Occurrence and Biosynthetic Mechanism of Poly-Gamma-Glutamic Acid

Brown Algae as a Valuable Substrate for the Cost-Effective Production of Poly-γ-Glutamic Acid for Applications in Cream Formulations

Poly-γ-glutamic acid (γ-PGA) is a carboxylic-acid-rich, bio-derived, water-soluble, edible, hydrating, non-immunogenic polymer produced naturally by several microorganisms. Here, we re-emphasise the ability of Bacillus subtilis natto to naturally produce γ-PGA on whole seaweed, as well as for the yields and chemical properties of the material to be affected by the presence of Mn(2+). Hyaluronic acid (HA) is an extracellular glycosaminoglycan which presents a high concentration of carboxylic acid and hydroxyl groups, being key in fulfilling numerous applications. Currently, there are strong environmental (solvent use), social (non-vegan extraction), and economic factors pushing for the biosynthesis of this material through prokaryotic microorganisms, which is not yet scalable or sustainable. Our study aimed to investigate an innovative raw material which can combine both superior hygroscopicity and UV protection to the cosmetic industry. Comparable hydration effect of commercially available γ-PGA to conventional moisturising agents (HA and glycerol) was observed; however, greater hydration capacity was observed from seaweed-derived γ-PGA. Herewith, successful incorporation of seaweed-derived γ-PGA (0.2-2 w/v%) was achieved for several model cream systems with absorbances reported at 300 and 400 nm. All γ-PGA-based creams displayed shear thinning behaviour as the viscosity decreased, following increasing shear rates. Although the use of commercial γ-PGA within creams did not suggest a significant effect in rheological behaviour, this was confirmed to be a result of the similar molecular weight. Seaweed-derived γ-PGA cream systems did not display any negative effect on model HaCaT keratinocytes by means of in vitro MTT analysis.

Poly (γ) glutamic acid: a unique microbial biopolymer with diverse commercial applicability

Microbial biopolymers have emerged as promising solutions for environmental pollution-related human health issues. Poly-γ-glutamic acid (γ-PGA), a natural anionic polymeric compound, is composed of highly viscous homo-polyamide of D and L-glutamic acid units. The extracellular water solubility of PGA biopolymer facilitates its complete biodegradation and makes it safe for humans. The unique properties have enabled its applications in healthcare, pharmaceuticals, water treatment, foods, and other domains. It is applied as a thickener, taste-masking agent, stabilizer, texture modifier, moisturizer, bitterness-reducing agent, probiotics cryoprotectant, and protein crystallization agent in food industries. γ-PGA is employed as a biological adhesive, drug carrier, and non-viral vector for safe gene delivery in tissue engineering, pharmaceuticals, and medicine. It is also used as a moisturizer to improve the quality of hair care and skincare cosmetic products. In agriculture, it serves as an ideal stabilizer, environment-friendly fertilizer synergist, plant-growth promoter, metal biosorbent in soil washing, and animal feed additive to reduce body fat and enhance egg-shell strength.

Poly-gamma-glutamic acid biopolymer: a sleeping giant with diverse applications and unique opportunities for commercialization

Poly-gamma-glutamic acid (γ-PGA) is a biodegradable, non-toxic, ecofriendly, and non-immunogenic biopolymer. Its phenomenal properties have gained immense attention in the field of regenerative medicine, the food industry, wastewater treatment, and even in 3D printing bio-ink. The γ-PGA has the potential to replace synthetic non-degradable counterparts, but the main obstacle is the high production cost and lower productivity. Extensive research has been carried out to reduce the production cost by using different waste; however, it is unable to match the commercialization needs. This review focuses on the biosynthetic mechanism of γ-PGA, its production using the synthetic medium as well as different wastes by L-glutamic acid-dependent and independent microbial strains. Furthermore, various metabolic engineering strategies and the recovery processes for γ-PGA and their possible applications are discussed. Finally, highlights on the challenges and unique approaches to reduce the production cost and to increase the productivity for commercialization of γ-PGA are also summarized.

Recent Advances in Microbial Synthesis of Poly-γ-Glutamic Acid: A Review

Poly-γ-glutamic acid (γ-PGA) is a natural, safe, non-immunogenic, biodegradable, and environmentally friendly glutamic biopolymer. γ-PGA has been regarded as a promising bio-based materials in the food field, medical field, even in environmental engineering field, and other industrial fields. Microbial synthesis is an economical and effective way to synthesize γ-PGA. Bacillus species are the most widely studied producing strains. γ-PGA biosynthesis involves metabolic pathway of racemization, polymerization, transfer, and catabolism. Although microbial synthesis of γ-PGA has already been used extensively, productivity and yield remain the major constraints for its industrial application. Metabolic regulation is an attempt to solve the above bottleneck problems and meet the demands of commercialization. Therefore, it is important to understand critical factors that influence γ-PGA microbial synthesis in depth. This review focuses on production strains, biosynthetic pathway, and metabolic regulation. Moreover, it systematically summarizes the functional properties, purification procedure, and industrial application of γ-PGA.

Poly(γ-glutamic acid) Nanocoating To Enhance the Viability of Pseudomonas stutzeri NRCB010 through Cell Surface Engineering

Microbial inoculants can enhance soil quality, promote plant nutrient acquisition, and alleviate problems caused by the excessive use of chemical fertilizers. However, susceptibility to harsh conditions during transport and storage, as well as the short shelf-life of plant growth-promoting rhizobacteria (PGPR), limit industrial application. Herein, a novel strategy to form nanocoating on bacterial surfaces to enhance viability was proposed. The nanocoating was composed of N-hydroxysuccinimide (NHS)-modified poly (γ-glutamic acid) (γ-PGA) and calcium ions, which could adhere to the surface of bacteria by forming covalent bonds and ionic bonds with the bacteria. The bacteria encapsulated in the coating had better resistance against harsh conditions than bare bacteria. The viability of coated bacteria was also increased by 2.38 times compared with bare bacteria after 4 weeks of storage. The pot experiment showed that coated Pseudomonas stutzeri NRCB010 had better growth-promoting properties compared with free P. stutzeri NRCB010. These results indicate that cell surface engineering is an effective method to enhance the resistance of bacteria against harsh conditions and is expected to promote the widespread use of PGPR.

Simultaneous production of poly-γ-glutamic acid and 2,3-butanediol by a newly isolated Bacillus subtilis CS13

Bacillus subtilis naturally produces large amounts of 2,3-butanediol (2,3-BD) as a main by-product during poly-γ-glutamic acid (γ-PGA) production. 2,3-BD is a promising platform chemical in various industries, and co-production of the two chemicals has great economic benefits. Co-production of γ-PGA and 2,3-BD by a newly isolated B. subtilis CS13 was investigated here. The fermentation medium and culture parameters of the process were optimized using statistical methods. It was observed that sucrose, L-glutamic acid, ammonium citrate, and MgSO₄·7H₂O were favorable for γ-PGA and 2,3-BD co-production at culture pH of 6.5 and 37 °C. An optimal medium composed of 119.8 g/L sucrose, 48.8 g/L L-glutamic acid, 21.1 g/L ammonium citrate, and 3.2 g/L MgSO₄·7H₂O was obtained by response surface methodology (RSM). The results show that the titers of γ-PGA and 2,3-BD reached 27.8 ± 0.9 g/L at 24 h and 57.1 ± 1.3 g/L at 84 h with the optimized medium, respectively. γ-PGA and 2,3-BD production by B. subtilis CS13 was significantly enhanced in fed-batch fermentations. γ-PGA (36.5 ± 1.1 g/L, productivity of 1.22 ± 0.04 g/L/h) and 2,3-BD concentrations (119.6 ± 2.8 g/L, productivity of 2.49 ± 0.66 g/L/h) were obtained in the optimized medium with feeding sucrose. The co-production of 2,3-BD and γ-PGA provides a new perspective for industrial production of γ-PGA and 2,3-BD. Key points • A strategy for co-production of γ-PGA and 2,3-BD was developed. • The culture parameters for the co-production of γ-PGA and 2,3-BD were studied. • RSM was used to optimize the medium for γ-PGA and 2,3-BD co-production. • 36.5 g/L γ-PGA and 119.6 g/L 2,3-BD were obtained from the optimum medium in fed-batch fermentation.

Enhanced Low Molecular Weight Poly-γ-Glutamic Acid Production in Recombinant Bacillus subtilis 1A751 with Zinc Ion

Poly-γ-glutamic acid (γ-PGA) is a novel biodegradable polyamide material. Microbial fermentation is the only way to produce γ-PGA, but the molecular weight of γ-PGA varied depending on different strains and culture conditions used. The molecular weight of γ-PGA is a main factor affecting the utilization of γ-PGA. It is urgent to find an efficient way to prepare γ-PGA with specific molecular weight, especially low molecular weight. Bacillus subtilis ECUST is a glutamate-dependent strain that produces γ-PGA. In this study, a recombinant B. subtilis harboring the γ-PGA synthase gene cluster pgsBCAE of our preciously identified γ-PGA-producing B. subtilis ECUST was constructed. Assay of γ-PGA contents and properties showed that recombinant B. subtilis 1A751-pBNS2-pgsBCAE obtained the ability to synthesize γ-PGA with low molecular weight (about 10 kDa). The excessive addition of glutamate inhibited the γ-PGA synthesis, while the addition of Zn²⁺ could promote the synthesis of γ-PGA by increasing the transcription of pgsB but had no effect on the molecular weight of synthesized γ-PGA. Under optimized conditions, γ-PGA produced by recombinant B. subtilis 1A751-pBNS2-pgsBCAE increased from initial 0.54 g/L to 3.9 g/L, and the glutamate conversion rate reached 78%. Recombinant B. subtilis 1A751-pBNS2-pgsBCAE has the potential for efficient preparation of low molecular weight γ-PGA.

Cryoprotection of probiotic bacteria with poly-γ-glutamic acid produced by Bacillus subtilis and Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is a naturally occurring biopolymer made up of repeating units of glutamic acid and can be potentially used for multiple applications. This study compared the production of γ-PGA by Bacillus subtilis and Bacillus licheniformis in GS and E media. The highest γ-PGA production was achieved using initial glycerol concentration of 40 and 80 g/l, ammonium chloride as the nitrogen source, 20 g/l glutamic acid at pH 6.5 for 72 h using E medium. On characterization, it was observed that glutamic acid was the sole component of the purified material. It contained a mixture of Na-γ-PGA and H⁺-γ-PGA. The survival of probiotics during freeze drying was improved by combining them with γ-PGA polymer. For Lactobacilli, 10% γ-PGA protected the cells significantly than 10% sucrose during freeze drying. γ-PGA protection was shown to improve the viability of probiotic bacteria in orange juice for 40 days. No considerable change was observed in the concentrations of citric acid, malic acid and ascorbic acid when probiotic bacteria and γ-PGA were introduced into orange juice and hence, it could be used as a non-dairy delivery platform for these bacteria.

Intracellular synthesis of glutamic acid in Bacillus methylotrophicus SK19.001, a glutamate-independent poly(γ-glutamic acid)-producing strain

BACKGROUND

Bacillus methylotrophicus SK19.001 is a glutamate-independent strain that produces poly(γ-glutamic acid) (γ-PGA), a polymer of D- and L-glutamic acids that possesses applications in food, the environment, agriculture, etc. This study was undertaken to explore the synthetic pathway of intracellular L- and D-glutamic acid in SK19.001 by investigating the effects of tricarboxylic acid cycle intermediates and different amino acids as metabolic precursors on the production of γ-PGA and analyzing the activities of the enzymes involved in the synthesis of L- and D-glutamate.

RESULTS

Tricarboxylic acid cycle intermediates and amino acids could participate in the synthesis of γ-PGA via independent pathways in SK19.001. L-Aspartate aminotransferase, L-glutaminase and L-glutamate synthase were the enzymatic sources of L-glutamate. Glutamate racemase was responsible for the formation of D-glutamate for the synthesis of γ-PGA, and the synthetase had stereoselectivity for glutamate substrate.

CONCLUSION

The enzymatic sources of L-glutamate were investigated for the first time in the glutamate-independent γ-PGA-producing strain, and multiple enzymatic sources of L-glutamate were verified in SK19.001, which will benefit efforts to improve production of γ-PGA with metabolic engineering strategies.

Effect of Poly(γ-glutamic acid) on the Physiological Responses and Calcium Signaling of Rape Seedlings (Brassica napus L.) under Cold Stress

Cold stress adversely affects plant growth and development. Poly(γ-glutamic acid) (γ-PGA) is a potential plant growth regulator that may be an effective cryoprotectant that prevents crops from damage during cold weather. In this study, the effects of γ-PGA on the physiological responses of rape seedlings subject to cold stress were investigated using hydroponic experiments. We determined that the malondialdehyde content was decreased by 33.4% and the proline content was increased by 62.5% by γ-PGA after 144 h under cold stress. Antioxidant enzymes activities were also evidently enhanced after treatment with γ-PGA. These responses counteracted increases in the fresh weight and chlorophyll content of rape seedlings, which increased by 24.5 and 50.9%, respectively, after 144 h, which meant that growth inhibition caused by cold was mitigated by γ-PGA. Our results also showed that γ-PGA also regulated Ca(2+) concentrations in the cytoplasm and calcium-dependent protein kinases, which are associated with cold resistance. In conclusion, we suggest that the Ca(2+)/CPKs signal pathway is involved in the γ-PGA-mediated enhancement of cold resistance in rape seedlings.

Enhanced poly(γ-glutamic acid) production by H2 O2 -induced reactive oxygen species in the fermentation of Bacillus subtilis NX-2

Effects of reactive oxygen species (ROS) on cell growth and poly(γ-glutamic acid) (γ-PGA) synthesis were studied by adding hydrogen peroxide to a medium of Bacillus subtilis NX-2. After optimizing the addition concentration and time of H₂ O₂ , a maximum concentration of 33.9 g/L γ-PGA was obtained by adding 100 µM H₂ O₂ to the medium after 24 H. This concentration was 20.6% higher than that of the control. The addition of diphenyleneiodonium chloride (ROS inhibitor) can interdict the effect of H₂ O₂ -induced ROS. Transcriptional levels of the cofactors and relevant genes were also determined under ROS stress to illustrate the possible metabolic mechanism contributing to the improve γ-PGA production. The transcriptional levels of genes belonging to the tricarboxylic acid cycle and electron transfer chain system were significantly increased by ROS, which decreased the NADH/NAD⁺ ratio and increased the ATP levels, thereby providing more reducing power and energy for γ-PGA biosynthesis. The enhanced γ-PGA synthetic genes also directly promoted the formation of γ-PGA. This study was the first to use the ROS control strategy for γ-PGA fermentation and provided valuable information on the possible mechanism by which ROS regulated γ-PGA biosynthesis in B. subtilis NX-2.

Enhanced poly(γ-glutamic acid) fermentation by Bacillus subtilis NX-2 immobilized in an aerobic plant fibrous-bed bioreactor

To enhance poly(γ-glutamic acid) (PGA) production, a novel aerobic plant fibrous-bed bioreactor (APFB) was constructed for immobilized fermentation. Based on the analysis of the kinetics of immobilized-cell fermentation using the APFB and conventional free-cell fermentation, immobilized-cell fermentation exhibited more efficient PGA production. Furthermore, repeated fed-batch cultures for PGA production were conducted to evaluate the stability of the APFB system. Average final PGA concentration and productivity of 71.21±0.83g/L and 1.246±0.008g/L/h were respectively achieved by cells immobilized in bagasse during APFB, which was reused eight times over a period of 457±18h. Analysis of the membrane phospholipids and the key enzyme activities indicated that APFB-adapted cells had better productivity than original cells. Thus, this study demonstrated the significant potential of the APFB culture system in future industrial applications.

Microbial production and chemical transformation of poly-γ-glutamate

Poly-γ-glutamate (PGA), a novel polyamide material with industrial applications, possesses a nylon-like backbone, is structurally similar to polyacrylic acid, is biodegradable and is safe for human consumption. PGA is frequently found in the mucilage of natto, a Japanese traditional fermented food. To date, three different types of PGA, namely a homo polymer of D-glutamate (D-PGA), a homo polymer of L-glutamate (L-PGA), and a random copolymer consisting of D- and L-glutamate (DL-PGA), are known. This review will detail the occurrence and physiology of PGA. The proposed reaction mechanism of PGA synthesis including its localization and the structure of the involved enzyme, PGA synthetase, are described. The occurrence of multiple carboxyl residues in PGA likely plays a role in its relative unsuitability for the development of bio-nylon plastics and thus, establishment of an efficient PGA-reforming strategy is of great importance. Aside from the potential applications of PGA proposed to date, a new technique for chemical transformation of PGA is also discussed. Finally, some techniques for PGA and its derivatives in advanced material technology are presented.

Pivotal Enzyme in Glutamate Metabolism of Poly-g-Glutamate-Producing Microbes

The extremely halophilic archaeon Natrialba aegyptiaca secretes the L-homo type of poly-g-glutamate (PGA) as an extremolyte. We examined the enzymes involved in glutamate metabolism and verified the presence of L-glutamate dehydrogenases, L-aspartate aminotransferase, and L-glutamate synthase. However, neither glutamate racemase nor D-amino acid aminotransferase activity was detected, suggesting the absence of sources of D-glutamate. In contrast, D-glutamate-rich PGA producers mostly possess such intracellular sources of D-glutamate. The results of our present study indicate that the D-glutamate-anabolic enzyme "glutamate racemase" is pivotal in the biosynthesis of PGA.

Economical production of poly(γ-glutamic acid) using untreated cane molasses and monosodium glutamate waste liquor by Bacillus subtilis NX-2

The production of poly(γ-glutamic acid) by Bacillus subtilis NX-2 from cane molasses and monosodium glutamate waste liquor (MGWL) was studied for the first time in this work. When batch fermentation was carried out with untreated molasses, 33.6±0.37 g L(-1) PGA was obtained with a productivity of 0.46±0.006 g L(-1) h(-1). In order to minimize the substrate inhibition, fed-batch fermentation was performed with untreated or hydrolyzed molasses in 7.5 L bioreactor, giving 50.2±0.53 and 51.1±0.51 g L(-1) of PGA at 96 h, respectively. Further studies were carried out by using MGWL as another carbon source, resulting in a PGA concentration of 52.1±0.52 g L(-1) with a productivity of 0.54±0.003 g L(-1) h(-1). These results suggest that the low-cost cane molasses and MGWL can be used for the environmental-friendly and economical production of PGA by B. subtilis NX-2.

Dynamics of bacterial communities during solid-state fermentation using agro-industrial wastes to produce poly-γ-glutamic acid, revealed by real-time PCR and denaturing gradient gel electrophoresis (DGGE)

The dynamics of bacterial communities play an important role in solid-state fermentation (SSF). Poly-γ-glutamic acid (γ-PGA) was produced by Bacillus amyloliquefaciens C1 in SSF using dairy manure compost and monosodium glutamate production residuals as basic substrates. The production of γ-PGA reached a maximum of 0.6% after 20 days fermentation. Real-time polymerase chain reaction showed the amount of total bacteria reached 3.95 × 10(9) 16S rDNA copies/g sample after 30 days, which was in good accordance with the 4.80 × 10(9) CFU/g obtained by plate counting. Denaturing gradient gel electrophoresis profile showed a reduction of microbial diversity during fermentation, while the inoculum, B. amyloliquefaciens C1, was detected as the dominant organism through the whole process. In the mesophilic phase of SSF, Proteobacteria was the dominant microbial, which was replaced by Firmicutes and Actinobacteria in the thermophilic phase. The molecular analysis of the bacterial diversity has significant potential for instructing the maturing process of SSF to produce γ-PGA at a large-scale level, which could be a benefit in the production of high quality and stable SSF products.