Analysis of Carbon Metabolism and Improvement of γ-Polyglutamic Acid Production from Bacillus subtilis NX-2

Untargeted metabolomics revealed the effect of soybean metabolites on poly(γ-glutamic acid) production in fermented natto and its metabolic pathway

BACKGROUND

Natto mucus is mainly composed of poly(γ-glutamic acid) (γ-PGA), which affects the sensory quality of natto and has some effective functional activities. The soybean metabolites that cause different γ-PGA contents in different fermented natto are unclear.

RESULTS

In this study, we use untargeted metabolomics to analyze the metabolites of high-production γ-PGA natto and low-production γ-PGA natto and their fermented substrate soybean. A total of 257 main significantly different metabolites with the same trend among the three comparison groups were screened, of which 114 were downregulated and 143 were upregulated. Through the enrichment of metabolic pathways, the metabolic pathways with significant differences were purine metabolism, nucleotide metabolism, fructose and mannose metabolism, anthocyanin biosynthesis, isoflavonoid biosynthesis and the pentose phosphate pathway.

CONCLUSION

For 114 downregulated main significantly different metabolites with the same trend among the three comparison groups, Bacillus subtilis (natto) may directly decompose them to synthesize γ-PGA. Adding downregulated substances before fermentation or cultivating soybean varieties with the goal of high production of such substances has a great effect on the production of γ-PGA by natto fermentation. The enrichment analysis results showed the main pathways affecting the production of γ-PGA by Bacillus subtilis (natto) using soybean metabolites, which provides a theoretical basis for the production of γ-PGA by soybean and promotes the diversification of natto products. © 2023 Society of Chemical Industry.

Effects of Fe2+ addition to sugarcane molasses on poly-γ-glutamic acid production in Bacillus licheniformis CGMCC NO. 23967

BACKGROUND

Poly-γ-glutamic acid (γ-PGA) is biodegradable, water-soluble, environment-friendly, and edible. Consequently, it has a variety of industrial applications. It is crucial to control production cost and increase output for industrial production γ-PGA.

RESULTS

Here γ-PGA production from sugarcane molasses by Bacillus licheniformis CGMCC NO. 23967 was studied in shake-flasks and bioreactors, the results indicate that the yield of γ-PGA could reach 40.668 g/L in a 5L stirred tank fermenter. Further study found that γ-PGA production reached 70.436 g/L, γ-PGA production and cell growth increased by 73.20% and 55.44%, respectively, after FeSO₄·7H₂O was added. Therefore, we investigated the metabolomic and transcriptomic changes following FeSO₄·7H₂O addition. This addition resulted in increased abundance of intracellular metabolites, including amino acids, organic acids, and key TCA cycle intermediates, as well as upregulation of the glycolysis pathway and TCA cycle.

CONCLUSIONS

These results compare favorably with those obtained from glucose and other forms of biomass feedstock, confirming that sugarcane molasses can be used as an economical substrate without any pretreatment. The addition of FeSO₄·7H₂O to sugarcane molasses may increase the efficiency of γ-PGA production in intracellular.

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.

Efficient molasses utilization for low-molecular-weight poly-γ-glutamic acid production using a novel Bacillus subtilis stain

Background

Poly-γ-glutamic acid (γ-PGA) is a biopolymer and has various applications based on its biocompatibility, non-toxicity, and edibility. Low-molecular-weight (Mw)-γ-PGA has promising applications in agriculture and pharmaceuticals. It is traditionally produced by enzymatic hydrolysis. Cost-effective bioproduction of low-Mw-γ-PGA is essential for commercial application of γ-PGA.

Results

Bacillus subtilis 242 is a newly isolated low-Mw-γ-PGA-producing strain. To develop cost-effective production of γ-PGA using this newly isolated strain, cane molasses and corn steep liquor were used to produce γ-PGA. The concentration of cane molasses was optimized and 100 g/L cane molasses resulted in high γ-PGA production. The effects of yeast extract and corn steep liquor on γ-PGA yield were investigated. High concentration of γ-PGA was obtained in the medium with corn steep liquor. A concentration of 32.14 g/L γ-PGA was achieved in fed-batch fermentation, with a productivity of 0.67 g/L/h and a percentage yield (g_γ-PGA/g_glutamate) of 106.39%. The Mw of γ-PGA was 27.99 kDa.

Conclusion

This study demonstrated the potential application of B. subtilis 242 for cost-effective production of low-Mw-γ-PGA from cane molasses.

Low cost and sustainable hyaluronic acid production in a manufacturing platform based on Bacillus subtilis 3NA strain

Hyaluronic acid (HA) is a high value glycosaminoglycan mostly used in health and cosmetic applications. Commercial HA is produced from animal tissues or in toxigenic bacteria of the genus Streptococcus grown in complex media, which are expensive and raise environmental concerns due to the disposal of large amounts of broth with high organic loads. Other microorganisms were proposed as hosts for the heterologous production of HA, but the methods are still costly. The extraordinary capacity of this biopolymer to bind and retain water attracts interest for large-scale applications where biodegradable materials are needed, but its high cost and safety concerns are barriers for its adoption. Bacillus subtilis 3NA strain is prototrophic, amenable for genetic manipulation, GRAS, and can rapidly reach high cell densities in salt-based media. These phenotypic traits were exploited to create a platform for biomolecule production using HA as a proof of concept. First, the 3NA strain was engineered to produce HA; second, a chemically defined medium was formulated using commodity-priced inorganic salts combined at the stoichiometric ratios needed to build the necessary quantities of biomass and HA; and third, a scalable fermentation process, where HA can be produced at the maximum volumetric productivity (VP), was designed. A comparative economic analysis against other methods indicates that the new process may increase the operating profit of a manufacturing plant by more than 100%. The host, the culture medium, and the rationale employed to develop the fermentation process described here, introduce an IP-free platform that could be adaptable for production of other biomolecules. KEY POINTS: • A biomolecule production platform based on B. subtilis 3NA strain and a synthetic medium was tested for hyaluronic acid biosynthesis • A fermentation process with the maximum volumetric productivity was designed • A techno-economic analysis forecasts a significant reduction in the manufacturing cost compared to the current methods.

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.

High-level production of poly-γ-glutamic acid from untreated molasses by Bacillus siamensis IR10

Background

Poly-γ-glutamic acid (γ-PGA) is a promising biopolymer and has been applied in many fields. Bacillus siamensis SB1001 was a newly isolated poly-γ-glutamic acid producer with sucrose as its optimal carbon source. To improve the utilization of carbon source, and then molasses can be effectively used for γ-PGA production, ⁶⁰cobalt gamma rays was used to mutate the genes of B. siamensis SB1001.

Results

Bacillus siamensis IR10 was screened for the production of γ-PGA from untreated molasses. In batch fermentation, 17.86 ± 0.97 g/L γ-PGA was obtained after 15 h, which is 52.51% higher than that of its parent strain. Fed-batch fermentation was performed to further improve the yield of γ-PGA with untreated molasses, yielding 41.40 ± 2.01 g/L of γ-PGA with a productivity of 1.73 ± 0.08 g/L/h. An average γ-PGA productivity of 1.85 g/L/h was achieved in the repeated fed-batch fermentation. This is the first report of such a high γ-PGA productivity. The analysis of the enzyme activities showed that they were affected by the carbon sources, enhanced ICDH and GDH, and decreased ODHC, which are important for γ-PGA production.

Conclusion

These results suggest that untreated molasses can be used for economical and industrial-scale production of γ-PGA by B. siamensis IR10.

Identification of Key Metabolites in Poly-γ-Glutamic Acid Production by Tuning γ-PGA Synthetase Expression

Poly-γ-glutamic acid (γ-PGA) production is commonly achieved using glycerol, citrate, and L-glutamic acid as substrates. The constitutive expression of the γ-PGA synthetase enabled γ-PGA production with Bacillus subtilis from glucose only. The precursors for γ-PGA synthesis, D- and L-glutamate, are ubiquitous metabolites. Hence, the metabolic flux toward γ-PGA directly depends on the concentration and activity of the synthetase and thereby on its expression. To identify pathway bottlenecks and important metabolites that are highly correlated with γ-PGA production from glucose, we engineered B. subtilis strains with varying γ-PGA synthesis rates. To alter the rate of γ-PGA synthesis, the expression level was controlled by two approaches: (1) Using promoter variants from the constitutive promoter P veg and (2) Varying induction strength of the xylose inducible promoter P xyl . The variation in the metabolism caused by γ-PGA production was investigated using metabolome analysis. The xylose-induction strategy revealed that the γ-PGA production rate increased the total fluxes through metabolism indicating a driven by demand adaption of the metabolism. Metabolic bottlenecks during γ-PGA from glucose were identified by generation of a model that correlates γ-PGA production rate with intracellular metabolite levels. The generated model indicates the correlation of certain metabolites such as phosphoenolpyruvate with γ-PGA production. The identified metabolites are targets for strain improvement to achieve high level γ-PGA production from glucose.

Comparison of Isomerase and Weimberg Pathway for γ-PGA Production From Xylose by Engineered Bacillus subtilis

The production of poly-γ-glutamic acid (γ-PGA), a biopolymer consisting of D- and L-glutamic acid monomers, currently relies on L-glutamate, or citrate as carbon substrates. Here we aimed at using plant biomass-derived substrates such as xylose. γ-PGA producing microorganisms including Bacillus subtilis natively metabolize xylose via the isomerase pathway. The Weimberg pathway, a xylose utilization pathway first described for Caulobacter crescentus, offers a carbon-efficient alternative converting xylose to 2-oxoglutarate without carbon loss. We engineered a recombinant B. subtilis strain that was able to grow on xylose with a growth rate of 0.43 h^-1 using a recombinant Weimberg pathway. Although ion-pair reversed-phase LC/MS/MS metabolome analysis revealed lower concentrations of γ-PGA precursors such as 2-oxoglutarate, the γ-PGA titer was increased 6-fold compared to the native xylose isomerase strain. Further metabolome analysis indicates a metabolic bottleneck in the phosphoenolpyruvate-pyruvate-oxaloacetate node causing bi-phasic (diauxic) growth of the recombinant Weimberg strain. Flux balance analysis (FBA) of the γ-PGA producing B. subtilis indicated that a maximal theoretical γ-PGA yield is achieved on D-xylose/ D-glucose mixtures. The results of the B. subtilis strain harboring the Weimberg pathway on such D-xylose/ D-glucose mixtures demonstrate indeed resource efficient, high yield γ-PGA production from biomass-derived substrates.

Investigation of Glutamate Dependence Mechanism for Poly-γ-glutamic Acid Production in Bacillus subtilis on the Basis of Transcriptome Analysis

The development of commercial poly-γ-glutamic acid (γ-PGA) production by glutamate-dependent strains requires understanding the glutamate dependence mechanism in the strains. Here, we first systematically analyzed the response pattern of Bacillus subtilis to glutamate addition by comparative transcriptomics. Glutamate addition induced great changes in intracellular metabolite concentrations and significantly upregulated genes involved in the central metabolic pathways. Subsequent gene overexpression experiments revealed that only the enhancement of glutamate synthesis pathway successfully led to γ-PGA accumulation without glutamate addition, indicating the key role of intracellular glutamate for γ-PGA synthesis in glutamate-dependent strains. Finally, by a combination of metabolic engineering targets, the γ-PGA titer reached 10.21 ± 0.42 g/L without glutamate addition. Exogenous glutamate further enhanced the γ-PGA yield (35.52 ± 0.26 g/L) and productivity (0.74 g/(L h)) in shake-flask fermentation. This work provides insights into the glutamate dependence mechanism in B. subtilis and reveals potential molecular targets for increasing economical γ-PGA production.

Deciphering metabolic responses of biosurfactant lichenysin on biosynthesis of poly-γ-glutamic acid

Poly-γ-glutamic acid (γ-PGA) is an extracellularly produced biodegradable polymer, which has been widely used as agricultural fertilizer, mineral fortifier, cosmetic moisturizer, and drug carrier. This study firstly discovered that lichenysin, as a biosurfactant, showed the capability to enhance γ-PGA production in Bacillus licheniformis. The exogenous addition of lichenysin improved the γ-PGA yield up to 17.9% and 21.9%, respectively, in the native strain B. licheniformis WX-02 and the lichenysin-deficient strain B. licheniformis WX02-ΔlchAC. The capability of intracellular biosynthesis of lichenysin was positively correlated with γ-PGA production. The yield of γ-PGA increased by 25.1% in the lichenysin-enhanced strain B. licheniformis WX02-Psrflch and decreased by 12.2% in the lichenysin-deficient strain WX02-ΔlchAC. Analysis of key enzyme activities and gene expression in the TCA cycle, precursor glutamate synthesis, and γ-PGA synthesis pathway revealed that the existence of lichenysin led to increased γ-PGA via shifting the carbon flux in the TCA cycle towards glutamate and γ-PGA biosynthetic pathways, minimizing by-product formation, and facilitating the uptake of extracellular substrates and the polymerization of glutamate to γ-PGA. Insight into the mechanisms of enhanced production of γ-PGA by lichenysin would define the essential parameters involved in γ-PGA biosynthesis and provide the basis for large-scale production of γ-PGA.

Optimization of γ-polyglutamic acid synthesis using response surface methodology of a newly isolated glutamate dependent Bacillus velezensis Z3

A new glutamate-dependent γ-polyglutamic acid (γ-PGA) producer Z3 isolated from soil samples in Daxinganling forest region of China was identified, and its optimal medium components were investigated using response surface methodology. Strain Z3 was identified as Bacillus velezensis by physiology and biochemistry and 16S rDNA sequence analysis. This is the first report of glutamate-dependent B. velezensis with the ability to synthesize γ-PGA. Then, the optimum γ-PGA yield (5.58 g/L) was achieved with glutamate 86 g/L, glucose 36 g/L, yeast extract powder 5.5 g/L, and NaH₂PO₄ 7.5 g/L. Furthermore, activities of enzymes participating in glutamate synthesis were assessed, and the results showed that lower ketoglutaric dehydrogenase activity (KGDH) and higher glutamate dehydrogenase activity (GDH) resulted in higher γ-PGA yield. Identification of glutamate-dependent γ-PGA producer named B. velezensis Z3 enriches microbiological resources with γ-PGA-producing capacity. B. velezensis optimization of nutrients and analysis of enzymes activities will not only help to increase γ-PGA productivity but also to understand the γ-PGA synthesis mechanism in B. velezensis Z3.

Poly-γ-glutamic Acid Synthesis, Gene Regulation, Phylogenetic Relationships, and Role in Fermentation

Poly-γ-glutamic acid (γ-PGA) is a biodegradable biopolymer produced by several bacteria, including Bacillus subtilis and other Bacillus species; it has good biocompatibility, is non-toxic, and has various potential biological applications in the food, pharmaceutical, cosmetic, and other industries. In this review, we have described the mechanisms of γ-PGA synthesis and gene regulation, its role in fermentation, and the phylogenetic relationships among various pgsBCAE, a biosynthesis gene cluster of γ-PGA, and pgdS, a degradation gene of γ-PGA. We also discuss potential applications of γ-PGA and highlight the established genetic recombinant bacterial strains that produce high levels of γ-PGA, which can be useful for large-scale γ-PGA production.

Improvement of glycerol catabolism in Bacillus licheniformis for production of poly-γ-glutamic acid

Bacillus licheniformis WX-02 is a well-studied strain to produce poly-γ-glutamic acid (γ-PGA) with numerous applications. This study is to improve WX-02 strain's capability of assimilating glycerol, a major byproduct of biofuels industries, through metabolic manipulation. Through gene knockout, the GlpK pathway was identified as the sole functional glycerol catabolism pathway, while the DhaK pathway was inactive for this strain under either aerobic or anaerobic conditions. The enhancement of glycerol utilization was attempted by substituting the native glpFK promoter with the constitutive promoter (P43), ytzE promoter (PytzE), and bacABC operon promoter (PbacA), respectively. The glycerol consumptions of the corresponding mutant strains WX02-P43glpFK, WX02-PytzEglpFK, and WX02-PbacAglpFK were 30.9, 26.42, and 18.8% higher than that of the WX-02 strain, respectively. The γ-PGA concentrations produced by the three mutant strains were 33.71, 23.39, and 30.05% higher than that of WX-02 strain, respectively. When biodiesel-derived crude glycerol was used as the carbon source, the mutant WX02-P43glpFK produced 16.63 g L^-1 of γ-PGA, with a productivity of 0.35 g L^-1 h^-1. Collectively, this study demonstrated that glycerol can be used as an effective substrate for producing γ-PGA by metabolic engineering B. licheniformis strains.

Effect of glucose on poly-γ-glutamic acid metabolism in Bacillus licheniformis

Background

Poly-gamma-glutamic acid (γ-PGA) is a promising macromolecule with potential as a replacement for chemosynthetic polymers. γ-PGA can be produced by many microorganisms, including Bacillus species. Bacillus licheniformis CGMCC2876 secretes γ-PGA when using glycerol and trisodium citrate as its optimal carbon sources and secretes polysaccharides when using glucose as the sole carbon source. To better understand the metabolic mechanism underlying the secretion of polymeric substances, SWATH was applied to investigate the effect of glucose on the production of polysaccharides and γ-PGA at the proteome level.

Results

The addition of glucose at 5 or 10 g/L of glucose decreased the γ-PGA concentration by 31.54 or 61.62%, respectively, whereas the polysaccharide concentration increased from 5.2 to 43.47%. Several proteins playing related roles in γ-PGA and polysaccharide synthesis were identified using the SWATH acquisition LC–MS/MS method. CcpA and CcpN co-enhanced glycolysis and suppressed carbon flux into the TCA cycle, consequently slowing glutamic acid synthesis. On the other hand, CcpN cut off the carbon flux from glycerol metabolism and further reduced γ-PGA production. CcpA activated a series of operons (glm and epsA-O) to reallocate the carbon flux to polysaccharide synthesis when glucose was present. The production of γ-PGA was influenced by NrgB, which converted the major nitrogen metabolic flux between NH₄⁺ and glutamate.

Conclusion

The mechanism by which B. licheniformis regulates two macromolecules was proposed for the first time in this paper. This genetic information will facilitate the engineering of bacteria for practicable strategies for the fermentation of γ-PGA and polysaccharides for diverse applications.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-017-0642-8) contains supplementary material, which is available to authorized users.

Significantly enhancing recombinant alkaline amylase production in Bacillus subtilis by integration of a novel mutagenesis-screening strategy with systems-level fermentation optimization

Background

Alkaline amylase has significant potential for applications in the textile, paper and detergent industries, however, low yield of which cannot meet the requirement of industrial application. In this work, a novel ARTP mutagenesis-screening method and fermentation optimization strategies were used to significantly improve the expression level of recombinant alkaline amylase in B. subtilis 168.

Results

The activity of alkaline amylase in mutant B. subtilis 168 mut-16# strain was 1.34-fold greater than that in the wild-type, and the highest specific production rate was improved from 1.31 U/(mg·h) in the wild-type strain to 1.57 U/(mg·h) in the mutant strain. Meanwhile, the growth of B. subtilis was significantly enhanced by ARTP mutagenesis. When the agitation speed was 550 rpm, the highest activity of recombinant alkaline amylase was 1.16- and 1.25-fold of the activities at 450 and 650 rpm, respectively. When the concentration of soluble starch and soy peptone in the initial fermentation medium was doubled, alkaline amylase activity was increased 1.29-fold. Feeding hydrolyzed starch and soy peptone mixture or glucose significantly improved cell growth, but inhibited the alkaline amylase production in B. subtilis 168 mut-16#. The highest alkaline amylase activity by feeding hydrolyzed starch reached 591.4 U/mL, which was 1.51-fold the activity by feeding hydrolyzed starch and soy peptone mixture. Single pulse feeding-based batch feeding at 10 h favored the production of alkaline amylase in B. subtilis 168 mut-16#.

Conclusion

The results indicated that this novel ARTP mutagenesis-screening method could significantly improve the yield of recombinant proteins in B. subtilis. Meanwhile, fermentation optimization strategies efficiently promoted expression of recombinant alkaline amylase in B. subtilis 168 mut-16#. These findings have great potential for facilitating the industrial-scale production of alkaline amylase and other enzymes, using B. subtilis cultures as microbial cell factories.

Improvement of Bacillus subtilis for poly-γ-glutamic acid production by genome shuffling

Poly‐γ‐glutamic acid (γ‐PGA) is a promising microbial polymer with potential applications in industry, agriculture and medicine. The use of high γ‐PGA‐producing strains is an effective approach to improve productivity of γ‐PGA. In this study, we developed a mutant, F3‐178, from Bacillus subtilis GXA‐28 using genome shuffling. The morphological characteristics of F3‐178 and GXA‐28 were not identical. Compared with GXA‐28 (18.4 ± 0.8 g l⁻¹), the yield of γ‐PGA was 1.9‐fold higher in F3‐178 (34.3 ± 1.2 g l⁻¹). Results from batch fermentation in 3.7 l fermenter showed that F3‐178 was satisfactory for industrial production of γ‐PGA. Metabolic studies suggested that the higher γ‐PGA yield in F3‐178 could be attributed to increased intracellular flux and uptake of extracellular glutamate. Real‐time PCR indicated that mRNA level of pgsB in F3‐178 was 18.8‐fold higher than in GXA‐28, suggesting the higher yield might be related to the overexpression of genes involved in γ‐PGA production. This study demonstrated that genome shuffling can be used for rapid improvement of γ‐PGA strains, and the possible mechanism for the improved phenotype was also explored at the metabolic and transcriptional levels.

Physiological and metabolic analysis of nitrate reduction on poly-gamma-glutamic acid synthesis in Bacillus licheniformis WX-02

Nitrate is an important nitrogen source for organism, but whether and how nitrate improves poly-γ-glutamic acid (γ-PGA) production of bacterial is not clear. The effect of nitrate on γ-PGA production of Bacillus licheniformis WX-02 was investigated. By addition of 50 mmol/L nitrate, the γ-PGA yield reached 12.3 ± 0.21 g/L, which increased 2.3-fold compared to the control. The mechanism of enhanced γ-PGA production was further investigated by analysis of nitrate reduction, physiology, pyruvate overflow metabolism and energy synthesis. Nitrate reduction was only carried out in the middle stage of γ-PGA fermentation. The result of consumption of nutrients showed that glucose uptake was not effected and the L-glutamic acid utilization efficiency increased from 48.3 to 77.0 %. The date of overflow metabolism obtained from high-performance liquid chromatography showed that the metabolism of pyruvate, formate, lactate and acetoin was both heightened by nitrate reduction, while the 2,3-butanediol biosynthesis was decreased. Meanwhile, the change of energy indicated that more ATP was synthesized during nitrate reduction. In summary, nitrate was a positive effector of γ-PGA biosynthesis in B. licheniformis WX-02 and nitrate reduction affected multi-metabolism pathways, including glycolysis, overflow metabolism and energy metabolism.

In vitro evaluation of new functional properties of poly-γ-glutamic acid produced by Bacillus subtilis D7

We investigated the functionality of poly-γ-glutamic acid (γ-PGA), which is produced by Bacillus subtilis D7, for its potential applications in medicine and cosmetics. The γ-PGA had angiotensin-converting enzyme (ACE) inhibition activity. ACE inhibition activity was dependent on the γ-PGA concentration; the highest ACE inhibition activity was observed at 1.25 mg/l of γ-PGA. IC50 (0.108 mg/ml) of the γ-PGA was lower than that of standard ACE inhibitory drug, N-[(S)-mercapto-2-methylpropionyl]-L-proline (0.247 mg/ml). The γ-PGA also had water-holding capacity and hygroscopicity. Furthermore, the γ-PGA inhibited growth of some pathogenic bacteria, including Listeria monocytogenes, Salmonella typhimurium, Staphylococcus aureus, Klebsiella pneumonia and Esherichia coli. The γ-PGA exhibited a good metal adsorption capacity; Cr (VI) adsorption capacity of γ-PGA increased with decreasing pH, and the maximal adsorption was observed at pH 2. Our results suggest that γ-PGA may be expected to be widely applied in cosmetics, biomedical and environmental industries with the feature of being less harmful to humans and the environment.

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.

Poly (glutamic acid)--an emerging biopolymer of commercial interest

Poly (γ-glutamic acid) (PGA) is water-soluble, anionic, biodegradable, and edible biopolymer produced by Bacillus subtilis. It has multifarious potential applications in foods, pharmaceuticals, healthcare, water treatment and other fields. The production of PGA has already been established on the industrial scale. Various studies regarding the fermentative production, downstream processing and characterization of PGA have been reported in the literature. This review provides updated information on fermentative production of PGA by various bacterial strains and effect of fermentation conditions and media component on production of PGA in submerged as well as solid state fermentation. Information on the application of genetic engineering for enhancement of yield of PGA, kinetic studies for production of PGA in submerged fermentation and recovery and purification of PGA is included. An attempt has also been made to review the current and potential applications of PGA. This review may contribute to further development of this commercially and academically interesting biopolymer.