Genome Shuffling and Gentamicin-Resistance to Improve ε-Poly-l-Lysine Productivity of Streptomyces albulus W-156

Combinatorial strain improvement and bioprocess development for efficient production of ε-poly-L-lysine in Streptomyces albulus

ε-Poly-L-lysine (ε-PL) is an amino acid homopolymer with diverse potential applications in the food, pharmaceutical and cosmetic industries. To improve its biomanufacturing efficiency, strain engineering and bioprocess optimization were combined in this study. Firstly, a cocktail strain breeding strategy was employed to generate a ε-PL high-production mutant, Streptomyces albulus GS114, with enhanced L-lysine uptake capability. Subsequently, the L-lysine feeding conditions during fed-batch fermentation were systematically optimized to improve the L-lysine supply, resulting in ε-PL production reaching 73.1 ± 1.4 g/L in 5 L bioreactor. Finally, an engineered strain, S. albulus L2, with enhanced uptake capability and polymerization ability of L-lysine was constructed, achieving ε-PL production of 81.4 ± 5.2 g/L by fed-batch fermentation. This represents the highest reported production of ε-PL to date. This study provided an efficient production strategy for ε-PL and valuable insights into the high-value utilization of L-lysine.

Multiple Functions of Compatible Solute Ectoine and Strategies for Constructing Overproducers for Biobased Production

Ectoine and its derivative 5-hydroxyectoine are compatible solutes initially found in the hyperhalophilic bacterium Ectothiorhodospira halochloris, which inhabits the desert in Egypt. The habitat of ectoine producers implies the primary function of ectoine as a cytoprotectant against harsh conditions such as high salinity, drought, and high radiation. More extensive and in-depth studies have revealed the multiple functions of ectoine in its native producer bacterial cells and other types of cells and its biomolecular components (such as proteins and DNA) as a general protective agent. Its chemical properties as a bio-based amino acid derivative make it attractive for basic scientific research and related industries, such as the food/agricultural industry, cosmetic manufacturing, biologics, and therapeutic agent preparation. This article first discusses the functions and applications of ectoine and 5-hydroxyectoine. Subsequently, more emphasis was placed on advances in bio-based ectoine and/or 5-hydroxyectoine production. Strategies for developing more robust cell factories for highly efficient ectoine and/or 5-hydroxyectoine production are further discussed. We hope this review will provide a valuable reference for studies on the bio-based production of ectoine and 5-hydroxyectoine.

Enhancement of poly‑γ‑L‑diaminobutanoic acid production in Bacillus pumilus by repeated pH shocks

This study investigated the effect of pH on poly-γ-L-diaminobutanoic acid (γ-PAB) production by Bacillus pumilus in batch fermentation. In the natural fermentation where pH was not controlled, pH decreased from initial 7.0 to 3.0 in 18 h and γ-PAB production was 428.6 mg/L. In the pH-controlled fermentation, B. pumilus tended to proliferation at higher pH, while γ-PAB synthesis was favorable at lower pH, in which the optimal pH for γ-PAB production was 4.2, and γ-PAB yield reached 2284.5 mg/L. Adopting a pH shock strategy which lasted 9 h in the pre-fermentation phase, biomass (OD600) and γ-PAB yield of B. pumilus were obtained as 61.3 and 2794.6 mg/L, respectively, which were 10.8% and 22.4% higher than those in batch fermentation without pH shock. Subsequent fermentation of repeated pH shocks showed that a further higher productivity could be achieved, in which the final OD600 reached 65.1, and γ-PAB production reached as high as 3482.3 mg/L, which were increased by 6.2% and 17.1% compared with those in single pH shock, respectively. This study demonstrated that B. pumilus can synthesize more γ-PAB at suboptimal pH and provided a novel approach to regulate γ-PAB synthesis.

Enhanced ε‑poly‑L‑lysine production in Streptomyces species by combining interspecific hybridization with multiple antibiotic resistance

To improve the ε-PL production in wild-type strains of Streptomyces. albulus, Streptomyces. noursei, Streptomyces. rochei and Streptomyces. yunnanensis, the interspecific hybridization based on protoplast fusion was first performed. Two-species hybridizations failed to obtain hybrids with significant increase in ε-PL production, but four-species hybridizations succeed in acquiring many high-yield hybrids. 16S rDNA homology alignment and RAPD confirmed that the hybrid HX17 was restructured by integrating gene fragments from S. albulus and S. rochei with S. noursei as the carrier. S. noursei HX17 was subsequently suffered from mutagenesis and genome shuffling combining with multiple antibiotic resistance, and a mutant S. noursei GX6 was obtained with ε-PL yield of 2.23 g/L in shake-flask fermentation. In fed-batch fermentation, the ε-PL production of GX6 reached 47.2 g/L, which was increased by 95.6% to 136.8% over the wild parents. Ribosomal genes associated with antibiotics were sequenced and majority of mutant strains had mutations at different sites, indicating that the increase of antibiotic resistance was strongly associated with them. This research proved that combining interspecific hybridization with multiple antibiotic resistance was as an effective approach to rapidly improve the ε-PL production in Streptomyces species.

Design and optimization of ε-poly-l-lysine with specific functions for diverse applications

ε-Poly-l-lysine (ε-PL) is a natural homo-poly(amino acid) which can be produced by microorganisms. With the advantages in broad-spectrum antimicrobial activity, biodegradability, and biocompatibility, ε-PL has been widely used as a preservative in the food industry. Different molecular architectures endow ε-PL and ε-PL-based materials with versatile applications. However, the microbial synthesis of ε-PL is currently limited by low efficiencies in genetic engineering and molecular architecture modification. This review presents recent advances in ε-PL production and molecular architecture modification of microbial ε-PL, with a focus on the current challenges and solutions for the improvement of the productivity and diversity of ε-PL. In addition, we highlight recent examples where ε-PL has been applied to expand the versability of edible films and nanoparticles in various applications. Commercial production and the challenges and future research directions in ε-PL biosynthesis are also discussed. Currently, although the main use of ε-PL is as a food preservative, ε-PL and ε-PL-based polymers have shown excellent application potential in biomedical fields. With the development of synthetic biology, the design and synthesis of ε-PL with a customized molecular architecture are possible in the near future. ε-PL-based polymers with specific functions will be a new trend in biopolymer manufacturing.

Antibacterial activity of epsilon-poly-l-lysine produced by Stenotrophomonas maltophilia HS4 and Paenibacillus polymyxa HS5, alone and in combination with bacteriophages

Over the past decades, antibiotic resistance has become a major clinical problem, and searching for new therapeutic strategies seems to be necessary. Using novel natural compounds, antimicrobial peptides, and bacteriophages is the most promising solution. In this study, various cationic metabolite-producer bacteria were isolated from different soil samples. Two isolates were identified as Stenotrophomonas maltophilia HS4 (accession number: MW791428) and Paenibacillus polymyxa HS5 (accession number: MW791430) based on biochemical characteristics and phylogenetic analysis using 16S rRNA gene sequences. The cationic compound in the fermentation broth was precipitated and purified with sodium tetraphenylborate salt. The purified cationic peptide was confirmed to be epsilon-poly-l-lysine by structural and molecular analysis using High-Performance Liquid Chromatography, Sodium dodecyl-sulfate-polyacrylamide gel electrophoresis, and Fourier-transform infrared spectroscopy. The antibacterial activity of epsilon-poly-l-lysine was evaluated against Staphylococcus aureus ATCC 25923, Escherichia coli ATCC 25922, Enterococcus faecalis ATCC 29212, Serratia marcescens ATCC 13880, and Klebsiella pneumoniae ATCC 13883 by microdilution method. Furthermore, the antibacterial effects of purified epsilon-poly-l-lysine in combination with two long non-contractile tail bacteriophages against vancomycin-resistant Enterococcus faecalis and colistin-resistant Klebsiella pneumoniae were investigated. The results indicated great antibacterial activity of epsilon-poly-l-lysine which was produced by two novel bacteria. The epsilon-poly-l-lysine as a potent cationic antimicrobial peptide is demonstrated to possess great antimicrobial activity against pathogenic and also antibiotic-resistant bacteria.

Engineering Streptomyces albulus to enhance ε-poly-L-lysine production by introducing a polyphosphate kinase-mediated ATP regeneration system

BACKGROUND

ε-Poly-L-lysine (ε-PL) is a natural and safe food preservative that is mainly produced by filamentous and aerobic bacteria Streptomyces albulus. During ε-PL biosynthesis, a large amount of ATP is used for the polymerization of L-lysine. A shortage of intracellular ATP is one of the major factors limiting the increase in ε-PL production. In previous studies, researchers have mainly tried to increase the oxygen supply to enhance intracellular ATP levels to improve ε-PL production, which can be achieved through the use of two-stage dissolved oxygen control, oxygen carriers, heterologous expression of hemoglobin, and supplementation with auxiliary energy substrates. However, the enhancement of the intracellular ATP supply by constructing an ATP regeneration system has not yet been considered.

RESULTS

In this study, a polyphosphate kinase (PPK)-mediated ATP regeneration system was developed and introduced into S. albulus to successfully improve ε-PL production. First, polyP:AMP phosphotransferase (PAP) from Acinetobacter johnsonii was selected for catalyzing the conversion of AMP into ADP through an in vivo test. Moreover, three PPKs from different microbes were compared by in vitro and in vivo studies with respect to catalytic activity and polyphosphate (polyP) preference, and PPK2B^cg from Corynebacterium glutamicum was used for catalyzing the conversion of ADP into ATP. As a result, a recombinant strain PL05 carrying coexpressed pap and ppk2B^cg for catalyzing the conversion of AMP into ATP was constructed. ε-PL production of 2.34 g/L was achieved in shake-flask fermentation, which was an increase of 21.24% compared with S. albulus WG608; intracellular ATP was also increased by 71.56%. In addition, we attempted to develop a dynamic ATP regulation route, but the result was not as expected. Finally, the conditions of polyP₆ addition were optimized in batch and fed-batch fermentations, and the maximum ε-PL production of strain PL05 in a 5-L fermenter was 59.25 g/L by fed-batch fermentation, which is the highest ε-PL production reported in genetically engineered strains.

CONCLUSIONS

In this study, we proposed and developed a PPK-mediated ATP regeneration system in S. albulus for the first time and significantly enhanced ε-PL production. The study provides an efficient approach to improve the production of not only ε-PL but also other ATP-driven metabolites.

Improved Production of ε-Poly-L-Lysine in Streptomyces albulus Using Genome Shuffling and Its High-Yield Mechanism Analysis

ε-Poly-L-lysine (ε-PL), a natural food preservative, has recently gained interest and mainly produced by Streptomyces albulus. Lacking of efficient breeding methods limit ε-PL production improving, knockout byproducts and increase of main product flux strategies as a logical solution to increase yield. However, removing byproduct formation and improving main product synthesis has seen limited success due to the genetic background of ε-PL producing organism is not clear. To overcome this limitation, random mutagenesis continues to be the best way towards improving strains for ε-PL production. Recent advances in Illumina sequencing opened new avenues to understand improved strains. In this work, we used genome shuffling on strains obtained by ribosome engineering to generate a better ε-PL producing strain. The mutant strain SG-86 produced 144.7% more ε-PL than the parent strain M-Z18. Except that SG-86 displayed obvious differences in morphology and ATP compared to parent strain M-Z18. Using Illumina sequencing, we mapped the genomic changes leading to the improved phenotype. Sequencing two strains showed that the genome of the mutant strain was about 2.1 M less than that of the parent strain, including a large number of metabolic pathways, secondary metabolic gene clusters, and gene deletions. In addition, there are many SNPs (single nucleotide polymorphisms) and InDels (insertions and deletions) in the mutant strain. Based on the results of data analysis, a mechanism of ε-PL overproduction in S. albulus SG-86 was preliminarily proposed. This study is of great significance for improving the fermentation performance and providing theoretical guidance for the metabolic engineering construction of ε-PL producing strains.

Recent advances in microbial ε-poly-L-lysine fermentation and its diverse applications

The naturally occurring homo-polyamide biopolymer, ε-poly-L-lysine (ε-PL) consists of 25-35 L-lysine residues with amide linkages between α-carboxyl groups and ε-amino groups. ɛ-PL exhibits several useful properties because of its unusual structure, such as biodegradability, water solubility, no human toxicity, and broad-spectrum antibacterial activities; it is widely applied in the fields of food, medicine, clinical chemistry and electronics. However, current industrial production of ε-PL is only performed in a few countries. Based on an analysis of the physiological characteristics of ε-PL fermentation, current advances that enhance ε-PL fermentation, from strain improvement to product isolation are systematically reviewed, focusing on: (1) elucidating the metabolic pathway and regulatory mechanism of ε-PL synthesis; (2) enhancing biosynthetic performance through mutagenesis, fermentation optimization and metabolic engineering; and (3) understanding and improving the biological activity and functional properties of ε-PL. Finally, perspectives on engineering and exploiting ε-PL as a source material for the production of various advanced materials are also discussed, providing scientific guidelines for researchers to further improve the ε-PL fermentation process.

Biotechnological production and application of epsilon-poly-L-lysine (ε-PL): biosynthesis and its metabolic regulation

Epsilon-poly-L-lysine (ε-PL) is an unusual biopolymer composed of L-lysine produced by several microorganisms, especially by the genus Streptomyces. Due to its excellent antimicrobial activity, good water solubility, high safety, and biodegradable nature, ε-PL with a GRAS status has been widely used in food and pharmaceutical industries. In the past years, studies have focused on the biotechnological production of ɛ-PL, the biosynthetic mechanism of microbial ɛ-PL, and its application. To provide new perspectives from recent advances, the review introduced the methods for the isolation of ɛ-PL producing strains and the biosynthetic mechanism of microbial ɛ-PL. We summarized the strategies for the improvement of ɛ-PL producing strains, including physical and chemical mutagenesis, ribosome engineering and gene engineering, and compared the different metabolic regulation strategies for improving ɛ-PL production, including medium optimization, nutrient supply, pH control, and dissolved oxygen control. Then, the downstream purification methods of ɛ-PL and its recent applications in food and medicine industries were introduced. Finally, we also proposed the potential challenges and the perspectives for the production of ε-PL.

Strategic Development of Aurantiochytrium sp. Mutants With Superior Oxidative Stress Tolerance and Glucose-6-Phosphate Dehydrogenase Activity for Enhanced DHA Production Through Plasma Mutagenesis Coupled With Chemical Screening

Thraustochytrids, such as Aurantiochytrium and Schizochytrium, have been shown as a promising sustainable alternative to fish oil due to its ability to accumulate a high level of docosahexaenoic acid (DHA) from its total fatty acids. However, the low DHA volumetric yield by most of the wild type (WT) strain of thraustochytrids which probably be caused by the low oxidative stress tolerance as well as a limited supply of key precursors for DHA biosynthesis has restricted its application for industrial application. Thus, to enhance the DHA production, we aimed to generate Aurantiochytrium SW1 mutant with high tolerance toward oxidative stress and high glucose-6 phosphate dehydrogenase (G6PDH) activities through strategic plasma mutagenesis coupled with chemical screening. The WT strain (Aurantiochytrium sp. SW1) was initially exposed to plasma radiation and was further challenged with zeocin and polydatin, generating a mutant (YHPM1) with a 30, 65, and 80% higher overall biomass, lipid, and DHA production in comparison with the parental strains, respectively. Further analysis showed that the superior growth, lipid, and DHA biosynthesis of the YHMP1 were attributed not only to the higher G6PDH and enzymes involved in the oxidative defense such as superoxide dismutase (SOD) and catalase (CAT) but also to other key metabolic enzymes involved in lipid biosynthesis. This study provides an effective approach in developing the Aurantiochytrium sp. mutant with superior DHA production capacity that has the potential for industrial applications.

AdpA, a developmental regulator, promotes ε-poly-l-lysine biosynthesis in Streptomyces albulus

Background

AdpA is a global regulator of morphological differentiation and secondary metabolism in Streptomyces, but the regulatory roles of the Streptomyces AdpA family on the biosynthesis of the natural product ε-poly-l-lysine (ε-PL) remain unidentified, and few studies have focused on increasing the production of ε-PL by manipulating transcription factors in Streptomyces.

Results

In this study, we revealed the regulatory roles of different AdpA homologs in ε-PL biosynthesis and morphological differentiation and effectively promoted ε-PL production and sporulation in Streptomycesalbulus NK660 by heterologously expressing adpA from S.neyagawaensis NRRLB-3092 (adpA_Sn). First, we identified a novel AdpA homolog named AdpA_Sa in S.albulus NK660 and characterized its function as an activator of ε-PL biosynthesis and morphological differentiation. Subsequently, four heterologous AdpA homologs were selected to investigate their phylogenetic relationships and regulatory roles in S.albulus, and AdpA_Sn was demonstrated to have the strongest ability to promote both ε-PL production and sporulation among these five AdpA proteins. The ε-PL yield of S.albulus heterologously expressing adpA_Sn was approximately 3.6-fold higher than that of the control strain. Finally, we clarified the mechanism of AdpA_Sn in enhancing ε-PL biosynthesis and its effect on ε-PL polymerization degree using real-time quantitative PCR, microscale thermophoresis and MALDI-TOF–MS. AdpA_Sn was purified, and its seven direct targets, zwf, tal, pyk2, pta, ack, pepc and a transketolase gene (DC74_2409), were identified, suggesting that AdpA_Sn may cause the redistribution of metabolic flux in central metabolism pathways, which subsequently provides more carbon skeletons and ATP for ε-PL biosynthesis in S.albulus.

Conclusions

Here, we characterized the positive regulatory roles of Streptomyces AdpA homologs in ε-PL biosynthesis and their effects on morphological differentiation and reported for the first time that AdpA_Sn promotes ε-PL biosynthesis by affecting the transcription of its target genes in central metabolism pathways. These findings supply valuable insights into the regulatory roles of the Streptomyces AdpA family on ε-PL biosynthesis and morphological differentiation and suggest that AdpA_Sn may be an effective global regulator for enhanced production of ε-PL and other valuable secondary metabolites in Streptomyces.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-022-01785-6.

Epsilon-poly-L-lysine: Recent Advances in Biomanufacturing and Applications

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.

Efficient production of ε-poly-l-lysine using cassava starch and fish meal by Streptomyces albulus FQC-24

ε-Poly-l-lysine (ε-PL) is used as a natural food preservative which consists of l-lysine units connected. However, due to the expensive culture medium, the production cost of ε-PL remains high. In this study, cheap raw materials cassava starch (CS) and fish meal (FM) were employed by S. albulus FQC-24 for ε-PL production. In the single factor experiment, the maximum ε-PL production reached 0.97 g/L at 60 g/L CS and 15 g/L FM. The results of screening experiments by Plackett-Burman design showed that three main components affecting ε-PL production were CS, FM, and (NH₄)₂SO₄. And the standardized effects of CS, FM, and (NH₄)₂SO₄ were 0.13, -0.22, and -0.2, respectively. The optimum fermentation medium developed by response surface methodology for ε-PL production contained (g/L) CS, 67.56; FM, 14.70 and (NH₄)₂SO₄, 5.41. Under the optimum conditions, the ε-PL production was achieved 1.30 g/L, with 34.02% higher than that before optimization. Moreover, ε-PL productions of batch and fed-batch fermentation in a 7-L fermentor were improved to 2.13 and 17.17 g/L respectively, which increased by 0.64 and 12.2 times compared with the shake flask culture. The results indicated that FM and CS are promising substrates for the efficient production of ε-PL.

Metabolomic Analysis of Biosynthesis Mechanism of ε-Polylysine Produced by Streptomyces diastatochromogenes

ε-Polylysine (ε-PL), a natural preservative with broad-spectrum antimicrobial activity, has been widely used as a green food additive, and it is now mainly produced by Streptomyces in industry. In the previous study, strain 6#-7 of high-yield ε-PL was obtained from the original strain TUST by mutagenesis. However, the biosynthesis mechanism of ε-PL in 6#-7 is still unclear. In this study, the metabolomic analyses of the biosynthesis mechanism of ε-PL in both strains are investigated. Results show that the difference in metabolisms between TUST and 6#-7 is significant. Based on the results of both metabolomic and enzymatic activities, a metabolic regulation mechanism of the high-yield strain is revealed. The transport and absorption capacity for glucose of 6#-7 is improved. The enzymatic activity benefits ε-PL synthesis, such as pyruvate kinase and aspartokinase, is strengthened. On the contrary, the activity of homoserine dehydrogenase in the branched-chain pathways is decreased. Meanwhile, the increase of trehalose, glutamic acid, etc. makes 6#-7 more resistant to ε-PL. Thus, the ability of the mutagenized strain 6#-7 to synthesize ε-PL is enhanced, and it can produce more ε-PLs compared with the original strain. For the first time, the metabolomic analysis of the biosynthesis mechanism of ε-PL in the high-yield strain 6#-7 is investigated, and a possible mechanism is then revealed. These findings provide a theoretical basis for further improving the production of ε-PL.

Expanding the lysine industry: biotechnological production of l-lysine and its derivatives

l-lysine is an essential amino acid that contains various functional groups including α-amino, ω-amino, and α-carboxyl groups, exhibiting high reaction potential. The derivatization of these functional groups produces a series of value-added chemicals, such as cadaverine, glutarate, and d-lysine, that are widely applied in the chemical synthesis, cosmetics, food, and pharmaceutical industries. Here, we review recent advances in the biotechnological production of l-lysine and its derivatives and expatiate key technological strategies. Furthermore, we also discuss the existing challenges and potential strategies for more efficient production of these chemicals.

Short-Chain Poly(γ-diaminobutanoic acid), A Poly(amino acid) Produced by a Marine Bacteria Bacillus pumilus

One marine bacteria Bacillus pumilus was isolated using allura red as ε-poly-L-lysine (ε-PL) secretion indicator. But actually the product was identified as poly-γ-L-diaminobutanoic acid (γ-PAB) by ionization-time-of-flight mass spectrometry, not coproduced with ε-PL. The polymerization degree of γ-PAB was 4-22, namely short-chain γ-PAB, compared with that in S. celluloflavus, and it exhibited stronger inhibitory activities against yeasts than long-chain γ-PAB but weaker activities against bacteria. The fermentative behavior of B. pumilus was investigated, and the γ-PAB production was 38.6 mg/L in shake flask and was enhanced to 284.2 mg/L in 5-L bioreactor by a pH control strategy. Interestingly, the suitable pH for B. pumilus to produce γ-PAB was 4.8, different from 4.0 for current Streptomyces strains, which suggests a potential new metabolic mechanism in B. pumilus as a novel γ-PAB producer. No studies on short-chain γ-PAB production in bacteria have been reported previously and we considered that this is a new discovery in the field of homopolymer research.

Enhanced poly-γ-L-diaminobutanoic acid production in Bacillus pumilus by combining genome shuffling with multiple antibiotic-resistance

A breeding approach combining genome shuffling with multiple antibiotic-resistance including gentamicin, rifampin and lincomycin, was developed in this research to improve the poly-γ-L-diaminobutanoic acid (γ-PAB) production in Bacillus pumilus LS-1. By this unique strategy, recombinants from the third round of genome shuffling could tolerate higher concentration of compound antibiotics and exhibited higher γ-PAB production as 392.4 mg/L in shake-flask fermentation, tenfold over the parent. In batch fermentation, B. pumilus GS3-M7 could produce γ-PAB as high as 2316.4 mg/L in two days, 5.4-fold higher than the control, which was the highest productivity ever reported. In addition, the optimal pH in B. pumilus for γ-PAB synthesis was decreased after ARTP mutagenesis and protoplast fusion, because the lower pH environment is favorable for accumulation of intracellular ATP. Some key enzymes in GS3-M7 showed higher activities than those in the parent, suggesting a greater flux to TCA circle and DAP pathway, which was a reason for enhanced γ-PAB production.

Enhancement of ε-poly-L-lysine production in Streptomyces griseofuscus by addition of exogenous astaxanthin

Addition of exogenous astaxanthin for improving ε-poly-L-lysine (ε-PL) production in Streptomyces griseofuscus was investigated in this study. By this unique strategy, the ε-PL production in shaker-flask fermentation was 2.48 g/L, which was 67.5% higher than the control at the addition dosage of 1.0 g/L, owing to the oxidation resistance of astaxanthin. In fed-batch fermentation, the ε-PL production reached 36.1 g/L, a 36.3% increase compared to the control. Intracellular response for oxidation in S. griseofuscus such as ROS generation and lipid peroxidation was reduced by astaxanthin addition. Illumina RNA deep sequencing (RNA-seq) technology further revealed that S. griseofuscus with astaxanthin addition showed down-regulated transcriptions of genes involved in oxidative stress. This research proved that the beneficial effect of astaxanthin addition was far better than glutathione (GSH) owing to the stronger antioxidant capacity, and provided a novel approach to regulate ε-PL synthesis.

Mechanisms of response to pH shock in microbial fermentation

The following review highlights pH shock, a novel environmental factor, as a tool for the improvement of fermentation production. The aim of this review is to introduce some recent original studies on the enhancement of microbial fermentation production by pH shock. Another purpose of this review is to improve the understanding of the processes that underlie physiological and genetic differences, which will facilitate future research on the improvement of fermentation production and reveal the associated molecular mechanisms. This understanding will simultaneously promote the application of this strategy to other microbial fermentation systems. Furthermore, improvement of the cellular tolerance of genetically engineered bacteria can also be a new field of research in the future to enhance fermentation production.

Sequential improvement of rimocidin production in Streptomyces rimosus M527 by introduction of cumulative drug-resistance mutations

Rimocidin is a polyene macrolide that exhibits a strong inhibitory activity against a broad range of plant-pathogenic fungi. In this study, fermentation optimization and ribosome engineering technology were employed to enhance rimocidin production in Streptomyces rimosus M527. After the optimization of fermentation, rimocidin production in S. rimosus M527 increased from 0.11 ± 0.01 to 0.23 ± 0.02 g/L during shake-flask experiments and reached 0.41 ± 0.05 g/L using 5-L fermentor. Fermentation optimization was followed by the generation of mutants of S. rimosus M527 through treatment of the strain with different concentrations of gentamycin (Gen) or rifamycin. One Gen^r mutant named S. rimosus M527-G37 and one Rif^r mutant named S. rimosus M527-R5 showed increased rimocidin production. Double-resistant (Gen^r and Rif^r) mutants were selected using S. rimosus M527-G37 and S. rimosus M527-R5, and subsequently tested. One mutant, S. rimosus M527-GR7, which was derived from M527-G37, achieved the greatest cumulative improvement in rimocidin production. In the 5-L fermentor, the maximum rimocidin production achieved by S. rimosus M527-GR7 was 25.36% and 62.89% greater than those achieved by S. rimosus M527-G37 and the wild-type strain S. rimosus M527, respectively. Moreover, in the mutants S. rimosus M527-G37 and S. rimosus M527-GR7 the transcriptional levels of ten genes (rimA_sr to rimK_sr) located in the gene cluster involved in rimocidin biosynthesis were all higher than those in the parental strain M527 to varying degrees. In addition, after expression of the single rimocidin biosynthetic genes in S. rimosus M527 a few recombinants showed an increase in rimocidin production. Expression of rimE led to the highest production.

Differential protein expression of a streptomycin-resistantStreptomyces albulusmutant in high yield production of ε-poly-l-lysine: a proteomics study

ε-Poly-l-lysine (ε-PL), produced by Streptomyces albulus, is an excellent antimicrobial agent which has been extensively used in the field of food and medicine. In our previous study, we have improved ε-PL production by S. albulus M-Z18 through iterative introduction of streptomycin resistance. To decipher the overproduction mechanism of high-yielding mutant S. albulus SS-62, we conducted a comparative proteomics analysis between S. albulus SS-62 and its parent strain S. albulus M-Z18. Approximately 11.5% of the predicted S. albulus proteome was detected and 401 known or putative regulatory proteins showed statistically differential expression levels. Expression levels of proteins involved in ε-PL precursor metabolism and energy metabolism, and proteins in the pathways related to transcriptional regulation and translation were up-regulated. It was indicated that mutant SS-62 could not only strengthen the ε-PL precursor metabolism and energy metabolism but also tune the pathways related to transcriptional regulation and translation, suggesting a better intracellular metabolic environment for the synthesis of ε-PL in mutant SS-62. To confirm these bioinformatics analyses, qRT-PCR was employed to investigate the transcriptional levels of pls, frr and hrdD and their transcription levels were found to have increased more than 4-fold. Further, overexpression of pls and frr resulted in an increase in ε-PL titer and the yield of ε-PL per unit cell. This report not only represents the first comprehensive study on comparative proteomics in S. albulus, but it would also guide strain engineering to further improve ε-PL production.

ε-Poly-l-lysine (ε-PL), produced by Streptomyces albulus, is an excellent antimicrobial agent which has been extensively used in the field of food and medicine.

Efficiently activated ε-poly-L-lysine production by multiple antibiotic-resistance mutations and acidic pH shock optimization in Streptomyces albulus

ε‐Poly‐L‐lysine (ε‐PL) is a food additive produced by Streptomyces and is widely used in many countries. Working with Streptomyces albulus FEEL‐1, we established a method to activate ε‐PL synthesis by successive introduction of multiple antibiotic‐resistance mutations. Sextuple mutant R6 was finally developed by screening for resistance to six antibiotics and produced 4.41 g/L of ε‐PL in a shake flask, which is 2.75‐fold higher than the level produced by the parent strain. In a previous study, we constructed a double‐resistance mutant, SG‐31, with high ε‐PL production of 3.83 g/L and 59.50 g/L in a shake flask and 5‐L bioreactor, respectively. However, we found that R6 did not show obvious advantages in fed‐batch fermentation when compared with SG‐31. For further activation of ε‐PL synthesis ability, we optimized the fermentation process by using an effective acidic pH shock strategy, by which R6 synthetized 70.3 g/L of ε‐PL, 2.79‐fold and 1.18‐fold greater than that synthetized by FEEL‐1 and SG‐31, respectively. To the best of our knowledge, this is the highest reported ε‐PL production to date. This ε‐PL overproduction may be due to the result of R99P and Q856H mutations in ribosomal protein S12 and RNA polymerase, respectively, which may be responsible for the increased transcription of the ε‐poly‐lysine synthetase gene (pls) and key enzyme activities in the Lys synthesis metabolic pathway. Consequently, ε‐PL synthetase activity, intracellular ATP, and Lys concentrations were improved and directly contributed to ε‐PL overproduction. This study combined ribosome engineering, high‐throughput screening, and targeted strategy optimization to accelerate ε‐PL production and probe the fermentation characteristics of hyperyield mutants. The information presented here may be useful for other natural products produced by Streptomyces.

Enhancement of ε-poly-L-lysine synthesis in Streptomyces by exogenous glutathione

Our previous work indicated that the vigor of Streptomyces decreased at the later stage of ε-poly-L-lysine (ε-PL) fermentation. In this study, we observed that the level of reactive oxygen species (ROS) in vivo increased sharply after 24 h, and the addition of an antioxidant glutathione (GSH) before this increase in ROS stimulated ε-PL synthesis in shake-flask fermentation. The enhancement of ε-PL production by GSH was further verified in batch and fed-batch fermentations. On a 5-l fermenter scale, the highest increasement was 68.8% in batch fermentation and the highest ε-PL level was 46.5 g l^- 1 in fed-batch fermentation. The RT-qPCR analysis showed that the transcriptional level of the catalase gene was down-regulated, and the decrease in cell activity was alleviated by the addition of GSH. The results revealed that exogenous antioxidant might maintain the cell vigor by reducing the excess ROS which provided a novel approach to regulate ε-PL synthesis.

Development of Microtiter Plate Culture Method for Rapid Screening of ε-Poly-L-Lysine-Producing Strains

ε-Poly-L-lysine (ε-PL) produced by Streptomyces albulus possesses a broad spectrum of antimicrobial activity and is widely used as a food preservative. To extensively screen ε-PL-overproducing strain, we developed an integrated high-throughput screening assay using ribosome engineering technology. The production protocol was scaled down to 24- and 48-deep-well microtiter plates (MTPs). The microplate reader assay was used to monitor ε-PL production. A good correlation was observed between the fermentation results obtained in both 24-(48)-deep-well MTPs and conventional Erlenmeyer flasks. Using this protocol, the production of ε-PL in an entire MTP was determined in <5 min without compromising on accuracy. The high-yielding strain selected through this protocol was also tested in Erlenmeyer flasks. The result showed that the ε-PL production of the high-yielding mutants was nearly 45% higher than that of the parent stain. Thus, development of this protocol is expected to accelerate the selection of ε-PL-overproducing strains.