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

Bacillus subtilis NX-2 produces γ-polyglutamic acid (γ-PGA) when using glucose and l-glutamate as carbon sources. The conversion of carbon sources into γ-PGA was analyzed with the ¹³C-NMR method after enriching the media with ¹³C-labeled glucose. The results showed that the percentage of γ-PGA monomers derived from glucose was relatively low, approximately 6% and 9%, respectively, with an initial glucose concentration of 30 and 40 g L⁻¹. It was concluded that glucose was utilized mainly as the growth-limiting substrate for cell growth and supplied the required energy during γ-PGA biosynthesis, while l-glutamate was preferred as the main substrate for γ-PGA formation. To achieve an efficient conversion of l-glutamate and enhance the γ-PGA production, a fed-batch culture was proposed by feeding of glucose. By this method, supplied l-glutamate (40 g L⁻¹) was completely depleted, and γ-PGA yield was attained 42 g L⁻¹.