| ?-poly-l-lysine(?-PL)is an isomeric amino acid polymer produced by microbial fermentation.Due to its good antibacterial property and high safety,it has been widely used in a variety of ways.In order to improve the fermentation yield of?-PL,more and more research has been conducted on its fermentation process in recent years.The acid pH shock strategy is a control strategy for efficient fermentation production of?-PL established by our laboratory when optimizing the fermentation production of?-PL by Streptomyces albulus M-Z18.However,in practical applications,we often adopt the strategy of dissolved oxygen level regulation through dynamic pH,which dynamically regulates pH according to the dissolved oxygen situation to maintain dissolved oxygen in an appropriate range.However,combined with the dynamic pH regulation dissolved oxygen strategy,the cell viability in the middle and late stage of fermentation could be enhanced to further improve the yield of?-PL.Therefore,studying the reasons for the dynamic pH regulation dissolved oxygen strategy to improve the yield of?-PL has important academic and practical application value.In this paper,the strategy of dissolved oxygen level regulation through dynamic pH was used as the research object.firstly,the reasons why this strategy promoted the synthesis of?-PL were analyzed from the perspective of bacterial oxidative damage.secondly,the sources of oxidative damage were analyzed from the two factors of pH value and?-PL concentration.finally,the total antioxidant capacity of the cells was improved by exogenous addition of antioxidant(glutathione)to achieve the purpose of improving the fermentation performance of S.albulus GS114.The main research results are as follows:(1)In this paper,the reason for improving the ability of S.albulus GS114 to synthesize?-PL was studied from the point of view of bacterial oxidative damage.Comparing the fermentation results of dissolved oxygen level regulation through dynamic pH with that of unregulated fermentation,it was found that the?-PL yield of dissolved oxygen regulation reached 60.2 g·L-1,which was 39%higher than that of unregulated fermentation.According to the detection of indicators related to bacterial oxidative damage,it was found that the intracellular ROS level,oxidative damage degree(malondialdehyde and carbonyl content),catalase(CAT)and total antioxidant capacity(T-AOC)of S.albulus GS114 were improved to varying degrees in the the strategy of dissolved oxygen level regulation through dynamic pH,especially the ROS level and T-AOC increased most significantly.Therefore,the increase of ROS level and T-AOC may be the main reason for the promotion of?-PL synthesis by this strategy.(2)Induction of elevated ROS did not increase the yield of?-PL,so the change in ROS level was not the reason why the the strategy of dissolved oxygen level regulation through dynamic pH improved the fermentation performance of S.albulus GS114.The addition of exogenous hydrogen peroxide to stimulate the increase in intracellular ROS level of S.albulus GS114 during the fermentation process would result in the inhibition of the growth of bacteria cells and the significant reduction of?-PL yield.The reasons for the increase of ROS level were discussed from two aspects of pH value and?-PL concentration.It was found that the intracellular ROS level of S.albulus GS114 was always positively correlated with its growth state,regardless of whether the fermentation under different pH conditions or the addition of different?-PL exogenous to change the?-PL concentration in the fermentation broth.Therefore,it was concluded that intracellular ROS level was related to the respiratory activity of bacteria.The higher the respiratory activity of bacteria,the higher the ROS level was.(3)The addition of glutathione improved the fermentation performance of S.albulus GS114 and confirmed that the increase in total antioxidant capacity was the main reason for the increase in?-PL synthesis under this strategy.With the yield of?-PL as the index,the antioxidant substances including cysteine,thiourea,dimethyl sulfoxide,2-2-bipyridine,salicylic acid,mannitol,resveratrol,lipoic acid,melatonin and glutathione were screened by shaking flask.Shaking flask results showed that 5mmol/L-1 of glutathione was the best choice.The fermentation was amplified in a 5 L tank and the results showed that the total antioxidant capacity of S.albulus GS114 was five times that of the control,and the yield of?-PL increased from 44.14 g·L-1 to 56.62g·L-1,an increase of 28.27%.It could be seen that the improvement of the total antioxidant capacity of Streptomyces albulus was the main reason for the promotion of?-PL synthesis by the the strategy of dissolved oxygen level regulation through dynamic pH. |
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