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

Poly-gamma-glutamic acid (gamma-PGA) is a natural biopolymer of glutamic acid. The repeating units of gamma-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 alpha-amino group and the gamma-carboxylic acid group. gamma-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 gamma-PGA via genetically and metabolically engineered recombinant bacteria. Strategies for improving production of gamma-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 gamma-PGA in industrial bacterial hosts that do not naturally produce gamma-PGA is discussed. Emerging trends and the challenges affecting the production of gamma-PGA are reviewed.