Effects of cell physiological structure on the fermentation broth viscosity during poly-γ-glutamic acid production by Bacillus subtilis GXA-28

The high viscosity of fermentation broth limited the further improvement of PGA titer. Our previous studies indicated that adding KCl to the medium could decrease the fermentation broth viscosity and improve the PGA titer. In order to clarify the reason, effects of cell physiological structure on the fermentation broth viscosity were investigated. Results from cell morphology observation showed that the reduction of cell aggregation caused by the weakened cross-linking between PGA and cells might be an important reason for the decrease in the fermentation broth viscosity. Besides, when 201.2 mM KCl was added to the medium, the zeta potential of cell surface decreased from − 70.48 ± 3.35 mV to − 81 ± 2.46 mV. The cell membrane integrity was reduced and permeability was enhanced. Furthermore, the percentage of lauric acid C12:0 in cell membrane increased by 12.36%, but palmitic acid C16:0 and stearic acid C18:0 decreased by 6.83% and 5.64%, respectively, which improved the fluidity of cell membrane. The above changes in cell membrane further affect the cross-linking between PGA and cells, thereby playing an important role in reducing the fermentation broth viscosity. This study provided some novel information for understanding the decrease of PGA fermentation broth viscosity by KCl.