. niger. The acidogenic growth of the fungus thus appears to mimic a condition of nutrient limitation and is associated with an extended autophagy-like state. This crucial role of autophagy in acidogenic A. niger physiology could be of value in improving citric acid fermentation.

Itaconic acid degradation in Aspergillus niger: the role of unexpected bioconversion pathways

BACKGROUND

Itaconic acid (IA), a C5-dicarboxylic acid, has previously been identified as one of the top twelve biochemicals that can be produced by biotechnological means. IA is naturally produced by Aspergillus terreus, however, heterologous production in the related species Aspergillus niger has been proposed earlier. Remarkably, we observed that during high producing conditions and elevated titers A. niger detoxifies the extracellular medium of IA. In order to determine the genes responsible for this decline in IA titers a transcriptome analysis was performed.

RESULTS

Transcriptome analysis has led to the identification of two novel and previously unknown IA bioconversion pathways in A. niger. One pathway is proposed to convert IA into pyruvate and acetyl-CoA through the action of itaconyl-CoA transferase (IctA), itaconyl-CoA hydratase (IchA) and citramalyl-CoA lyase, similar to the pathway identified in A. terreus. Another pathway putatively converts IA into 1-methyl itaconate through the action of trans-aconitate methyltransferase (TmtA). Upon deleting the key genes ictA and ichA we have observed increased IA production and titers and cessation of IA bioconversion. Surprisingly, deletion of tmtA lead to strong reduction of heterologous IA production.

CONCLUSION

Heterologous IA production in A. niger induces the expression of IA bioconversion pathways. These pathways can be inhibited by deleting the key genes ictA, ichA and tmtA. Deletion of ictA and ichA resulted in increased IA production. Deletion of tmtA, however, resulted in almost complete cessation of IA production.

Untargeted metabolite profiling for koji-fermentative bioprocess unravels the effects of varying substrate types and microbial inocula

Untargeted metabolomics unraveled the effects of varying substrates (soybean, wheat, and rice) and inocula (Aspergillus oryzae and Bacillus amyloliquefaciens) on metabolite compositions of koji, a starter ingredient in various Asian fermented foods. Multivariate analyses of the hyphenated mass spectrometry datasets for different koji extracts highlighted 61 significantly discriminant primary metabolites (sugars and sugar alcohols, organic acids, amino acids, fatty acids, nucleosides, phenolic acids, and vitamins) according to varying substrates and inocula combinations. However, 59 significantly discriminant secondary metabolites were evident for koji-types with varying substrates only, viz., soybean (flavonoids, soyasaponins, and lysophospholipids), wheat (flavones and lysophospholipids), and rice (flavonoids, fatty acids derivatives, and lysophospholipids). Independently, the substrates influenced primary metabolite compositions in koji (soybean > wheat, rice). The inocula choice of A. oryzae engendered higher carbohydrates, organic acids, and lipid derivative levels commensurate with high α-amylase and β-glucosidase activities, while B. amyloliquefaciens affected higher amino acids levels, in respective koji types.

Metabolomics profiling reveals the mechanism of increased pneumocandin B0 production by comparing mutant and parent strains

Metabolic profiling was used to discover mechanisms of increased pneumocandin B₀ production in a high-yield strain by comparing it with its parent strain. Initially, 79 intracellular metabolites were identified, and the levels of 15 metabolites involved in six pathways were found to be directly correlated with pneumocandin B₀ biosynthesis. Then by combining the analysis of key enzymes, acetyl-CoA and NADPH were identified as the main factors limiting pneumocandin B₀ biosynthesis. Other metabolites, such as pyruvate, α-ketoglutaric acid, lactate, unsaturated fatty acids and previously unreported metabolite γ-aminobutyric acid were shown to play important roles in pneumocandin B₀ biosynthesis and cell growth. Finally, the overall metabolic mechanism hypothesis was formulated and a rational feeding strategy was implemented that increased the pneumocandin B₀ yield from 1821 to 2768 mg/L. These results provide practical and theoretical guidance for strain selection, medium optimization, and genetic engineering for pneumocandin B₀ production.

Diversity, Application, and Synthetic Biology of Industrially Important Aspergillus Fungi

The filamentous fungal genus Aspergillus consists of over 340 officially recognized species. A handful of these Aspergillus fungi are predominantly used for food fermentation and large-scale production of enzymes, organic acids, and bioactive compounds. These industrially important Aspergilli primarily belong to the two major Aspergillus sections, Nigri and Flavi. Aspergillus oryzae (section Flavi) is the most commonly used mold for the fermentation of soybeans, rice, grains, and potatoes. Aspergillus niger (section Nigri) is used in the industrial production of various enzymes and organic acids, including 99% (1.4 million tons per year) of citric acid produced worldwide. Better understanding of the genomes and the signaling mechanisms of key Aspergillus species can help identify novel approaches to enhance these commercially significant strains. This review summarizes the diversity, current applications, key products, and synthetic biology of Aspergillus fungi commonly used in industry.

A Polyketide Synthase Encoded by the Gene An15g07920 Is Involved in the Biosynthesis of Ochratoxin A in Aspergillus niger

The polyketide synthase gene An15g07920 was known in Aspergillus niger CBS 513.88 as putatively involved in the production of ochratoxin A (OTA). Genome resequencing analysis revealed that the gene An15g07920 is also present in the ochratoxin-producing A. niger strain 1062. Disruption of An15g07920 in A. niger 1062 removed its capacity to biosynthesize ochratoxin β (OTβ), ochratoxin α (OTα), and OTA. These results indicate that the polyketide synthase encoded by An15g07920 is a crucial player in the biosynthesis of OTA, in the pathway prior to the phenylalanine ligation step. The gene An15g07920 reached its maximum transcription level before OTA accumulation reached its highest level, confirming that gene transcription precedes OTA production. These findings will not only help explain the mechanism of OTA production in A. niger but also provide necessary information for the development of effective diagnostic, preventive, and control strategies to reduce the risk of OTA contamination in foods.

Metabolomic Profiles of Aspergillus oryzae and Bacillus amyloliquefaciens During Rice Koji Fermentation

Rice koji, used early in the manufacturing process for many fermented foods, produces diverse metabolites and enzymes during fermentation. Using gas chromatography time-of-flight mass spectrometry (GC-TOF-MS), ultrahigh-performance liquid chromatography linear trap quadrupole ion trap tandem mass spectrometry (UHPLC-LTQ-IT-MS/MS), and multivariate analysis we generated the metabolite profiles of rice koji produced by fermentation with Aspergillus oryzae (RK_AO) or Bacillus amyloliquefaciens (RK_BA) for different durations. Two principal components of the metabolomic data distinguished the rice koji samples according to their fermenter species and fermentation time. Several enzymes secreted by the fermenter species, including α-amylase, protease, and β-glucosidase, were assayed to identify differences in expression levels. This approach revealed that carbohydrate metabolism, serine-derived amino acids, and fatty acids were associated with rice koji fermentation by A. oryzae, whereas aromatic and branched chain amino acids, flavonoids, and lysophospholipids were more typical in rice koji fermentation by B. amyloliquefaciens. Antioxidant activity was significantly higher for RK_BA than for RK_AO, as were the abundances of flavonoids, including tricin, tricin glycosides, apigenin glycosides, and chrysoeriol glycosides. In summary, we have used MS-based metabolomics and enzyme activity assays to evaluate the effects of using different microbial species and fermentation times on the nutritional profile of rice koji.