ε-poly-L-lysine (ε-PL) is a homopoly(amino acids) produced by Streptomyces albulus, which is currently approved as an excellent natural food preservative in many countries due to its broad antibacterial activity. To improve the ε-PL production by S. albulus GS114, twelve commonly used organic nitrogen types and their concentrations were compared and systematically optimized in shake flask, batch and fed-batch fermentation, respectively. The yeast extract FM760, with the addition concentration of 9.27 g/L, was considered as the optimal organic nitrogen for ε-PL production. Subsequently, the amino acid composition of FM760 was analyzed and compared with the control to explain the reason for the enhanced ε-PL production. The result showed that in shake flask fermentation, the usage of fast-acting organic nitrogen source was more favorable for the biosynthesis of ε-PL. Considering together the ε-PL production and ε-PL biosynthesis capacity per unit biomass, the organic nitrogen sources FP103, FM760 and FM408 were all better than the control FM902. The highest ε-PL production reached (2.60±0.02) g/L by FM760, which was 22.07% higher than that of the control organic nitrogen. In a pH-controlled batch fermentation, the ε-PL production, dry weight, yield, glucose conversion rate and ε-PL synthesis capacity per unit biomass were compared in a pH-controlled batch fermentation, and FM760 is also turned out to be the best organic nitrogen source for ε-PL production in S. albulus GS114 fermentation. The ε-PL production reaching (6.41±0.23) g/L, 42.64% higher than that of the control organic nitrogen. In fed-batch fermentation, we optimized the addition concentration of FM760, and determined the optimal addition concentration for ε-PL synthesis was 9.27 g/L. Under this condition, the ε-PL production and glucose conversion rate reached 62.38 g/L and 9.76% respectively, which was 18.80% and 25.77% higher than those of the control. The above results demonstrated that choosing 9.27 g/L FM760 as the organic nitrogen source not only obtained more than 60 g/L of ε-PL production, but also significantly improved the substrate conversion rate and achieved the unification of fermentation production and fermentation economy. Furthermore, the possible key amino acids for improving ε-PL fermentation efficiency were analyzed by comparing the difference of amino acid components between FM760 and control FM902. The amino acid addition-validated experiment showed that the improved ε-PL production by yeast extract FM760 might be related to the isoleucine, leucine and alanine. From the changes of free amino acids concentrations in the fermentation broth during the fermentation of different concentrations of FM760, the isoleucine concentration was almost zero during the fermentation process and did not have an increasing trend, which mean that isoleucine was the possible factor limiting the cell growth and ε-PL synthesis. The content of alanine and leucine had an increasing trend in the middle phase and dropped to a low level in the late phase of the fermentation. As is known, the addition of alanine may lead to the oxidative damage in bacteria, while the addition of leucine may result in the metabolic feedback inhibition, which together suggested that lower levels of leucine and alanine may be more favorable for ε-PL synthesis. The present study is of great significance for the selection of organic nitrogen source in the industrial production of ε-PL.
LIU Yuxiang
,
LIU Tianyi
,
DENG Yue
,
WANG Liang
,
ZHANG Hongjian
,
ZHANG Jianhua
,
CHEN Xusheng
. Improvement of ε-poly-L-lysine yield through organic nitrogen optimization by Streptomyces albulus[J]. Food and Fermentation Industries, 2022
, 48(13)
: 1
-8
.
DOI: 10.13995/j.cnki.11-1802/ts.030845
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