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

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TL;DR

This review summarizes the microbial production, biosynthesis, and diverse applications of poly-γ-glutamic acid (γ-PGA), a biodegradable and non-toxic biopolymer. Key findings include that γ-PGA yields up to 101.1 g/L have been achieved using optimized fermentation with Bacillus subtilis, and that the polymer shows promise in drug delivery, tissue engineering, cosmetics, food preservation, and agriculture. The clinical significance lies in its potential as a safe, biocompatible material for pharmaceutical and medical uses, though high production costs remain a barrier to commercialization.

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**Background:** Microbial biopolymers are gaining attention as eco-friendly alternatives to synthetic polymers. Poly-γ-glutamic acid (γ-PGA) is a natural anionic homopolyamide composed of D- and L-glutamic acid units linked via amide bonds between α-amino and γ-carboxylic acid groups. It is water-soluble, biodegradable, edible, non-immunogenic, and harmless to humans. γ-PGA is produced primarily by Bacillus species, including B. subtilis and B. licheniformis. Its unique properties enable applications in healthcare, pharmaceuticals, cosmetics, food, agriculture, and water treatment. However, high production costs and low yields limit its commercialization. This review covers γ-PGA-producing microorganisms, biosynthesis mechanisms, production optimization, downstream processing, characterization, and applications.

**Methods:** This is a narrative review that synthesizes findings from multiple studies on γ-PGA production and applications. The authors describe various fermentation strategies, including solid-state and submerged fermentation, use of different carbon and nitrogen sources, metabolic precursors, and genetic engineering approaches. They also discuss downstream processing methods such as solvent precipitation, metal ion precipitation, and ultrafiltration. Characterization techniques include gel permeation chromatography (GPC), Fourier-transform infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and amino acid analysis. The review compiles data from published studies on γ-PGA yields under different conditions.

**Key Results:** The highest reported γ-PGA yield is 101.1 g/L using B. subtilis ZJU-7 in a fed-batch bioreactor with glucose, L-glutamate, and yeast extract (Huang et al., 2011). Other notable yields include 73.0 g/L with B. subtilis NX-2 using rice straw hydrolysate, 68.7 g/L with B. subtilis MJ80, and 60.00 g/L with B. subtilis CCTCC202048 using swine manure. For B. licheniformis, yields of 98.64 mg/g dry solids (solid-state) and 35.75 g/L (submerged) were achieved with metabolic precursors (Bajaj and Singhal, 2009). Genetic manipulation, such as double deletion of ggt and pgdS genes in B. subtilis 168, increased yield to 40 g/L (Scoffone et al., 2013). The molecular weight of γ-PGA ranges from 10^5 to 10^6 Da, and can be reduced by enzymatic or ultrasonic degradation. γ-PGA exhibits antimicrobial activity against gram-positive and gram-negative bacteria, with MIC <2.5 mg/mL for E. coli and B. subtilis when molar mass >700 kDa (Yu et al., 2023). In agriculture, γ-PGA at ≥0.2 g/kg soil reduces nitrogen leaching and enhances nutrient uptake. In food, 1% γ-PGA reduces oil uptake in doughnuts from 0.7 to 0.2 g/g dough. In cosmetics, γ-PGA acts as a super moisturizer, absorbing 5000 times its weight in water.

**Clinical Implications:** γ-PGA has significant potential in medicine as a drug carrier, anticancer agent, tissue engineering scaffold, and wound dressing. For example, PGA-paclitaxel conjugate (paclitaxel poliglumex) improves water solubility, stability, and tumor selectivity compared to standard paclitaxel (Singer, 2005). γ-PGA induces NK-cell-mediated antitumor immunity in mice (Kim et al., 2007). Chitosan/γ-PGA polyelectrolyte complexes promote wound healing and reduce inflammation (Tsao et al., 2011). γ-PGA-Phe nanoparticles effectively deliver dexamethasone to retinal microglia and macrophages, offering a safer alternative to steroids for retinal diseases (Ryu et al., 2011). γ-PGA also enhances calcium absorption in postmenopausal women and reduces postprandial blood glucose in mice (Tanimoto et al., 2007; Tamura et al., 2020). Despite these benefits, clinical translation requires further evaluation of biodistribution, toxicity, and pharmacokinetics. The main barrier to widespread use is the high production cost, which may be reduced by using low-cost substrates (e.g., agricultural wastes) and genetically engineered strains.