Efficient production of poly(γ-glutamic acid) by newly isolated Bacillus subtilis NX-2

A bacterium with high poly(γ-glutamic acid) (γ-PGA) productivity was isolated from soil. The bacterial strain NX-2 was identified taxonomically as Bacillus subtilis with the exception that it could not assimilate citric acid. The production of γ-PGA was investigated. Exogenous glutamic acid and additional carbon source was necessary for the production of γ-PGA. The suitable carbon sources were glucose, sucrose, maltose, starch and glycerol. However, γ-PGA could not be produced when citric acid was used as carbon sources, which is different from the other bacteria reported before. The amount of γ-PGA increased upon addition of glutamic acid to the medium. When 70 g/l of l-glutamic acid was added to the medium, the highest γ-PGA concentration of 41 g/l was obtained. But the apparent conversion rate to l-glutamic acid substrate was relatively low, only 58% in this case. The amount of γ-PGA reached 30.2 g/l in the presence of 30 g/l l-glutamic acid after the short incubation time of 24 h, resulting in the high apparent conversion rate of 101% and high productivity of 1.26 g/l/h. The total amount of glutamic acid including that accumulated in γ-PGA and residual glutamic acid in broth was much more than the glutamic acid supplied to the medium. The yield of γ-PGA also exceeded the amount of glutamic acid came from glucose conversion. This showed that the glutamic acid units in γ-PGA not only came from the glutamic acid added to the medium but also from glucose through the de novo pathway of glutamic acid synthesis.