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ε-poly-L-lysine (ε-PL) is a naturally occurring poly(amino acid) of varying polymerization degree, which possesses excellent antimicrobial activity and has been widely used in food and pharmaceutical industries. To provide new perspectives from recent advances, this review compares several conventional and advanced strategies for the discovery of wild strains and development of high-producing strains, including isolation and culture-based traditional methods as well as genome mining and directed evolution. We also summarize process engineering approaches for improving production, including optimization of environmental conditions and utilization of industrial waste. Then, efficient downstream purification methods are described, including their drawbacks, followed by the brief introductions of proposed antimicrobial mechanisms of ε-PL and its recent applications. Finally, we discuss persistent challenges and future perspectives for the commercialization of ε-PL.

ε-Poly-L-lysine (ε-PL) is produced by Streptomyces as a secondary metabolite with wide industrial applications, but its production still needs to be further enhanced. Environmental stress is an important approach for the promotion of secondary metabolites production by Streptomyces. In this study, the effect of acidic pH shock on enhancing ε-PL production by Streptomyces sp. M-Z18 was investigated in a 5-L fermenter. Based on the evaluation of acidic pH shock on mycelia metabolic activity and shock parameters optimization, an integrated pH-shock strategy was developed as follows: pre-acid-shock adaption at pH 5.0 to alleviate the damage caused by the followed pH shock, and then acidic pH shock at 3.0 for 12 h (including pH decline from 4.0 to 3.0) to positively regulate mycelia metabolic activity, finally restoring pH to 4.0 to provide optimal condition for ε-PL production. After 192 h of fed-batch fermentation, the maximum ε-PL production and productivity reached 54.70 g/L and 6.84 g/L/day, respectively, which were 52.50 % higher than those of control without pH shock. These results demonstrated that acidic pH shock is an efficient approach for improving ε-PL production. The information obtained should be useful for ε-PL production by other Streptomyces.

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Small pellets with loose interior were obtained with talc. Consequently, oxygen and nutrition supply was enhanced, which promoted mycelia activity.

The productivity of ε-poly-L: -lysine (ε-PL) in currently reported wild-type strains is low. Here we improved glucose tolerance of a Streptomyces graminearus strain LS-B1 by genome shuffling while simultaneously enhancing the ε-PL productivity. The starting population was generated by ultraviolet irradiation and nitrosoguanidine mutagenesis and then subjected for recursive protoplast fusion. The positive colonies from library, created by fusing the inactivated protoplasts were screened on agar plates containing different concentrations of glucose. Characterization of all recombinants and wild-type strain in shake-flask fermentation indicated the compatibility of two phenotypes of glucose tolerance and ε-PL yield enhancement. The best performing recombinant, F3-4, was isolated after three rounds of genome shuffling, whose ε-PL production was about 88% higher than that of the parent strain. In batch fermentation test, the ε-PL concentration was obtained as 2.4 g/L by F3-4 compared with 1.6 g/L of wild type. Fed-batch fermentation by F3-4 was carried out and the ε-PL production accumulated to 13.5 g/L when initial glucose concentration was improved from 50 to 85 g/L. Enzyme activities of hexokinase, pyruvate kinase, and citrate synthase revealed that the glycolytic pathway and tricarboxylic acid circle way in F3-4 were more active than those in wild type, which was a possible reason for enhanced ε-PL production.

The production of ε-poly-L: -lysine (ε-PL) by Streptomyces sp. M-Z18 from glycerol was investigated in a 5-L jar-fermenter. Batch fermentations by Streptomyces sp. M-Z18 at various pH values ranging from 3.5 to 4.5 were studied. Based on the analysis of the time course of specific cell growth rate and specific ε-PL formation rate, a novel two-stage pH control strategy was developed to improve ε-PL production by shifting the culture pH from 3.5 to 3.8 after 36 h of cultivation. By applying the strategy, the maximal ε-PL concentration and productivity had a significant improvement and reached 9.13 g L(-1) and 4.76 g L(-1) day(-1), respectively, compared with those in one-stage pH control process where the pH value is controlled at 3.5 (7.83 g L(-1) and 3.13 g L(-1) day(-1)). Fed-batch fermentation with two-stage pH control strategy was also applied to produce ε-PL; final ε-PL concentration of 30.11 g L(-1) was obtained, being 3.3-fold greater than that of batch fermentation. To our knowledge, it is the first report on production of ε-PL from glycerol in fermenter scale and achievement of high ε-PL production with two-stage pH control strategy.

ε-Poly-L-lysine (ε-PL), as a food additive, has been widely used in many countries. However, its production still needs to be improved. We successfully enhanced ε-PL production of Streptomyces albulus FEEL-1 by introducing mutations related to antibiotics, such as streptomycin, gentamicin, and rifampin. Single- and double-resistant mutants (S-88 and SG-31) were finally screened with the improved ε-PL productions of 2.81 and 3.83 g/L, 1.75- to 2.39-fold compared with that of initial strain FEEL-1. Then, the performances of mutants S-88 and SG-31 were compared with the parent strain FEEL-1 in a 5-L bioreactor under the optimal condition for ε-PL production. After 174-h fed-batch fermentation, the ε-PL production and productivity of hyper-strain SG-31 reached the maximum of 59.50 g/L and 8.21 g/L/day, respectively, which was 138 and 105% higher than that of FEEL-1. Analysis of streptomycin-resistant mutants demonstrated that a point mutation occurred in rpsL gene (encoding the ribosomal protein S12). These single and double mutants displayed remarkable increases of the activities and transcriptional levels of key enzymes in ε-PL biosynthesis pathway, which may be responsible for the enhanced mycelia viability, respiratory activity, and ε-PL productions of SG-31. These results showed that the new breeding method, called ribosome engineering, could be a novel and effective breeding strategy for the evolution of ε-PL-producing strains.