Efficient production of poly-γ-glutamic acid from cane molasses by Bacillus subtilis NX-2 immobilized on chemically modified sugarcane bagasse

BACKGROUND: Poly-γ-glutamic acid (PGA), a naturally occurring homopolymer, has gained increased interest worldwide because of its extensive applications. In this study, sugarcane bagasse chemically modified with poly-ε-_L-lysine (PL) was used in an aerobic plant fibrous-bed bioreactor (APFB) to enhance PGA fermentation by Bacillus subtilis NX-2. RESULTS: 5gL^-1 of PL was optimized to modify native sugarcane bagasse, and 45.7±0.6gL^-1 PGA was obtained with PL-bagasse immobilized cells in batch fermentation, with a productivity of 1.08±0.02gL^-1 h^-1. These results were 1.23 and 1.4 times higher than those with unmodified sugarcane bagasse. The effect of dissolved oxygen (DO) on PGA fermentation was further investigated, and suitable conditions were 45% DO with the PGA concentration 55.4±0.4gL^-1. In 10 batches of repeated-batch fermentation, PGA production significantly increased and the average PGA yield was maintained at 90.6±0.5gL^-1 in the three later batches and was increased by 8.7% compared with that in the first batch. CONCLUSION: The improved APFB system was proved to be a green, economic, and effective bioprocess for PGA fermentation, and this system could also provide a suitable microenvironment for B. subtilis NX-2 to induce its evolution.