Statistical Optimization of γ-Polyglutamic Acid Production by Bacillus licheniformis DPC6338

Poly-γ-glutamic acid (γ-PGA) is an important biopolymer produced by various species of Bacillus. Novel γ-PGA producers have shown strain-dependent nutritional and culture requirement that must be characterised and optimised to improve γ-PGA yields. The optimal nutritional and cultural condition for maximum γ-PGA titre in a newly identified γ-PGA producing strain Bacillus licheniformis DPC6338 was determined using one factor at a time (OFAT) and design of experiments (DOE). The optimal nutritional and culture condition for maximum γ–PGA titre in B. licheniformis DPC6338 was 67g/L glutamic acid, 32g/L tryptone, 75g/L glucose, 5g/L citric acid, 2g/L K₂HPO₄, 0.5g/L MgSO₄·7H₂O, 0.02 g/L FeCl₂·4H₂O, 0.1g/L CaCl₂·2H₂O, 0.5 g/L MnSO₄·H₂O, 2g/L ZnSO₄·7H₂O, 42°C, pH 6.5 – 7.0, 1% inoculum, at 250 rpm. Under optimised conditions in shake flask, maximum γ–PGA titre 75.35 ± 0.38 was obtained after 96h while peak productivity of 1.3 g/L/h occurred at 48 h, representing a 27% and 4% improvement in titre and productivity compared to the screening medium. Scale-up to bioreactor conditions significantly enhanced the final titre γ-PGA and early-phase volumetric productivity by ~30% and ~80% respectively. The results obtained in this study highlight the potential of B. licheniformis DPC6338 for industrial γ-PGA producing strain.