Effects Of Vc Regulated Oxidative Stress On ε-PL Fermentation

ε-poly-L-lysine（ε-PL）,as a food-grade preservative,is produced by the liquid fermentation of Streptomyces sp.Enhancingε-PL synthesis capability through various breeding methods has become an important way to increase product level;however,few studies have focused on how to eliminate unfavorable factors in the production process and create a favorable environment forε-PL synthesis.In this study,the process parameters were monitored duringε-PL fermentation by Streptomyces sp.AF3-44,and the fermentation process was optimized through single factor experiment.The relationship among oxidative damage,ε-PL and bacterial viability was discussed from the perspective of physiological and biochemical characteristics of AF3-44.By adding suitable exogenous substances,the viability of AF3-44 was improved and the yield ofε-PL was increased.The main contents and results are as follows:（1）A single factor optimization experiment was conducted in a 5 L fermentor to determine that the optimal dissolved oxygen（DO）of fermentation for strain AF3-44 was 20%with an optimal pH of 4.0.Under this optimal condition,the maximum titer ofε-PL in batch fermentation reached 5.45±0.11 g·L^-1,and the dry cell weight（DCW）was 14.54 g·L^-1.Compared with other conditions,theε-PL yield,theε-PL synthetic capacity per gram of cell,and the substrate conversion rate were all the highest.Experiment results showed that high pH was beneficial to the growth of AF3-44 but not toε-PL synthesis,while the low pH severely affected the growth and metabolism of AF3-44,and the titer ofε-PL and DCW decreased simultaneously.（2）Through cell viability staining and ROS staining of AF3-44,it was found that ROS gradually accumulated with the prolongation of fermentation time,and the viability of the bacteria continued to decrease.Exogenous addition of 10 mmol·L^-1 hydrogen peroxide did not reduce the production ofε-PL.Instead,it increased the activity of antioxidative enzymes,especially catalase.The addition of catalase prosthetic precursors also successfully increased the catalase activity and enhanced the resistance of the AF3-44 to oxidative stress.（3）The effects of adding vitamin C（Vc）on the fermentation ofε-PL were studied.The results showed that the addition of Vc improved the total antioxidant capacity of the whole fermentation process,increased the unsaturation of cell membrane fatty acids and alleviated the oxidative damage of cells meanwhile enhanced cell viability to ensure normal cell survival and function.The yield ofε-PL was 7.73 g·L^-1,which was 50.34%higher than the control(5.14 g·L^-1)at the end of batch fermentation in the 5 L fermentor.Theε-PL production was 34.17±0.97 g·L^-1,an increase of 24.75%compared to the control after the fed-batch fermentation for 7 days.