Streptomyces albulus is an industrial strain for ε-poly-L-lysine (ε-PL) production. ε-PL is a homopoly (amino acids) produced by S. albulus,which is currently used as an excellent natural food preservative in many countries due to its eminent antibacterial activity. It often suffers low-pH stress during fermentation, which easily cause a decrease in the capacity of ε-PL biosynthesis. As a result, the adaptive evolution strategy with increasing the selection pressure step by step was introduced to improve the low-pH tolerance of S. albulus GS114 in this study. We first determine the initial transfer pH (pH 4.0) and transfer time (36 h) in adaptive evolution strategy. After 93 days of adaptive evolution, the tolerance pH value of S. albulus GS114 is reduced from 4.0 to 3.6. After a lot of strain screening in each selection pH (i.e. pH 4.0, 3.8, 3.6) in acid tolerance and ε-PL yield, we obtained S. albulus ALE4.0, S. albulus ALE3.8 and S. albulus ALE3.6. We chose spore plate coating and ε-PL shake flask fermentation to evaluate the characteristics of the strains after adaptive evolution. Combined with low pH tolerance and shaker fermentation experiments, it was determined that the optimal adaptive evolutionary strain is S. albulus ALE3.6. Further study investigated the influence of pH on the production of ε-PL by S. albulus ALE3.6. Results showed that S. albulus ALE3.6 had significant fermentation advantages under different pH values compared with parent strain S. albulus GS114. In terms of ε-PL production, dry cell weight, ε-PL yield, and ε-PL productivity, S. albulus ALE3.6 had different degrees of advantages. The ε-PL production of S. albulus ALE3.6 in batch fermentation at pH 4.0, 3.8, and 3.6 were reached 4.63, 6.14, and 7.2 g/L, respectively, which were enhanced by 13.5%, 8.9% and decreased by 7.7%, compared with the original strain S. albulus GS114. Meanwhile, the dry cell weight of S. albulus ALE3.6 was achieved by 16.8, 11.8, and 14.7 g/L, respectively, which were decreased to varying degrees compared to the original strain. After comparing the fermentation processes of different pH, we chose pH 4.0 for follow-up study from the point of view of the average specific synthesis rate of the product. Finally, the ε-PL production of S. albulus ALE3.6 in fed-batch fermentation at pH 4.0 were investigated. In a 5 L scale fermentor, ε-PL production of S. albulus ALE3.6 was reached by 43.7 g/L in 192 h, which is 63% higher than the original strain S. albulus GS114. The dry cell weight was attained by 39.8 g/L, which was 30% lower than the original strain. S. albulus ALE3.6 had higher ability of cell synthesis per unit and average specific synthesis rate of products in this fermentation process than the original strain. The acquisition of the S. albulus ALE3.6 provided an object for the analysis of the mechanism of acid tolerance. Furthermore, this result showed that the low-pH tolerance of S. albulus can be significantly improved through the adaptive evolution method, and enhancing the low-pH tolerance of S. albulus is an effective way to improve its ε-PL production.
LIU Tianyi
,
ZHANG Yue
,
WANG Liang
,
ZHANG Hongjian
,
ZHANG Jianhua
,
CHEN Xusheng
. Improvement of ε-poly-L-lysine production by Streptomyces albulus based on low-pH adaptive evolution strategy[J]. Food and Fermentation Industries, 2024
, 50(1)
: 14
-21
.
DOI: 10.13995/j.cnki.11-1802/ts.035126
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