Efficient Production of Polymalic Acid by a Novel Isolated Aureobasidium pullulans Using Metabolic Intermediates and Inhibitors

Advances in Aureobasidium research: Paving the path to industrial utilization

We here explore the potential of the fungal genus Aureobasidium as a prototype for a microbial chassis for industrial biotechnology in the context of a developing circular bioeconomy. The study emphasizes the physiological advantages of Aureobasidium, including its polyextremotolerance, broad substrate spectrum, and diverse product range, making it a promising candidate for cost-effective and sustainable industrial processes. In the second part, recent advances in genetic tool development, as well as approaches for up-scaled fermentation, are described. This review adds to the growing body of scientific literature on this remarkable fungus and reveals its potential for future use in the biotechnological industry.

Efficient Production of Melanin by Aureobasidium Melanogenum Using a Simplified Medium and pH-Controlled Fermentation Strategy with the Cell Morphology Analysis

Natural melanin is a biopolymer with wide application prospects in medicine, food, cosmetics, environmental protection, agriculture, and so on. Microbial fermentation is an important and effective way to produce melanin. In this study, Aureobasidium melanogenum, known as black yeast with cellular pleomorphism, was used for the production of melanin. Based on the characteristic of A. melanogenum secreting melanin under oligotrophic stress, a simple medium containing only glucose, MgSO4·7H2O, and KCl was constructed for the production of melanin. The melanin titer of 6.64 ± 0.22 g/L was obtained after 20 days of fermentation without pH control. The cell morphological changes of A. melanogenum during the production of melanin were recorded, and the results showed that chlamydospore might be the most favorable cell morphology for melanin synthesis. Then, different fermentation strategies with cell morphology analysis were developed to further improve the production of melanin in a 5-L fermenter. Results showed that the maximum titer of melanin reached 18.50 g/L by using the fermentation strategy integrating pH control, ammonium salt addition, and H2O2 stimulation, which increased by 178.6% than that of the strategy without pH control. Furthermore, the melanin obtained from the fermentation broth was characterized as eumelanin containing an indole structure. This study provided a potentially feasible fermentation strategy for the industrial production of melanin.

Biosynthesis of poly(β-L-malic acid) from rubberwood enzymatic hydrolysates in co-fermentation by Aureobasidium pullulans

Co-fermentation of multiple substrates has emerged as the most effective method to improve the yield of bioproducts. Herein, sustainable rubberwood enzymatic hydrolysates (RWH) were co-fermented by Aureobasidium pullulans to produce poly(β-L-malic acid) (PMA), and RWH + glucose/xylose was also investigated as co-substrates. Owing to low inhibitor concentration and abundant natural nitrogen source content of RWH, a high PMA yield of 0.45 g/g and a productivity of 0.32 g/L/h were obtained by RWH substrate fermentation. After optimization, PMA yields following the fermentation of RWH + glucose and RWH + xylose reached 59.92 g/L and 53.71 g/L, respectively, which were 52 % and 36 % higher than that after the fermentation of RWH. RWH + glucose more significantly affected the correlation between PMA yield and substrate concentration than RWH + xylose. The results demonstrated that the co-fermentation of RWH co-substrate is a promising method for the synthesis of bioproducts.

Engineering the glyoxylate cycle for chemical bioproduction

With growing concerns about environmental issues and sustainable economy, bioproduction of chemicals utilizing microbial cell factories provides an eco-friendly alternative to current petro-based processes. Creating high-performance strains (with high titer, yield, and productivity) through metabolic engineering strategies is critical for cost-competitive production. Commonly, it is inevitable to fine-tuning or rewire the endogenous or heterologous pathways in such processes. As an important pathway involved in the synthesis of many kinds of chemicals, the potential of the glyoxylate cycle in metabolic engineering has been studied extensively these years. Here, we review the metabolic regulation of the glyoxylate cycle and summarize recent achievements in microbial production of chemicals through tuning of the glyoxylate cycle, with a focus on studies implemented in model microorganisms. Also, future prospects for bioproduction of glyoxylate cycle-related chemicals are discussed.

Curcumin-laden hyaluronic acid-co-Pullulan-based biomaterials as a potential platform to synergistically enhance the diabetic wound repair

Injectable hydrogel with multifunctional tunable properties comprising biocompatibility, anti-oxidative, anti-bacterial, and/or anti-infection are highly preferred to efficiently promote diabetic wound repair and its development remains a challenge. In this study, we report hyaluronic acid and Pullulan-based injectable hydrogel loaded with curcumin that could potentiate reepithelization, increase angiogenesis, and collagen deposition at wound microenvironment to endorse healing cascade compared to other treatment groups. The physical interaction and self-assembly of hyaluronic acid-Pullulan-grafted-pluronic F127 injectable hydrogel were confirmed using nuclear magnetic resonance (¹H NMR) and Fourier transformed infrared spectroscopy (FT-IR), and cytocompatibility was confirmed by fibroblast viability assay. The CUR-laden hyaluronic acid-Pullulan-g-F127 injectable hydrogel promptly undergoes a sol-gel transition and has proved to potentiate wound healing in a streptozotocin-induced diabetic rat model by promoting 93% of wound closure compared to other groups having 35%, 38%, and 62%. The comparative in vivo study and histological examination was conducted which demonstrated an expeditious recovery rate by significantly reducing the wound healing days i.e. 35 days in a control group, 33 days in the CUR suspension group, 21 days in unloaded injectable, and 13 days was observed in CUR loaded hydrogel group. Furthermore, we suggest that the injectable hydrogel laden with CUR showed a prompt wound healing potential by increasing the cell proliferation and serves as a drug delivery platform for sustained and targeted delivery of hydrophobic moieties.

Enhanced lipid production by addition of malic acid in fermentation of recombinant Mucor circinelloides Mc-MT-2

In our previous work, we reported a novel approach for increasing lipid production in an oleaginous fungus Mucor circinelloides by overexpression of mitochondrial malate transporter protein. This transporter plays a vital role in fatty acid biosynthesis during malate and citrate transport systems in oleaginous fungi. In this study, the controlling metabolic supplementation strategy was used to improve the lipid production by overexpression of malate transporter protein in M. circinelloides strain coded as Mc-MT-2. The effects of different metabolic intermediates on lipid production in batch fermentation by Mc-MT-2 were investigated. The optimal lipid production was obtained at 0.8% malic acid after 24 h of fermentation. Furthermore, in fed-batch bioreactors containing glucose as a carbon source supplemented with malic acid, the highest cell growth, and lipid production were achieved. The resulting strain showed the fungal dry biomass of 16 g/L, a lipid content of 32%, lipid yield of 5.12 g/L in a controlled bench-top bioreactor, with 1.60-, 1.60- and 2.56-fold improvement, respectively, compared with the batch control without supplementation of malic acid. Our findings revealed that the addition of malic acid during fermentation might play an important role in lipid accumulation in the recombinant M. circinelloides Mc-MT-2. This study provides valuable insights for enhanced microbial lipid production through metabolic supplementation strategy in large scale and industrial applications.

Polymalate (PMA) biosynthesis and its molecular regulation in Aureobasidium spp

It has been well documented that different strains of Aureobasidium spp. can synthesize and secrete over 30.0 g/L of polymalate (PMA) and the produced PMA has many potential applications in biomaterial, medical and food industries. The substrates for PMA biosynthesis include glucose, xylose, fructose, sucrose and glucose or fructose or xylose or sucrose-containing natural materials from industrial and agricultural wastes. Malate, the only monomer for PMA biosynthesis mainly comes from TCA cycle, cytosolic reduction TCA pathway and the glyoxylate cycle. The PMA synthetase (a NRPS) containing A like domain, T domain and C like domain is responsible for polymerization of malate into PMA molecules by formation of ester bonds between malates. PMA biosynthesis is regulated by the transcriptional activator Crz1 from Ca²⁺ signaling pathway, the GATA-type transcription factor Gat1 from nitrogen catabolite repression and the GATA-type transcription factor NsdD.

Poly(malic acid) production from liquefied corn starch by simultaneous saccharification and fermentation with a novel isolated Aureobasidium pullulans GXL-1 strain and its techno-economic analysis

In this study, a novel Aureobasidium pullulans GXL-1 strain without melanin secretion was isolated for efficient polymalic acid (PMA) production. The PMA produced by GXL-1 was characterized, and its molecular mass was determined to be 1.621 kDa by gel permeation chromatography. Liquefied corn starch was shown to replace glucose for PMA production by GXL-1 through simultaneous saccharification and fermentation. The PMA titer obtained from batch fermentation was up to 49.0 ± 1.6 g/L in a 10 L fermentor, and the PMA yield and productivity obtained from repeated-batch fermentation were up to 0.50 g/g and 0.34 g/L·h, respectively. Furthermore, process design and techno-economic analysis were performed at an annual output level of 5000 metric tons by SuperPro Designer. Results showed that the production cost of $2.046/kg and payback period of 6.9 years were achieved by repeated-batch fermentation; this provides an economically feasible strategy for industrial-scale production of PMA.

Membrane-assisted β-poly(L-malic acid) production from bagasse hydrolysates by Aureobasidium pullulans ipe-1

Membrane-assisted β-poly(L-malic acid) (PMLA) production from bagasse hydrolysates was developed. For the first time, it was found that mixing the acid and enzyme hydrolysates was unfavorable for PMLA production because too high hexose: pentose ratio and glucose concentration in the mixed sugar could inhibit the assimilation of pentose. 120 g/L sugar concentrations in the acid hydrolysate was suitable for PMLA production with 23.2 g/L PMLA and 34.7 g/L biomass. Moreover, an integrated membrane process consisting of ultrafiltration, nanofiltration and reverse osmosis membranes could concentrate sugars and adjust acetic acid concentration prior to fermentation of lignocellulosic sugars. Meanwhile, it was found that 1.46 g/L acetic acid was preferred for PMLA production from enzyme hydrolysate or sole glucose which respectively increased PMLA production and cell growth by 25.4% and 5.9% from sole glucose, while it showed no significant enhancement in PMLA production with a higher cell growth and productivity from acid hydrolysate.

Efficient β-poly(l-malic acid) production from Jerusalem artichoke by Aureobasidium pullulans ipe-1 immobilized in luffa sponge matrices

β-poly(l-malic acid) (PMLA) production by Aureobasidium pullulans ipe-1 using Jerusalem artichoke tuber (JA) hydrolysate as a low cost carbon source was developed. The PMLA production was favored by JA pretreated with 0.06 M nitric acid without adding exogenous nitrogen sources into fermentation medium. With an initial 130 g/L total sugar of the JA hydrolysate, the highest PMLA productivity 0.52 g/L·h was achieved, which was increased by 2.0 folds compared to that with sole glucose case. To further enhance PMLA productivity, the cells were immobilized in luffa sponge matrices, and repeated batch culture was carried out for 4 cycles. The resulting PMLA productivity was further enhanced by 50% compared with the batch culture. The cost of PMLA production in the JA case was only 5.4% of that in the glucose case. The outcomes of this work provided a strategy of PMLA production on a commercial scale.

A Novel Strategy to Regulate 1-Deoxynojirimycin Production Based on Its Biosynthetic Pathway in Streptomyces lavendulae

This study characterized the biosynthetic pathway of the secondary metabolite 1-deoxynojirimycin (DNJ) from Streptomyces lavendulae. The results revealed that glucose was a preferable precursor for DNJ synthesis, and its carbon skeleton underwent a C2-N-C6 cyclization reaction during synthesis. The biosynthetic pathway was related to the glycolysis pathway, and started from fructose-6-phosphate, and involved amination, dephosphorylation, oxidation, cyclization, dehydration, and reduction reaction steps, yielding DNJ. Then, based on clarified biosynthetic pathway information, precursors, analogs, and metabolism inhibitors were used as novel regulators to enhance the production of DNJ. The results demonstrated that the titer of DNJ could reach 296.56 mg/L, which was 3.3-fold higher than that of a control group (90 mg/L) when sodium citrate (0 h, 5 g/L), sorbose (0 h, 1 g/L), iodoacetic acid (20 h, 50 mg/L), and glucose (26 h, 7 g/L) were added during the fermentation process. This study provides a new understanding of the biosynthetic pathway of DNJ, and also provides an efficient strategy to regulate the production of DNJ based on this biosynthetic pathway, which is a new perspective for the regulation of other secondary metabolites.