Epsilon-poly-l-lysine (ε-PL) exhibits multifaceted antifungal mechanisms of action that control postharvest Alternaria rot

Sanxiapeptin is an ideal preservative with a dual effect of controlling green mold and inducing systemic defense in postharvest citrus

Green mold is a common postharvest disease infected by Penicillium digitatum that causes citrus fruit decay, and severely affects fruit storage quality. This work aimed to investigate the antifungal activity of Sanxiapeptin against P. digitatum, and elucidate the possible mechanisms involved. Sanxiapeptin was capable of inhibiting spore germination, germ tube length and mycelial growth. The SYTOX green staining assay revealed that Sanxiapeptin targeted the fungal membrane, and changed the membrane permeability, leading to the leakage of cell constituents. Meanwhile, Sanxiapeptin could influence the cell wall permeability and integrity by increasing the activities of chitinase and glucanase, resulting in abnormal chitin consumption and the decrease of glucan. Intriguingly, Sanxiapeptin could effectively control postharvest decay in citrus fruits, and activate the host resistance responses by regulating the phenylpropanoid pathway. In conclusion, Sanxiapeptin exhibits multiphasic antifungal mechanisms of action to control green mold in citrus fruits, shows great potential as novel food preservatives.

Biopolymer-based packaging films/edible coatings functionalized with ε-polylysine: New options for food preservation

In light of the commendable advantages inherent in natural polymers such as biocompatibility, biodegradability, and cost-effectiveness, researchers are actively engaged in the development of biopolymer-based biodegradable food packaging films (BFPF). However, a notable limitation is that most biopolymers lack intrinsic antimicrobial activity, thereby restricting their efficacy in food preservation. To address this challenge, various active substances with antibacterial properties have been explored as additives to BFPF. Among these, ε-polylysine has garnered significant attention in BFPF applications owing to its outstanding antibacterial properties. This study provides a brief overview of the synthesis method and chemical properties of ε-polylysine, and comprehensively examines its impact as an additive on the properties of BFPF derived from diverse biopolymers, including polysaccharides, proteins, aliphatic polyesters, etc. Furthermore, the practical applications of various BFPF functionalized with ε-polylysine in different food preservation scenarios are summarized. The findings underscore that ε-polylysine, functioning as an antibacterial agent, not only directly enhances the antimicrobial activity of BFPF but also serves as a cross-linking agent, interacting with biopolymer molecules to influence the physical and mechanical properties of BFPF, thereby enhancing their efficacy in food preservation.

Antifungal efficiency and mechanisms of ethyl ferulate against postharvest pathogens

Postharvest loss caused by a range of pathogens necessitates exploring novel antifungal compounds that are safe and efficient in managing the pathogens. This study evaluated the antifungal activity of ethyl ferulate (EF) and explored its mechanisms of action against Alternaria alternata, Aspergillus niger, Botrytis cinerea, Penicillium expansum, Penicillium digitatum, Geotrichum candidum and evaluated its potential to inhibit postharvest decay. The results demonstrated that EF exerts potent antifungal activity against a wide board of postharvest pathogens. Results also revealed that its antifungal mechanism is multifaceted: EF may be involved in binding to and disturbing the integrity of the fungal plasma membrane, causing leakage of intracellular content and losing normal morphology and ultrastructure. EF also induced oxidative stress in the pathogen, causing membrane lipid peroxidation and malondialdehyde accumulation. EF inhibited the critical gene expression of the pathogen, affecting its metabolic regulation, antioxidant metabolism, and cell wall degrading enzymes. EF exhibited antifungal inhibitory activity when applied directly into peel wounds or after incorporation with chitosan coating. Due to its wide board and efficient antifungal activity, EF has the potential to provide a promising alternative to manage postharvest decay.

Inhibitory Effect of L-Methionine on Alternaria alternata Based on Metabolomics Analysis

Alternaria alternata is the main pathogenic fungus of postharvest black spots in fruits and vegetables. This study aimed to explore the antifungal activity of methionine on A. alternata in vitro and to reveal related antifungal mechanisms through a metabolomics analysis. The results showed that the inhibitory effects of L-methionine (Met) treatment on mycelium growth, spore germination, and the germ tube elongation of A. alternata were enhanced with an increase in the Met concentration, but the inhibitory effects decreased when the Met concentration was higher than 50 mmolL-1. The results of propidium iodide staining and scanning electron microscopy showed that the Met treatment damaged the plasma membrane integrity of the A. alternata spores and caused an irreversible deformation of mycelium. In addition, after the Met treatment, the leakage of electrolytes, nucleic acid, and proteins in the A. alternata cells was significantly higher than that in the control group, indicating that the Met treatment increased the permeability of the cell membranes. Eighty-one different metabolites, divided into seven categories, were identified through the metabolomics analysis, including forty-three downregulated metabolites and thirty-eight upregulated metabolites. Among them, these differential metabolites were mainly involved in amino acid synthesis and metabolism, the pentose phosphate pathway, and the TCA cycle. Therefore, the antifungal effect of the Met treatment on A. alternata was mainly to damage the integrity of the cell membranes, make nucleic acid and protein contents leak, and affect the TCA cycle, carbohydrate metabolism, amino acid synthesis metabolism, and the metabolic pathways associated with cell membrane biosynthesis. Thus, the growth and development of A. alternata were inhibited. The research enriched the investigation of the effect of the antifungal mechanism of Met treatment on A. alternata and provided a theoretical basis for the application of Met to prevent and treat postharvest black spots in fruits and vegetables.

Epsilon-poly-l-lysine alleviates brown blotch disease of postharvest Agaricus bisporus mushrooms by directly inhibiting Pseudomonas tolaasii and inducing mushroom disease resistance

The natural antimicrobial peptide, epsilon-poly-l-lysine (ε-PL), is widely acknowledged as a food preservative. However, its potential in managing bacterial brown blotch disease in postharvest edible mushrooms and the associated mechanism remain unexplored. In this study, concentrations of ε-PL ≥ 150 mg L-1 demonstrated significant inhibition effects, restraining over 80% of growth and killed over 99% of Pseudomonas tolaasii (P. tolaasii). This inhibition effect occurred in a concentration-dependent manner. The in vivo findings revealed that treatment with 150 mg L-1 ε-PL effectively inhibited P. tolaasii-caused brown blotch disease in Agaricus bisporus (A. bisporus) mushrooms. Plausible mechanisms underlying ε-PL's action against P. tolaasii in A. bisporus involve: (i) damaging the cell morphology and membrane integrity, and increasing uptake of propidium iodide and leakage of cellular components of P. tolaasii; (ii) interaction with intracellular proteins and DNA of P. tolaasii; (iii) inhibition of P. tolaasii-induced activation of polyphenol oxidase, elevation of antioxidative enzyme activities, stimulation of phenylpropanoid biosynthetic enzyme activities and metabolite production, and augmentation of pathogenesis-related protein contents in A. bisporus mushrooms. These findings suggest promising prospects for the application of ε-PL in controlling bacterial brown blotch disease in A. bisporus.

Development of bio-based PLA/cellulose antibacterial packaging and its application for the storage of shiitake mushroom

This study presents the extraction of cellulose from water bamboo byproducts to prepare polylactic acid (PLA)/cellulose antibacterial packaging material. The cellulose was modified using a silane coupling agent, which improved the interfacial compatibility between cellulose and PLA. Upon coating the PLA onto the modified cellulose sheet, the water contact angle of the composite material increased from 11.42° to 132.12° and the water absorption rate decreased from 182.52% to 55.71%, which improved the water resistance performance of the material. The addition of cinnamaldehyde in the PLA layer imparted antibacterial activity to the PLA/cellulose packaging material. This packaging material effectively inhibited the mycelial growth and spore germination of Aspergillus niger and Trichoderma harzianum isolated from shiitake mushroom. Additionally, the study investigated the effects of the composite on the postharvest quality of shiitake mushroom. Overall, the packaging material contributed to shiitake mushroom storage and can be applied to other perishable food products.

Culturable Bacterial Endophytes of Wild White Poplar (Populus alba L.) Roots: A First Insight into Their Plant Growth-Stimulating and Bioaugmentation Potential

The white poplar (Populus alba L.) has good potential for a green economy and phytoremediation. Bioaugmentation using endophytic bacteria can be considered as a safe strategy to increase poplar productivity and its resistance to toxic urban conditions. The aim of our work was to find the most promising strains of bacterial endophytes to enhance the growth of white poplar in unfavorable environmental conditions. To this end, for the first time, we performed whole-genome sequencing of 14 bacterial strains isolated from the tissues of the roots of white poplar in different geographical locations. We then performed a bioinformatics search to identify genes that may be useful for poplar growth and resistance to environmental pollutants and pathogens. Almost all endophytic bacteria obtained from white poplar roots are new strains of known species belonging to the genera Bacillus, Corynebacterium, Kocuria, Micrococcus, Peribacillus, Pseudomonas, and Staphylococcus. The genomes of the strains contain genes involved in the enhanced metabolism of nitrogen, phosphorus, and metals, the synthesis of valuable secondary metabolites, and the detoxification of heavy metals and organic pollutants. All the strains are able to grow on media without nitrogen sources, which indicates their ability to fix atmospheric nitrogen. It is concluded that the strains belonging to the genus Pseudomonas and bacteria of the species Kocuria rosea have the best poplar growth-stimulating and bioaugmentation potential, and the roots of white poplar are a valuable source for isolation of endophytic bacteria for possible application in ecobiotechnology.

Multi-arm ε-polylysines exhibit broad-spectrum antifungal activities against Candida species

Invasive fungal infections pose a crucial threat to public health and are an under-recognized component of antimicrobial resistance, which is an emerging crisis worldwide. Here we designed and synthesized a panel of multi-arm ε-polylysines (ε-mPLs, nR-Km) with a precise number of n = 3-6 arms of ε-oligo(L-lysine)s and a precise arm length of m = 3-7 ε-lysine residues. ε-mPLs have good biocompatibility and exhibited broad-spectrum antifungal activities towards Aspergillus, Mucorales and Candida species, and their antifungal activities increased with residue arm length. Among these ε-mPLs, 3R-K7 showed high antifungal activity against C. albicans with a MIC value of as low as 24 μg mL-1 (only 1/16th that of ε-PL) and also exhibited similar antifungal activity towards the clinically isolated multi-drug resistant (MDR) C. albicans strain. Furthermore, 3R-K7 could inhibit the formation of C. albicans biofilms and kill the cells within mature C. albicans biofilms. Mechanistic studies proved that 3R-K7 killed fungal cells by entering the cells to generate reactive oxygen species (ROS) and induce cell apoptosis. An in vivo study showed that 3R-K7 significantly increased the survival rate of mice in a systemic murine candidiasis model, demonstrating that ε-mPL has great potential as a new antifungal agent.

Fatty acid metabolism and C9 aldehyde biosynthesis are involved in ε-poly-l-lysine-induced citrus fruit resistance to Penicillium digitatum

Citrus fruit were easily infected by Penicillium digitatum, and caused green mold rapidly, resulting in enormous post-harvest losses. ε-poly-l-lysine (ε-PL) was generally regarded as a safe (GRAS) substance. Besides, it was proved to have a dual effect on harming fungi and triggering fruit defense responses. Fatty acid metabolism is closely related to fruit defense response. However, little is known about how ε-PL affected fatty acid metabolism in citrus fruit. Here, we found that ε-PL increased the expression of CsFATA, CsACSL, CsFAD2, CsFAD3, CsLOX2S, and CsHPL in fatty acid metabolism, decreasing oleic acid levels and enhancing linoleic and linolenic acid levels. Additionally, ε-PL enriched the activities of LOX and HPL during the oxidative decomposition of fatty acids, and activating C9 aldehyde biosynthesis. Interestingly, ε-PL combined with (2E,4E)-nonadienal (C9 aldehyde) would improve the inhibitory effect against Penicillium digitatum. And the combined bio-fungicide significantly delayed the citrus green mold compared to single concentrations of the individual components. These results suggested that ε-PL improved citrus fruit defense responses through fatty acid-mediated defense responses. Combined bio-fungicide consisting of ε-PL and (2E,4E)-nonadienal have an excellent prospect for controlling citrus green mold.

Insights into the Isolation, Identification, and Biological Characterization Analysis of and Novel Control Strategies for Diaporthe passiflorae in Postharvest Passion Fruit

Postharvest diseases seriously restrict developments in the passion fruit industry. In this study, we aimed to identify the postharvest pathogen affecting passion fruit, investigate its pathogenicity, and explore relevant control methods. The pathogen was isolated from rotting passion fruit and identified using morphological characteristics, ITS sequences, and phylogenetic tree analyses. Additionally, preliminary studies were conducted to assess the biological characteristics of the pathogen and evaluate the efficacy of various treatments for disease control. The fungus on the passion fruit called B4 was identified as Diaporthe passiflorae. Optimal conditions for mycelial growth were observed at 25-30 °C and pH 5-6, with starch as the carbon source and peptone as the nitrogen source. Infection by D. passiflorae accelerated fruit decay, reduced the h° value of the peel, and increased the peel cell membrane permeability when compared to the control. Notably, treatments with appropriate concentrations of ɛ-poly-l-lysine, salicylic acid, and melatonin showed inhibitory effects on the pathogen's growth in vitro and may thus be potential postharvest treatments for controlling brown rot caused by D. passiflorae in passion fruit. The results provide a scientific basis for the development of strategies to control postharvest decay and extend the storage period of passion fruit.

Synergistic Effects of 1-MCP Fumigation and ε-Poly-L-Lysine Treatments on Delaying Softening and Enhancing Disease Resistance of Flat Peach Fruit

Flat peach, a predominant fruit consumed in China, is highly susceptible to softening and perishable. The impact of 1-methylcycloproene (1-MCP) fumigation combined with ε-poly-L-lysine (ε-PL) on softening and postharvest reactive oxygen species (ROS) and phenylpropanoid pathway metabolisms in peaches and its relationship to disease resistance were investigated. Findings revealed that a combination of 1 µL L-1 1-MCP and 300 mg L-1 ε-PL effectively suppressed the activity of cell-wall-degrading enzymes and the disassembly of cell wall structure, thus maintaining higher firmness and lower decay incidence. Compared to the control group, the synergistic approach bolstered enzymatic responses linked to disease resistance and ROS-scavenge system, consistently preserving total phenolics, flavonoids, ascorbic acid, and glutathione levels. Concurrently, the accumulation of hydrogen peroxide and malondialdehyde was significantly diminished post-treatment. These results show that there is good synergistic effect between 1-MCP and ε-PL, which could effectively maintain the quality of flat peach fruit by modulating cell wall metabolism and enhancing the resistance.

Putrescine (1,4-Diaminobutane) enhances antifungal activity in postharvest mango fruit against Colletotrichum gloeosporioides through direct fungicidal and induced resistance mechanisms

Anthracnose decay caused by Colletotrichum gloeosporioides greatly shortens the shelf life and commercial quality of mango fruit. Putrescine (1,4-Diaminobutane) is involved in modulating plant defense to various environmental stresses. In this research, in vivo and in vitro tests were used to explore the antifungal activity and the underlying mechanism of putrescine against C. gloeosporioides in mango fruit after harvested. In vivo tests suggested that putrescine markedly delayed the occurrence of disease and limited the spots expansion on inoculated mango fruit. Further analysis exhibited that putrescine treatment enhanced disease resistance, along with enhanced activities of chitinase (CHI), β-1,3-glucanase (GLU), phenylalanine ammonia-lyase (PAL), cinnamate-4-hydroxylase (C4H), 4-coumarate coenzyme A ligase (4CL), polyphenol oxidase (PPO) and the accumulation of lignin, flavonoid, phenolics, and anthocyanin in infected mango fruit. In addition, in vitro tests showed that putrescine exerted strongly antifungal activity against C. gloeosporioides. Putrescine induced the production of reactive oxygen species (ROS) and severe lipid peroxidation damage in C. gloeosporioides mycelia, resulting in the leakage of soluble protein, soluble sugar, nucleic acids, K+ and Ca2+ of C. gloeosporioides mycelia. The mycelium treated with putrescine showed severe deformity and shrinkage, and even cracking. Taken together, putrescine could effectively reduce the incidence rate and severity of anthracnose disease possibly through direct fungicidal effect and indirect induced resistance mechanism, thus showing great potential to be applied to disease control.

Ursolic acid, the main component of blueberry cuticular wax, inhibits Botrytis cinerea growth by damaging cell membrane integrity

Cuticular wax has been reported to play an essential role in resisting pathogens in various fruits. This study investigated the antifungal ability of the components in blueberry cuticular wax. We showed that the cuticular wax of blueberry inhibited the growth of Botrytis cinerea and ursolic acid (UA) was the key antifungal compound. UA inhibited B. cinerea growth in vitro and in vivo. Furthermore, UA increased extracellular conductivity and cellular leakage in B. cinerea, deformed the mycelial morphology, and destroyed cell ultrastructure. We also demonstrated that UA stimulated the accumulation of reactive oxygen species (ROS) and inactivated ROS scavenging enzymes. These results indicate that UA may exert antifungal effects against B. cinerea by disrupting cell membrane integrity. Thus, UA has significant potential as an agent for the control of gray mold in blueberry.

Application of antimicrobial peptides in plant protection: making use of the overlooked merits

Pathogen infection is one of the major causes of yield loss in the crop field. The rapid increase of antimicrobial resistance in plant pathogens has urged researchers to develop both new pesticides and management strategies for plant protection. The antimicrobial peptides (AMPs) showed potential on eliminating plant pathogenic fungi and bacteria. Here, we first summarize several overlooked advantages and merits of AMPs, which includes the steep dose-response relations, fast killing ability, broad synergism, slow resistance selection. We then discuss the possible application of AMPs for plant protection with above merits, and highlight how AMPs can be incorporated into a more efficient integrated management system that both increases the crop yield and reduce resistance evolution of pathogens.

Biocontrol Microneedle Patch: A Promising Agent for Protecting Citrus Fruits from Postharvest Infection

With increasing human awareness of food safety, the replacement of highly toxic pesticides with biocompatible antimicrobials has become a trend. This study proposes a biocontrol microneedle (BMN) to expand the application of the food-grade preservative epsilon-poly-L-lysine (ε-PL) in fruit preservatives by utilizing a dissolving microneedle system. The macromolecular polymer ε-PL not only possesses broad-spectrum antimicrobial activity but also exhibits good mechanical properties. With the addition of a small amount of polyvinyl alcohol, the mechanical strength of the ε-PL-based microneedle patch could be further improved to achieve an enhanced failure force of needles at 1.6 N/needle and induce an approximately 96% insertion rate in citrus fruit pericarps. An ex vivo insertion test revealed that the microneedle tips could be effectively inserted into the citrus fruit pericarp, rapidly dissolve within 3 min, and produce inconspicuous needle holes. Moreover, the high drug loading capacity of BMN was observed to reach approximately 1890 μg/patch, which is essential for enhancing the concentration-dependent antifungal activity of ε-PL. The drug distribution study has confirmed the feasibility of mediating the local diffusion of EPL in the pericarp through BMN. Therefore, BMN has great potential to reduce the incidence of invasive fungal infections in local areas of citrus fruit pericarp.

ε-Polylysine Derived from Marine Bacteria-A Possible Natural Preservative for Raw Milk Storage

Despite the fact that researchers have been working on the preservation of raw milk at room temperature for several decades, most of the processes are limited to the use of chemical preservatives. One of the major problems of raw milk self-life is its spoilage at ambient temperature during the summer season. Therefore, in the present study, research has been conducted to control raw milk spoilage at 4 °C and 35 °C (considered in different regions’ ambient temperatures). ε-Polylysine, a natural preservative approved for food use, was isolated from the fermentation broth of Bacillus licheniformis PL26 grown in an M3G medium, and its antimicrobial preservation properties for milk applications were tested. The raw milk samples containing 0.02% w/v ε-polylysine could be stored at 4 °C for up to 16 days without spoilage, however, raw milk samples without ε-polylysine as preservative spoiled on the 8th day even at 4 °C refrigeration conditions. Raw milk containing 0.02% ε-polylysine in combination with 0.2% sodium bicarbonate (added to avoid acidification) could be stored at ambient temperature (35 °C) for up to 48 h. The changes in milk composition, especially of the casein, lactose, and fat stability, during storage under different conditions with/without ε-polylysine, were studied as well. The present study proves that ε-polylysine can be successfully used as a new biopreservative. Therefore, for the dairy industry, a natural preservative to store milk at room temperature during the summer season, replacing synthetic preservatives derived from renewable sources, can be proposed. Once again, marine bacteria seem to be one of the promising sustainable and renewable sources of biologically active compounds such as new food biopreservatives

Fabrication of CuO nanoparticles composite ε-polylysine-alginate nanogel for high-efficiency management of Alternaria alternate

Although ε-poly-l-lysine (ε-PL) has a good potential as a green fungicide, high concentration is usually required during its controlling of plant disease. On the other hand, same problems also appeared in the study of CuONP based nano pesticides. In this manuscript, a new composite alginate nanogel (ALGNP) that containing CuONP and ε-PL was fabricated via in situ reduction of CuONP in nanogel and ε-PL surface coating. Based on the chelation of amide bond of ε-PL and Cu²⁺ released by CuONP, the synergy effect between Cu²⁺ and ε-PL layer of the nanogel make the nanogel (CuONP@ALGNP@PL) performed high anti-fungal activity under low Cu²⁺ and ε-PL concentration (Cu concentration was 40.09 μg/mL, ε-PL concentration was 11.90 μg/mL). Study showed that the nanogel could more significantly destroy the fungal cell membrane than CuONP@ALGNP and ALGNP@PL, also better than commercial fungicide CuCaSO₄ (Cu concentration was 120 μg/mL). Furthermore, CuONP@ALGNP@PL could seriously affect the spore production, spore germination rate and bud tube elongation length of Alternaria alternate. Moreover, CuONP@ALGNP@PL also inhibit Botrytis cinerea, Phytophthora, Thanatephorus cucumeris and Fusarium graminearum. These results showed that composite of CuONP and ε-PL based on nanogel can decrease the raw materials application amount, and achieve a high disease controlling ability, which provides a new perspective for preventing fungal diseases.

Catalytic Production of Functional Monomers from Lysine and Their Application in High-Valued Polymers

Lysine is a key raw material in the chemical industry owing to its sustainability, mature fermentation process and unique chemical structure, besides being an important nutritional supplement. Multiple commodities can be produced from lysine, which thus inspired various catalytic strategies for the production of these lysine-based chemicals and their downstream applications in functional polymer production. In this review, we present a fundamental and comprehensive study on the catalytic production process of several important lysine-based chemicals and their application in highly valued polymers. Specifically, we first focus on the synthesis process and some of the current industrial production methods of lysine-based chemicals, including ε-caprolactam, α-amino-ε-caprolactam and its derivatives, cadaverine, lysinol and pipecolic acid. Second, the applications and prospects of these lysine-based monomers in functional polymers are discussed such as derived poly (lysine), nylon-56, nylon-6 and its derivatives, which are all of growing interest in pharmaceuticals, human health, textile processes, fire control and electronic manufacturing. We finally conclude with the prospects of the development of both the design and synthesis of new lysine derivatives and the expansion of the as-synthesized lysine-based monomers in potential fields.

Natamycin as a safe food additive to control postharvest green mold and sour rot in citrus

AIMS

The purpose of this study was to explore the potential inhibitory mechanism and assess the feasibility of natamycin as an antifungal agent in the utilization of citrus storage.

METHODS AND RESULTS

In this study, the mycelial growth, spore germination as well as germ tube elongations of Geotrichum citri-aurantii and Penicillium digitatum were significantly inhibited by natamycin treatment. The relative conductivities of G. citri-aurantii and P. digitatum mycelia were increased as time went by and the damages of plasma membranes were up to 17.43 % and 28.61 %. The mitochondria abnormalities and vacuolation were also observed in the TEM. Moreover, the sour rot and green mold decay incidences were reduced to 18.33 % and 10 % post-incubation with G. citri-aurantii and P. digitatum under 300 mg L^-1 natamycin application, respectively. For the citrus storage experiment, there was no significant difference in edible rate, juice yield, total soluble solid (TSS) content, titratable acid (TA) and decay incidences of the 'Newhall' navel orange fruit treated with 300 mg L^-1 natamycin stored for 90 d.

CONCLUSIONS

Natamycin could decrease the expansions of green mold and sour rot and maintain quality and improve storability on citrus fruit.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work explores the potential inhibition mechanism of natamycin G. citri-aurantii and P. digitatum, and assesses the feasibility of natamycin as an antifungal agent in the utilization of citrus storage.

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.

Involvement of Organic Acid in the Control Mechanism of ε-Poly-L-lysine (ε-PL) on Blue Mold Caused by Penicillium expansum in Apple Fruits

Blue mold is one of the most serious postharvest diseases in apples caused by Penicillium expansum. The purpose of this study is to determine the effect of ε-poly-L-lysine (ε-PL) on the pathogenicity of P. expansum and explore the potential mechanism from the perspective of organic acid. The study investigates the effect of ε-PL treatment on the growth and acid production of P. expansum in vitro and in vivo. When the concentration of ε-PL was 50 mg/L, the growth of P. expansum was inhibited and the decrease in pH value was delayed in the medium. For example, on the third day of culture, P. expansum reduced the pH of the medium from 6.1 to 4.15, and ε-PL inhibited the decrease in the pH value at most 34.4%. When the concentration reached 1000 or 2000 mg/L, the infection of P. expansum in fruits was effectively inhibited. During the growth and infection of P. expansum, gluconic acid is one of the main factors leading to the pH value falling in the local environment. After ε-PL treatment, the accumulation of gluconic acid decreased, the activity of glucose oxidase was suppressed, and then the decline in the local environmental pH slowed down. In addition, after ε-PL treatment, the activities of cell-wall-degrading enzymes, such as cellulase (CL) and polygalacturonase (PG), in the different areas of the P. expansum–apple interaction were also affected by pH change. The results show that ε-PL inhibited the pathogenicity of P. expansum by affecting the accumulation of gluconic acid and slowing the decline in pH in fruit tissues, so as to affect the pathogenicity of P. expansum. This is the first time that the mechanism of ε-PL interfering with the pathogenicity of P. expansum from the perspective of organic acids is clarified.

ε-Poly-l-Lysine Enhances Fruit Disease Resistance in Postharvest Longans (Dimocarpus longan Lour.) by Modulating Energy Status and ATPase Activity

ε-poly-l-lysine (ε-PL) holds a strong antibacterial property and is widely used for food preservation. However, the application of ε-PL to enhance fruit disease resistance in postharvest longans (Dimocarpus longan Lour.) has not been explored. The objective of this study was to explore the impact of ε-PL treatment on disease occurrence and energy metabolism of longans infected with Phomopsis longanae Chi (P. longanae). It was found that, in comparison with P. longanae-inoculated longans, ε-PL could decrease the fruit disease index and adenosine monophosphate (AMP) content, increase the amounts of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and energy charge, and enhance the activities of adenosine triphosphatase (ATPase) (such as H⁺-, Mg²⁺-, and Ca²⁺-ATPase) in the mitochondria, protoplasm, and vacuole. The results suggest that the higher levels of ATPase activity and energy status played essential roles in disease resistance of postharvest longan fruit. Therefore, the ε-PL treatment can be used as a safe and efficient postharvest method to inhibit the disease occurrence of longan fruit during storage at room temperature.

Editorial for special issue of food natural antimicrobials

This special issue compiled 13 research articles and one review analyzing natural antimicrobial agents applied in real food systems. The accepted submissions were received from 14 countries, including Spain, Brazil, Lebanon, United Arab Emirates, Jordan, Greece, Thailand, Turkey, Saudi Arabia, Italy, Argentina, Canada, Iran, and China. The studied antimicrobial substances included phenolic compounds from plant tissues, terpenes from plant essential oils, bacteriocins, and chitosan. The treated food matrices were fresh fruit, fruit juices, beef, chicken products, camel meat, cheese, fish, and yogurt. Most of the published papers directly applied the natural substances in the food matrices, and others use edible coatings, marinades, micro and nanocarriers. Also, hurdle technologies were used to increase the antimicrobial effect of the studied natural substances, including temperature, vacuum packaging, ultraviolet, and edible coatings. In conclusion, promising results were obtained to impulse the transitions of natural antimicrobials as effective agents in the food industry; some contributions to the mode of actions of natural antimicrobials in real food systems were also included.

Bioactive Peptides: A Promising Alternative to Chemical Preservatives for Food Preservation

Bioactive peptides used for food preservation can prolong the shelf life through bacteriostasis and antioxidation. On the one hand, bioactive peptides can inhibit lipid oxidation by scavenging free radicals, interacting with metal ions, and inhibiting lipid peroxidation. On the other hand, bioactive peptides can fundamentally inhibit the growth and reproduction of microorganisms by destroying their cell membranes or targeting intracellular components. Besides, bioactive peptides are biocompatible and biodegradable in vivo. Therefore, they are regarded as a promising alternative to chemical preservatives. However, bioactive peptides are easily affected by the external environment in practical application, which hinders their commercialization. Currently, the studies to overcome the weakness focus on encapsulation and chemical synthesis. Bioactive peptides have been applied to the preservation of various foods in experimental research, with good results. In the future, with the deepening understanding of their safety and structure-activity relationship, there may be more bioactive peptides as food preservatives.

ε-poly-L-lysine Affects the Vegetative Growth, Pathogenicity and Expression Regulation of Necrotrophic Pathogen Sclerotinia sclerotiorum and Botrytis cinerea

Microbial secondary metabolites produced by Streptomyces are applied to control plant diseases. The metabolite, ε-poly-l-lysine (ε-PL), is a non-toxic food preservative, but the potential application of this compound as a microbial fungicide in agriculture is rarely reported. In this study, the effect and mode of action of ε-PL on two necrotrophic pathogenic fungi, Sclerotinia sclerotiorum and Botrytis cinerea, were investigated. The results showed that ε-PL effectively inhibited the mycelial growth of S. sclerotiorum and B. cinerea with EC₅₀ values of 283 μg/mL and 281 μg/mL, respectively. In addition, ε-PL at the dose of 150 and 300 μg/mL reduced S. sclerotiorum sclerotia formation. The results of the RNA-seq and RT-qPCR validation indicated that ε-PL significantly regulated the gene expression of critical differential expressed genes (DEGs) involved in fungal growth, metabolism, pathogenicity, and induced an increase in the expression of the fungal stress responses and the detoxification genes. These results provided new insights for understanding the modes of action of ε-PL on S. sclerotiorum and B. cinerea and improved the sustainable management of these plant diseases.

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.

Interaction of Antimicrobial Peptide Ponericin W1, Thanatin, and Mastatopara-S with Geotrichum citri-aurantii Genomic DNA

Antimicrobial peptides of mastatopara-S (M-S), thanatin, and ponericin W1(P W1) were able to disrupt the membrane integrity and alter the morphology of the hyphae of Geotrichum citri-aurantii and then reduced the sour rot of citrus fruit. In order to understand the mechanisms of thanatin, P W1 and M-S other than membrane disruption, the interaction betwixt the peptides and G. citri-aurantii DNA were investigated in this research. The laser confocal microscopy found that P W1, thanatin, and M-S could penetrate the cell membrane. Gel retardation assay demonstrated that P W1, thanatin, and M-S could bind to the G. citri-aurantii genomic DNA in vitro. UV-visible spectra and fluorescence spectra analysis further confirmed that the peptides can bind to the DNA, and then insert into the base pairs in the DNA helix, followed by wrecking the double-helix structure. In addition, M-S, thanatin, and P W1 can suppress the synthesis of DNA and RNA of G. citri-aurantii.