Microbial synthesis of poly-γ-glutamic acid: current progress, challenges, and future perspectives

“Clickable” bacterial poly(γ-glutamic acid)

The controlled functionalization of bacterial γ-PGA is realized through sonication, solubilization using quaternary ammonium salts and click chemistry.

Isolation of a novel poly-γ-glutamic acid-producing Bacillus licheniformis A14 strain and optimization of fermentation conditions for high-level production

In the present study, bacteria producing poly-γ-glutamic acid were isolated from marine sands, and an efficient producer identified. γ-PGA was rapidly screened by thin-layer chromatography and UV spectrophotometer assay. Media optimization was carried out, and for the cost-effective production of γ-PGA, monosodium glutamate was used as the substrate for the synthesis of γ-PGA instead of glutamic acid. Lastly, Plackett-Buman design (PB) and Response surface methodology (RSM) were used to determine significant media components and their interaction effect to achieve maximum γ-PGA production. With this integrated method, a bacterial strain with a high yield of γ-PGA was obtained rapidly, and the production was increased up to 37.8 g/L after optimization.

The Depression and Adsorption Mechanism of Polyglutamic Acid on Chalcopyrite and Pyrrhotite Flotation Systems

The rejection of pyrrhotite and pyrite has become a long-standing problem in the copper ore industry. This paper describes the first successful depression and adsorption mechanism of a novel and non-hazardous reagent, polyglutamic acid (PGA), on pyrrhotite in the selective flotation of chalcopyrite with xanthate as the collector, making use of various laboratory-scale measurement techniques. The addition of PGA inhibited the flotation of pyrrhotite much more strongly than that of the chalcopyrite in a wide pH range. The prior addition of PGA achieved an improved selective flotation of chalcopyrite from pyrrhotite at pH 8, at which the grade and recovery of chalcopyrite in concentrate were over 80%. Surface measurement techniques including XPS spectral, IR spectral, zeta potential, and reagent adsorption analyses indicated that the PGA interacted differently with the two minerals, and had much greater affinity towards pyrrhotite than chalcopyrite. The presence of PGA blocked the electrochemical activity of the collector on the pyrrhotite surface and thus depressed its flotation, whereas the adsorption of the collector and its oxidation to dixanthogen were more effective on the chalcopyrite surface, indicating a weaker interaction of PGA with chalcopyrite. Our results suggest that the application of PGA could replace the toxic inorganic depressants in flotation technology, and could significantly reduce the environmental impacts of processing.

Engineering Corynebacterium glutamicum for the de novo biosynthesis of tailored poly-γ-glutamic acid

γ-Polyglutamic acid (γ-PGA) is a biodegradable polymer naturally produced by Bacillus spp. that has wide applications. Fermentation of γ-PGA using Bacillus species often requires the supplementation of L-glutamic acid, which greatly increases the overall cost. Here, we report a metabolically engineered Corynebacterium glutamicum capable of producing γ-PGA from glucose. The genes encoding γ-PGA synthase complex from B. subtilis (pgsB, C, and A) or B. licheniformis (capB, C, and A) were expressed under inducible promoter P_tac in a L-glutamic acid producer C. glutamicum ATCC 13032, which led to low levels of γ-PGA production. Subsequently, C. glutamicum F343 with a strong L-glutamic acid production capability was tested. C. glutamicum F343 carrying capBCA produced γ-PGA up to 11.4 g/L, showing a higher titer compared with C. glutamicum F343 expressing pgsBCA. By introducing B. subtilis glutamate racemase gene racE under P_tac promoter mutants with different expression strength, the percentage of L-glutamic acid units in γ-PGA could be adjusted from 97.1% to 36.9%, and stayed constant during the fermentation process, while the γ-PGA titer reached 21.3 g/L under optimal initial glucose concentrations. The molecular weight (M_w) of γ-PGA in the engineered strains ranged from 2000 to 4000 kDa. This work provides a foundation for the development of sustainable and cost-effective de novo production of γ-PGA from glucose with customized ratios of L-glutamic acid in C. glutamicum.

Knockout of pgdS and ggt gene changes poly-γ-glutamic acid production in Bacillus licheniformis RK14-46

Poly-gamma-glutamic acid (γ-PGA) is a water-soluble, nontoxic biocompatible polymer, which is extensively used in medicines, foodstuffs, cosmetics, and in water treatment. We previously isolated a novel γ-PGA producing strain Bacillus licheniformis RK14 from soil and developed a hyper-producing mutant strain RK14-46 by an ethyl methanesulfonate (EMS) treatment. In this study, endo-type (pgdS) and exo-type γ-PGA hydrolases (ggt) were disrupted by integrating plasmids into the genomic DNA of B. licheniformis RK14-46 strain. Unexpectedly, we observed strong inhibition of γ-PGA production following deletion of the pgdS gene, suggesting that pgdS is essential for γ-PGA biosynthesis in strain RK14-46, and in its parent strain RK14. In contrast, γ-PGA production increased by the deletion of the ggt gene and reached 39 g/L in the presence of 90 g/L glucose and elevated oxygen supply. Furthermore, γ-PGA from the ggt-disrupted mutant (Δggt) maintained a larger molecular mass throughout the culture period, whereas that from the original RK14-46 strain had degraded after glucose consumption. γ-PGA-containing culture supernatants from Δggt strain showed greater flocculation efficiency in sewage sludge than supernatants from the RK14-46 strain, reflecting greater production of γ-PGA with larger molecular mass by the Δggt strain. This is the first report concerning the deletion of pgdS and ggt genes in B. licheniformis strain and the properties of γ-PGA obtained from the mutant strain.

Poly-Gamma-Glutamic Acid Functions as an Eective Lubricant with Antimicrobial Activity in Multipurpose Contact Lens Care Solutions

In order to perform the multiple functions of disinfection, cleansing, and storage, preservatives are often added to contact lens care solutions. The disadvantage of adding preservatives is that this often causes various eye conditions. However, lens care solutions would not be able to disinfect in the absence of such preservatives. In addition, comfort is an important issue for contact lens wearers due to the long periods of time they are worn. It has been shown that lower friction coefficients are correlated with increased comfort. We have previously developed a multipurpose contact lens care solution in which poly-gamma-glutamic acid (γ-PGA) was the main ingredient. In this study, we investigated the antimicrobial activity and lubricating property of our care solution. We showed that there was a synergetic effect of γ-PGA and chlorine dioxide on antimicrobial activity. We also demonstrated that γ-PGA functioned as a lubricating agent. Our results provided evidence of γ-PGA acting as a multi-functional polymer that could be applied in contact lens care solutions.

Involvement of Peptide Epimerization in Poly-γ-glutamic Acid Biosynthesis

Poly-γ-glutamic acid (PGA) is a promising polymer that comprises d- and l-glutamic acid (Glu) connected via an amide bond. PGA is biosynthesized by a transmembrane enzyme complex composed of PgsB, PgsC, and PgsA. However, the detailed reaction, especially the mechanism for introducing d-Glu residues into PGA, remains elusive. We herein report isotope tracer experiments with deuterated l- and d-Glu and demonstrate that epimerization of a growing peptide is involved in PGA biosynthesis.

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.

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.

Bacterially Produced, Nacre-Inspired Composite Materials

The impressive mechanical properties of natural composites, such as nacre, arise from their multiscale hierarchical structures, which span from nano- to macroscale and lead to effective energy dissipation. While some synthetic bioinspired materials have achieved the toughness of natural nacre, current production methods are complex and typically involve toxic chemicals, extreme temperatures, and/or high pressures. Here, the exclusive use of bacteria to produce nacre-inspired layered calcium carbonate-polyglutamate composite materials that reach and exceed the toughness of natural nacre, while additionally exhibiting high extensibility and maintaining high stiffness, is introduced. The extensive diversity of bacterial metabolic abilities and the possibility of genetic engineering allows for the creation of a library of bacterially produced, cost-effective, and eco-friendly composite materials.

Properties of Poly-γ-Glutamic Acid Producing-Bacillus Species Isolated From Ogi Liquor and Lemon-Ogi Liquor

Poly-γ-glutamic acid (γPGA) is a natural and promising biopolymer synthesized by Bacillus spp. during fermentation. This study isolated Bacillus spp. from ogi steep liquor (OSL) and lemon-ogi steep liquor (LOSL) using standard methods and determined the γPGA-producing ability. The antimicrobial and angiotensin-converting enzyme (ACE) inhibitory activities of γPGA polymer were evaluated and isolates were sequenced. Four isolates (TA004, TA006, TA011, TA012) selected based on phenotypic characterization and stickiness (<15 cm) showed antibacterial activity against different pathogens with the highest activity found in TA004 (22.5 mm) and least in TA011 (16.6 mm). Furthermore, time-kill assay showed that the combined γPGA polymer was more effective and demonstrated bactericidal activity over individual γPGA which are bacteriostatic in nature. All γPGA polymer exhibited ACE properties except TA011. The highest IC50 was observed in TA006 (0.11 mg/ml) and least in TA004 (0.35 mg/ml). TA004 had the highest molecular weight (261 kDa) while TA011 had the least (194.97 kDa). In addition, all γPGA exhibited characteristic peaks at 3413-3268 cm-1 and 1722-1664 cm-1 that corresponded to amine N-H stretching intensities and C = O stretching in COOH. Bacillus isolates were identified as TA004 (B. subtilis-GenBank: MH782061), TA006 (B. amyloliquefaciens- GenBank: MH782075), TA011 (B. subtilis- GenBank: MH782088), TA012 (B. subtilis- GenBank: MH782083). OSL and LOSL have the potential for developing functional foods with a valuable effect on health.

Characterization of a Regulator pgsR on Endogenous Plasmid p2Sip and Its Complementation for Poly(γ-glutamic acid) Accumulation in Bacillus amyloliquefaciens

Bacillus amyloliquefaciens NX-2S154 is a promising poly(γ-glutamic acid) (γ-PGA) producing strain discovered in previous studies. However, the wild-type strain contains an unknown endogenous plasmid, p2Sip, which causes low transformation efficiency and instability of exogenous plasmids. In our study, p2Sip is 5622 bp with 41% G+C content and contains four putative open reading frames (ORFs), including genes repB, hsp, and mobB and γ-PGA-synthesis regulator, pgsR. Elimination of p2Sip from strain NX-2S154 delayed γ-PGA secretion and decreased production of γ-PGA by 18.1%. Integration of a pgsR expression element into the genomic BamHI locus using marker-free manipulation based on pheS* increased the γ-PGA titer by 8%. pgsR overexpression upregulated the expression of γ-PGA synthase pgsB, regulator degQ, and glutamic acid synthase gltA, thus increasing the γ-PGA production in B. amyloliquefaciens NB. Our results indicated that pgsR from p2Sip plays an important regulatory role in γ-PGA synthesis in B. amyloliquefaciens.

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.

Integration of enzymatic data in Bacillus subtilis genome-scale metabolic model improves phenotype predictions and enables in silico design of poly-γ-glutamic acid production strains

Background

Genome-scale metabolic models (GEMs) allow predicting metabolic phenotypes from limited data on uptake and secretion fluxes by defining the space of all the feasible solutions and excluding physio-chemically and biologically unfeasible behaviors. The integration of additional biological information in genome-scale models, e.g., transcriptomic or proteomic profiles, has the potential to improve phenotype prediction accuracy. This is particularly important for metabolic engineering applications where more accurate model predictions can translate to more reliable model-based strain design.

Results

Here we present a GEM with Enzymatic Constraints using Kinetic and Omics data (GECKO) model of Bacillus subtilis, which uses publicly available proteomic data and enzyme kinetic parameters for central carbon (CC) metabolic reactions to constrain the flux solution space. This model allows more accurate prediction of the flux distribution and growth rate of wild-type and single-gene/operon deletion strains compared to a standard genome-scale metabolic model. The flux prediction error decreased by 43% and 36% for wild-type and mutants respectively. The model additionally increased the number of correctly predicted essential genes in CC pathways by 2.5-fold and significantly decreased flux variability in more than 80% of the reactions with variable flux. Finally, the model was used to find new gene deletion targets to optimize the flux toward the biosynthesis of poly-γ-glutamic acid (γ-PGA) polymer in engineered B. subtilis. We implemented the single-reaction deletion targets identified by the model experimentally and showed that the new strains have a twofold higher γ-PGA concentration and production rate compared to the ancestral strain.

Conclusions

This work confirms that integration of enzyme constraints is a powerful tool to improve existing genome-scale models, and demonstrates the successful use of enzyme-constrained models in B. subtilis metabolic engineering. We expect that the new model can be used to guide future metabolic engineering efforts in the important industrial production host B. subtilis.

Efficient Biosynthesis of Low-Molecular-Weight Poly-γ-glutamic Acid by Stable Overexpression of PgdS Hydrolase in Bacillus amyloliquefaciens NB

Low-molecular-weight poly-γ-glutamic acid (LMW-γ-PGA) has attracted much attention owing to its great potential in food, agriculture, medicine, and cosmetics. Current methods of LMW-γ-PGA production, including enzymatic hydrolysis, are associated with low operational stability. Here, an efficient method for stable biosynthesis of LMW-γ-PGA was conceived by overexpression of γ-PGA hydrolase in Bacillus amyloliquefaciens NB. To establish stable expression of γ-PGA hydrolase (PgdS) during fermentation, a novel plasmid pNX01 was constructed with a native replicon from endogenous plasmid p2Sip, showing a loss rate of 4% after 100 consecutive passages. Subsequently, this plasmid was applied in a screen of high activity PgdS hydrolase, leading to substantial improvements to γ-PGA titer with concomitant decrease in the molecular weight. Finally, a satisfactory yield of 17.62 ± 0.38 g/L LMW-γ-PGA with a weight-average molecular weight of 20-30 kDa was achieved by direct fermentation of Jerusalem artichoke tuber extract. Our study presents a potential method for commercial production of LMW-γ-PGA.

Systematic unravelling of the inulin hydrolase from Bacillus amyloliquefaciens for efficient conversion of inulin to poly-(γ-glutamic acid)

BACKGROUND

Bacillus amyloliquefaciens NB is a newly discovered strain, which produces poly-(γ-glutamic acid) (γ-PGA) from raw extracted inulin of Jerusalem artichoke tubers; however, the underlying mechanisms remain unknown. To address this problem, we identified the inulin hydrolase in wild-type strain B. amyloliquefaciens NB.

RESULTS

The novel inulin hydrolase (CscA) was discovered from strain NB, with high inulinase activity (987.0 U/mg at 55 °C) and strong resistance at pH values between 8.0 and 11.0, suggesting the potential application of CscA in Jerusalem artichoke biorefinery. CscA exhibited a k _cat/K _m of (6.93 ± 0.27) × 10³ for inulin; its enzymatic activity was stimulated by metal ions, like K⁺, Mn²⁺, or Ca²⁺. Similar to their role in glycoside hydrolase 32 family enzymes, the conserved Asp37, Asp161, and Glu215 residues of CscA contribute to its catalytic activity. Targeted disruption of CscA gene suppressed inulin utilization by strain NB. Overexpression of CscA significantly enhanced the γ-PGA generation by 19.2% through enhancement in inulin consumption.

CONCLUSIONS

The inulin hydrolase CscA is critical for inulin metabolism in B. amyloliquefaciens and indicates potential application in Jerusalem artichoke biorefinery.

Stimulatory effects of amino acids on γ-polyglutamic acid production by Bacillus subtilis

This paper is about study to increase the γ-PGA yield by developing new methods. The effect of various amino acids on production of γ-PGA by Bacillus subtilis Z15 was investigated. The γ-PGA yield was increased 23.18%, 12.15% and 31.46%, respectively, with 3 g/L aspartic acid (0 h), 1.5 g/L phenylalanine (0 h) and 7 g/L glutamic acid (24 h). Additonally, crude extract of glutamic acid after isoelectric crystallization (CEGA)could be a replacement for glutamate for γ-PGA production. Then, response surface methodology (RSM) was used for further optimization. The final media ingredient of amino acids were obtained as follow: CEGA 9 g/L, aspartic acid 4 g/L, phenylalanine 1.55 g/L. By applying this receipt in 5-L bioreactor, the γ-PGA yield reached 42.92 ± 0.23 g/L after 44 h, which is 63.1% higher than the control without amino acids for production. In addition, amino acids could shorten the lag phase and the average fermentation time (44 h versus 48 h). Fermentation with amino acids addition can be an positive option for γ-PGA production.

Fabrication and evaluation of a γ-PGA-based self-assembly transferrin receptor-targeting anticancer drug carrier

Background

cis-Dichlorodiamineplatinum (CDDP) was one of the most common used drugs in clinic for cancer treatment. However, CDDP caused a variety of side effects. The abundant carboxyl groups on the surface of poly glutamic acid (PGA) could be modified with various kinds of targeted ligands. PGA delivery system loaded CDDP for cancer therapies possesses potential to overcome the side effects.

Materials and methods

In this study, we constructed a safe and efficient anticancer drug delivery system PGA–Asp–maleimide–cisplatin–peptide complex (PAMCP), which was loaded with CDDP and conjugated with the transferrin receptor (TFR)-targeting peptide through a maleimide functional linker. The size of PAMCP was identified by transmission electron microscopy (TEM) and dynamic light scattering (DLS). Fluorescence microscopy and flow cytometry methods were used to detect the cell targeting ability in vitro. The MTT assay was used to detect targeted toxicity in vitro. The in vivo acute toxicity was tested in Kun Ming (KM) mice. The tumor suppression activity and drug distribution was analyzed in nude mice bearing with HeLa tumor cells.

Results

The nano-size was 110±28 nm detected with TEM and 89±18 nm detected with DLS, respectively. Fluorescence microscopy and flow cytometry methods indicated that PAMCP possessed excellent cell targeting ability in vitro. The MTT assay suggested that PAMCP was excellent for targeted toxicity. The acute in vivo toxicity study revealed that the body mass index and serum index in the PAMCP-treated group were superior to those in the CDDP-treated group (P<0.001), and no obvious differences were detected on comparing with the PBS- or PGA–Asp–maleimide–P8 (PAMP)-treated groups. PAMCP reduced the toxicity of CDDP, suppressed tumor cell growth, and achieved efficient anti-tumor effects in vivo. After PAMCP treatment, the toxicity of CDDP was reduced and tumor growth was more remarkably inhibited compared with the free CDDP treatment group (P<0.01). Much stronger (5–10 folds) fluorescence intensity in tumor tissue was detected compared with the irrelevant-peptide group for drug distribution analysis detected with the frozen section approach.

Conclusion

Our data highlighted that PAMCP reduced the side effects of CDDP and exhibited stronger anti-tumor effects. Therefore, PAMCP presented the potential to be a safe and effective anticancer pharmaceutical formulation for future clinical applications.

Genome Features and Biochemical Characteristics of a Robust, Fast Growing and Naturally Transformable Cyanobacterium Synechococcus elongatus PCC 11801 Isolated from India

Cyanobacteria provide an interesting platform for biotechnological applications due to their efficient photoautotrophic growth, amenability to genetic engineering and the ability to grow on non-arable land. An ideal industrial strain of cyanobacteria would need to be fast growing and tolerant to high levels of temperature, light, carbon dioxide, salt and be naturally transformable. In this study, we report Synechococcus elongatus PCC 11801, a strain isolated from India that fulfills these requirements. The physiological and biochemical characteristics of PCC 11801 under carbon and light-limiting conditions were investigated. PCC 11801 shows a doubling time of 2.3 h, that is the fastest growth for any cyanobacteria reported so far under ambient CO₂ conditions. Genome sequence of PCC 11801 shows genome identity of ~83% with its closest neighbors Synechococcus elongatus PCC 7942 and Synechococcus elongatus UTEX 2973. The unique attributes of PCC 11801 genome are discussed in light of the physiological characteristics that are needed in an industrial strain. The genome of PCC 11801 shows several genes that do not have homologs in neighbor strains PCC 7942 and UTEX 2973, some of which may be responsible for adaptation to various abiotic stresses. The remarkably fast growth rate of PCC 11801 coupled with its robustness and ease of genetic transformation makes it an ideal candidate for the photosynthetic production of fuels and chemicals.

Rewiring glycerol metabolism for enhanced production of poly-γ-glutamic acid in Bacillus licheniformis

BACKGROUND

Poly-γ-glutamic acid (γ-PGA) is a natural polymer with great potential applications in areas of agriculture, industry, and pharmaceutical. The biodiesel-derived glycerol can be used as an attractive feedstock for γ-PGA production due to its availability and low price; however, insufficient production of γ-PGA from glycerol is limitation.

RESULTS

The metabolic pathway of Bacillus licheniformis WX-02 was rewired to improve the efficiency of glycerol assimilation and the supply of NADPH for γ-PGA synthesis. GlpK, GlpX, Zwf, and Tkt1 were found to be the key enzymes for γ-PGA synthesis using glycerol as a feedstock. Through combinational expression of these key enzymes, the γ-PGA titer increased to 19.20 ± 1.57 g/L, which was 1.50-fold of that of the wild-type strain. Then, we studied the flux distributions, gene expression, and intracellular metabolites in WX-02 and the recombinant strain BC4 (over-expression of the above quadruple enzymes). Our results indicated that over-expression of the quadruple enzymes redistributed metabolic flux to γ-PGA synthesis. Furthermore, using crude glycerol as carbon source, the BC4 strain showed a high productivity of 0.38 g/L/h, and produced 18.41 g/L γ-PGA, with a high yield of 0.46 g γ-PGA/g glycerol.

CONCLUSIONS

The approach to rewiring of metabolic pathways enables B. licheniformis to efficiently synthesize γ-PGA from glycerol. The γ-PGA productivity reported in this work is the highest obtained in glutamate-free medium. The present study demonstrates that the recombinant B. licheniformis strain shows significant potential to produce valuable compounds from crude glycerol.

Effective and safe in vivo gene delivery based on polyglutamic acid complexes with heterocyclic amine modified-polyethylenimine

Polyethylenimine (PEI) has been extensively used for non-viral gene delivery. Increasing the molecular weight of PEI often improves transfection efficiency, but enhances cytotoxicity and non-specific interaction with plasma proteins, limiting its use in clinical applications. In this study, poly-l-glutamic acid (L-PGA) as an anionic polymer, was introduced to piperazine-modified PEI to improve its in vivo properties. The physicochemical properties, cytotoxicity, in vitro and in vivo tranfection efficiency of these carriers were evaluated. Conjugation of 50% of primary amines of PEI 25 kDa with piperazine in the presence of PGA_1% (PEI₂₅Pip_50%/PGA_1%) could significantly increase transfection efficiency even in the presence of serum compared to PEI 25 kDa. Increasing the PGA content led to lower cytotoxicity of DNA/PEI₂₅Pip_50%/PGA_1% triplexes. Systemic administration of triplexes in Balb/c mice resulted in significant enhancement of luciferase gene expression in brain, spleen, and liver compared to PEI 25 kDa. In a 30-day survival study, no significant changes were observed in mice body weights in DNA/PEI₂₅Pip_50%/PGA_1% group. Moreover, this group exhibited a survival rate of 100% compared to 0% in mice receiving PEI 25 kDa. This novel PEI₂₅Pip_50%/PGA_1% carrier could be used to overcome the serum inhibitory effects on gene expression in vivo, providing a promising gene delivery system for tissue-specific targeting.

Poly (γ-Glutamic Acid) Promotes Enhanced Dechlorination of p-Chlorophenol by Fe-Pd Nanoparticles

Nanoscale zero-valent iron (nZVI) has shown considerable promise in the treatment of chlorinated organic compounds, but rapid aggregation and inactivation hinder its application. In this study, palladium-doped zero-valent iron nanoparticles involving poly (γ-glutamic acid) (Fe-Pd@PGA NPs) were synthesized. The nanoparticles were small (~100 nm), uniformly distributed, and highly stable. The dechlorination performance of Fe-Pd@PGA NPs was evaluated using p-CP as a model. The results demonstrated that Fe-Pd@PGA NPs show high activity even in weakly alkaline conditions. The maximum rate constant reached 0.331 min^- 1 at pH 9.0 with a Fe to p-CP ratio of 100. Additionally, the dechlorination activity of Fe-Pd@PGA NPs is more than ten times higher than that of the bare Fe-Pd NPs, demonstrating the crucial role of PGA in this system. Furthermore, we investigated the dechlorination performance in the presence of different anions. The results indicated that Fe-Pd@PGA NPs can maintain high activity in the presence of Cl^-, H₂PO₄^-, and humic acid, while HPO₄^2-and HCO₃^- ions slightly reduce the dechlorination activity. We believed that PGA is a promising stabilizer and promoter for Fe-Pd NPs and the Fe-Pd@PGA NPs have the potential for practical applications.

Enhanced production of poly-γ-glutamic acid by improving ATP supply in metabolically engineered Bacillus licheniformis

Poly-γ-glutamic acid (γ-PGA) is an important multifunctional biopolymer with various applications, for which adenosine triphosphate (ATP) supply plays a vital role in biosynthesis. In this study, the enhancement of γ-PGA production was attempted through various approaches of improving ATP supply in the engineered strains of Bacillus licheniformis. The first approach is to engineer respiration chain branches of B. licheniformis, elimination of cytochrome bd oxidase branch reduced the maintenance coefficient, leading to a 19.27% increase of γ-PGA yield. The second approach is to introduce Vitreoscilla hemoglobin (VHB) into recombinant B. licheniformis, led to a 13.32% increase of γ-PGA yield. In the third approach, the genes purB and adK in ATP-biosynthetic pathway were respectively overexpressed, with the AdK overexpressed strain increased γ-PGA yield by 14.69%. Our study also confirmed that the respiratory nitrate reductase, NarGHIJ, is responsible for the conversion of nitrate to nitrite, and assimilatory nitrate reductase NasBC is for conversion of nitrite to ammonia. Both NarGHIJ and NasBC were positively regulated by the two-component system ResD-ResE, and overexpression of NarG, NasC, and ResD also improved the ATP supply and the consequent γ-PGA yield. Based on the above individual methods, a method of combining the deletion of cydBC gene and overexpression of genes vgB, adK, and resD were used to enhance ATP content of the cells to 3.53 μmol/g of DCW, the mutant WX-BCVAR with this enhancement produced 43.81 g/L of γ-PGA, a 38.64% improvement compared to wild-type strain WX-02. Collectively, our results demonstrate that improving ATP content in B. licheniformis is an efficient strategy to improve γ-PGA production.

Factors increasing poly-γ-glutamic acid content of cheongguk-jang fermented by Bacillus subtilis 168

Cheongguk-jang is a Korean traditional food produced by natural fermentation of boiled soybean. In cheongguk-jang, bacilli are dominant bacteria and produce highly viscous poly-γ-glutamic acid (γ-PGA), which improves human health functions. The purpose of this experiment was to find maximum production condition for the γ-PGA content during fermentation of cheongguk-jang with Bacillus subtilis 168. The most viscous cheongguk-jang was produced when soybean was cooked at 121 °C for 60 min in the presence of 50%(w/w) added water, followed by fermentation at 40 °C for 2 days. Additional conditions for maximum production of γ-PGA were the addition of 0.1%(w/w) FeCl₃·6H₂O, 3.0%(w/w) lactose and 3.0%(w/w) yeast extract as nutrients of inorganic salts, carbon source and nitrogen source, respectively. The three conditions did not show cumulative effect on the γ-PGA production and the addition of iron salt induced the most γ-PGA (0.97 ± 0.05%(w/w)), which corresponded to 2.7 times of the content in control cheongguk-jang.

Poly-L-gamma-glutamic acid production by recombinant Bacillus subtilis without pgsA gene

Poly-gamma-glutamic acid (PGA) is a promising bio-based polymer that shares many functions with poly (acrylic acid) and its derivatives. Thus, technologies for efficient production and molecular size control of PGA are required to expand the application of this useful biopolymer. In Bacillus strains, PGA is synthesized by the PgsBCA protein complex, which is encoded by the pgsBCA gene operon, otherwise is known as ywsC and ywtAB operons and/or capBCA operon. Hence, we investigated responsible components of the PgsBCA complex in B. subtilis for over-production of PGA. In particular, we constructed genomic pgsBCA gene-deletion mutants of B. subtilis. And also, we assembled high copy-number plasmids harboring σA-dependent promoter, leading to high-level expression of all combinations of pgsBCA, pgsBC, pgsBA, pgsCA, pgsB, pgsC, and/or pgsA genes. Subsequently, PGA production of the transformed B. subtilis mutant was determined in batch fermentation using medium supplemented with l-glutamate. PGA production by the transformants introduced with pgsBC genes (lacking the genomic pgsBCA genes) was 26.0 ± 3.0 g L⁻¹, and the enantiomeric ratio of d- and l-glutamic acid (d/l-ratio) in the produced PGA was 5/95. In contrast, d/l-ratio of produced PGA by the transformants introduced with pgsBCA genes (control strains) was 75/25. In conclusion, B. subtilis without pgsA gene could over-produce PGA with an l-rich enantiomeric ratio.

Bacterial components as naturally inspired nano-carriers for drug/gene delivery and immunization: Set the bugs to work?

Drug delivery is a rapidly growing area of research motivated by the nanotechnology revolution, the ideal of personalized medicine, and the desire to reduce the side effects of toxic anti-cancer drugs. Amongst a bewildering array of different nanostructures and nanocarriers, those examples that are fundamentally bio-inspired and derived from natural sources are particularly preferred. Delivery of vaccines is also an active area of research in this field. Bacterial cells and their components that have been used for drug delivery, include the crystalline cell-surface layer known as "S-layer", bacterial ghosts, bacterial outer membrane vesicles, and bacterial products or derivatives (e.g. spores, polymers, and magnetic nanoparticles). Considering the origin of these components from potentially pathogenic microorganisms, it is not surprising that they have been applied for vaccines and immunization. The present review critically summarizes their applications focusing on their advantages for delivery of drugs, genes, and vaccines.

Genetic and metabolic engineering for microbial production of poly-γ-glutamic acid

Poly-γ-glutamic acid (γ-PGA) is a natural biopolymer of glutamic acid. The repeating units of γ-PGA may be derived exclusively from d-glutamic acid, or l-glutamic acid, or both. The monomer units are linked by amide bonds between the α-amino group and the γ-carboxylic acid group. γ-PGA is biodegradable, edible and water-soluble. It has numerous existing and emerging applications in processing of foods, medicines and cosmetics. This review focuses on microbial production of γ-PGA via genetically and metabolically engineered recombinant bacteria. Strategies for improving production of γ-PGA include modification of its biosynthesis pathway, enhancing the production of its precursor (glutamic acid), and preventing loss of the precursor to competing byproducts. These and other strategies are discussed. Heterologous synthesis of γ-PGA in industrial bacterial hosts that do not naturally produce γ-PGA is discussed. Emerging trends and the challenges affecting the production of γ-PGA are reviewed.

Enhanced Production of Poly-γ-glutamic acid by Overexpression of the Global Anaerobic Regulator Fnr in Bacillus licheniformis WX-02

Poly-γ-glutamic acid is a multi-functional biopolymer with various applications. ATP supply plays an important role in poly-γ-glutamic acid (γ-PGA) synthesis. Global anaerobic regulator Fnr plays a key role in anaerobic adaptation and nitrate respiration, which might affect ATP generation during γ-PGA synthesis. In this study, we have improved γ-PGA production by overexpression of Fnr in Bacillus licheniformis WX-02. First, the gene fnr was knocked out in WX-02, and the γ-PGA yields have no significant differences between WX-02 and the fnr-deficient strain WXΔfnr in the medium without nitrate (BFC medium). However, the γ-PGA yield of 8.95 g/L, which was produced by WXΔfnr in the medium with nitrate addition (BFCN medium), decreased by 74% compared to WX-02 (34.53 g/L). Then, the fnr complementation strain WXΔfnr/pHY-fnr restored the γ-PGA synthesis capability, and γ-PGA yield was increased by 13% in the Fnr overexpression strain WX/pHY-fnr (39.96 g/L) in BFCN medium, compared to WX/pHY300 (35.41 g/L). Furthermore, the transcriptional levels of narK, narG, and hmp were increased by 5.41-, 4.93-, and 3.93-fold in WX/pHY-fnr, respectively, which led to the increases of nitrate consumption rate and ATP supply for γ-PGA synthesis. Collectively, Fnr affects γ-PGA synthesis mainly through manipulating the expression level of nitrate metabolism, and this study provides a novel strategy to improve γ-PGA production by overexpression of Fnr.

DL-endopeptidases function as both cell wall hydrolases and poly-γ-glutamic acid hydrolases

Biopolymers on the cell surface are very important for protecting microorganisms from environmental stresses, as well as storing nutrients and minerals. Synthesis of biopolymers is well studied, while studies on the modification and degradation processes of biopolymers are limited. One of these biopolymers, poly-γ-glutamic acid (γ-PGA), is produced by Bacillus species. Bacillus subtilis PgdS, possessing three NlpC/P60 domains, hydrolyses γ-PGA. Here, we have demonstrated that several dl-endopeptidases with an NlpC/P60 domain (LytE, LytF, CwlS, CwlO, and CwlT) in B. subtilis digest not only an amide bond of d-γ-glutamyl-diaminopimelic acid in peptidoglycans but also linkages of γ-PGA produced by B. subtilis. The hydrolase activity of dl-endopeptidases towards γ-PGA was inhibited by IseA, which also inhibits their hydrolase activity towards peptidoglycans, while the hydrolysis of PgdS towards γ-PGA was not inhibited. PgdS hydrolysed only the d-/l-Glu‒d-Glu linkages of d-Glu-rich γ-PGA (d-Glu:l-Glu=7 : 3) and l-Glu-rich γ-PGA (d-Glu:l-Glu=1 : 9), indicating that PgdS can hydrolyse only restricted substrates. On the other hand, the dl-endopeptidases in B. subtilis cleaved d-/l-Glu‒d-/l-Glu linkages of d-Glu-rich γ-PGA (d-Glu:l-Glu=7 : 3), indicating that these enzymes show different substrate specificities. Thus, the dl-endopeptidases digest γ-PGA more flexibly than PgdS, even though they are annotated as "dl-endopeptidase, digesting the d-γ-glutamyl-diaminopimelic acid linkage (d‒l amino acid bond)".

Preparation of Chitosan/Poly‐γ‐Glutamic Acid Polyelectrolyte Multilayers on Biomedical Metals for Local Antibiotic Delivery

Polyelectrolyte multilayer assembly is one of the most widely applied biomaterial coatings for applications from surface modification, drug delivery, tissue engineering to biomimetic extracellular environment. In this research, we propose a simple layer-wise spin coating technique to prepare chitosan/poly-γ-glutamic acid (C/PGA) polyelectrolyte multilayers (PEMs) on two different biomedical metals, 316L stainless steel (316LSS) and titanium alloy (Ti6Al4V). The multilayer coating was fabricated using oppositely charged chitosan and poly--glutamic acid to deposit a total of 10, 20, or 30 multilayered films. Afterward, tetracycline was loaded by soaking the coated metals for 12 hours. The microstructure, mechanical properties, biocompatibility and drug release rate were investigated by scanning electron microscopy, contact angle measurement, MG63 cell viability and inhibition of Escherichia coli (E. coli) growth. Lastly, MG63 cell attachment was detected by fluorescence microscopy after staining with Hoechst 33258. This coating technique can prepare a layer of 2.2–6.9 m C/PGA PEMs favoring cell attachment and growth. Moreover, tetracycline was released from C/PGA PEMs and inhibited the growth of E. coli. The results suggest that C/PGA PEMs provide a useful platform for modulating the micro-environment for better cell adhesion and antibiotic delivery, which hold great potential for surface modification and drug loading for biomimetic materials.

The Synthesis and Structural Characterization of Graft Copolymers Composed of γ-PGA Backbone and Oligoesters Pendant Chains

The novel copolymers composed of poly-γ-glutamic acid (γ-PGA) and oligoesters have been developed. The structures of the obtained copolymers including variety of end groups were determined at the molecular level with the aid of electrospray ionization multistage mass spectrometry (ESI-MSⁿ). The fragmentation experiment performed for the selected sodium adducts of the copolymers confirmed that the developed methods lead to the formation of graft copolymers composed of poly-γ-glutamic acid (γ-PGA) backbone and oligoesters pendant chains. Moreover, it was established that fragmentation of selected sodium adducts of graft copolymers proceeded via random breakage of amide bonds along the backbone and ester bonds of the oligoesters pendant chains. Considering potential applications of the synthesized copolymers in the area of biomaterials, the hydrolytic degradation under laboratory conditions and in vitro cytotoxicity tests were performed. The ESI-MSⁿ technique applied in this study has been proven to be a useful tool in structural studies of novel graft copolymers as well as their degradation products. Graphical Abstract ᅟ.

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.

Bacteriomimetic poly-γ-glutamic acid surface coating for hemocompatibility and safety of nanomaterials

Poly-γ-glutamic acid (PGA), a major component of the bacterial capsule, is known to confer hydrophilicity to bacterial surfaces and protect bacteria from interactions with blood cells. We tested whether applying a bacteriomimetic surface coating of PGA modulates interactions of nanomaterials with blood cells or affects their safety and photothermal antitumor efficacy. Amphiphilic PGA (APGA), prepared by grafting phenylalanine residues to PGA, was used to anchor PGA to reduced graphene oxide (rGO) nanosheets, a model of hydrophobic nanomaterials. Surface coating of rGO with bacterial capsule-like APGA yielded APGA-tethered rGO nanosheets (ArGO). ArGO nanosheets remained stable in serum over 4 weeks, whereas rGO in plain form precipitated in serum within 5 minutes. Moreover, ArGO did not interact with blood cells, whereas rGO in plain form or as a physical mixture with PGA formed aggregates with blood cells. Mice administered ArGO at a dose of 50 mg/kg showed 100% survival and no hepatic or renal toxicity. No mice survived exposure at the same dose of rGO or a PGA/rGO mixture. Following intravenous administration, ArGO showed a greater distribution to tumors and prolonged tumor retention compared with other nanosheet formulations. Irradiation with near-infrared light completely ablated tumors in mice treated with ArGO. Our results indicate that a bacteriomimetic surface modification of nanomaterials with bacterial capsule-like APGA improves the stability in blood, biocompatibility, tumor distribution, and photothermal antitumor efficacy of rGO. Although APGA was used here to coat the surfaces of rGO, it could be applicable to coat surfaces of other hydrophobic nanomaterials.

Covalent nano delivery systems for selective imaging and treatment of brain tumors

Nanomedicine is a rapidly evolving form of therapy that holds a great promise for superior drug delivery efficiency and therapeutic efficacy than conventional cancer treatment. In this review, we attempt to cover the benefits and the limitations of current nanomedicines with special attention to covalent nano conjugates for imaging and drug delivery in the brain. The improvement in brain tumor treatment remains dismal despite decades of efforts in drug development and patient care. One of the major obstacles in brain cancer treatment is the poor drug delivery efficiency owing to the unique blood-brain barrier (BBB) in the CNS. Although various anti-cancer agents are available to treat tumors outside of the CNS, the majority fails to cross the BBB. In this regard, nanomedicines have increasingly drawn attention due to their multi-functionality and versatility. Nano drugs can penetrate BBB and other biological barriers, and selectively accumulate in tumor cells, while concurrently decreasing systemic toxicity.

A novel approach to improve poly-γ-glutamic acid production by NADPH Regeneration in Bacillus licheniformis WX-02

Poly-γ-glutamic acid (γ-PGA) is an important biochemical product with a variety of applications. This work reports a novel approach to improve γ-PGA through over expression of key enzymes in cofactor NADPH generating process for NADPH pool. Six genes encoding the key enzymes in NADPH generation were over-expressed in the γ-PGA producing strain B. licheniformis WX-02. Among various recombinants, the strain over-expressing zwf gene (coding for glucose-6-phosphate dehydrogenase), WX-zwf, produced the highest γ-PGA concentration (9.13 g/L), 35% improvement compared to the control strain WX-pHY300. However, the growth rates and glucose uptake rates of the mutant WX-zwf were decreased. The transcriptional levels of the genes pgsB and pgsC responsible for γ-PGA biosynthesis were increased by 8.21- and 5.26-fold, respectively. The Zwf activity of the zwf over expression strain increased by 9.28-fold, which led to the improvement of the NADPH generation, and decrease of accumulation of by-products acetoin and 2,3-butanediol. Collectively, these results demonstrated that NADPH generation via over-expression of Zwf is as an effective strategy to improve the γ-PGA production in B. licheniformis.

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.