Innovative approaches to nisin production

Harnessing the polysaccharide production potential to optimize and expand the application of probiotics

Certain probiotic microorganisms can synthesize important bioproducts, including polysaccharides as components of cellular structure or extracellular matrix. Probiotic-derived polysaccharides have been widely applied in food, pharmaceutical, and medical fields due to their excellent properties and biological activities. The development of polysaccharide production potential has become a driving force for facilitating biotechnological applications of probiotics. Based on technical advances in synthetic biology, significant progress has recently been made in engineering probiotics with efficient biosynthesis of polysaccharides. Herein, this review summarizes probiotics chassis and genetic tools used for polysaccharide production. Then, probiotic polysaccharides and relevant biosynthesis mechanisms are also clearly described. Next, we introduce strategies for preparing high-yield, controllable molecular weight or non-native polysaccharides by adjusting metabolic pathways and integrating expression elements in probiotics. Finally, some prospective and well-established contributions of exogenous and in situ polysaccharides in probiotics' stability, bioactivity, and therapeutic effects are presented. Our viewpoints on advancing the efficient biomanufacturing of valuable biopolymers in probiotics and engineering probiotics with customized features are provided to exploit probiotics' industrial and biomedical applications.

Efficient metabolic pathway modification in various strains of lactic acid bacteria using CRISPR/Cas9 system for elevated synthesis of antimicrobial compounds

Lactic acid bacteria (LAB) are known to exhibit various beneficial roles in fermentation, serving as probiotics, and producing a plethora of valuable compounds including antimicrobial activity such as bacteriocin-like inhibitory substance (BLIS) that can be used as biopreservative to improve food safety and quality. However, the yield of BLIS is often limited, which poses a challenge to be commercially competitive with the current preservation practice. Therefore, the present work aimed to establish an optimised two-plasmid CRISPR/Cas9 system to redirect the carbon flux away from lactate towards compounds with antimicrobial activity by disrupting lactate dehydrogenase gene (ldh) on various strains of LAB. The lactic acid-deficient (ldhΔ) strains caused a metabolic shift resulting in increased inhibitory activity against selected foodborne pathogens up to 78 % than the wild-type (WT) strain. The most significant effect was depicted by Enterococcus faecalis-ldh∆ which displayed prominent bactericidal effects against all foodborne pathogens as compared to the WT that showed no antimicrobial activity. The present work provided a framework model for economically important LAB and other beneficial bacteria to synthesise and increase the yield of valuable food and industrial compounds. The present work reported for the first time that the metabolism of selected LAB can be manipulated by modifying ldh to attain metabolites with higher antimicrobial activity.

Screening, characterization and mechanism of a potential stabiliser for nisin nanoliposomes with high encapsulation efficiency

The encapsulation efficiency (EE%) reflects the amount of bioactive components that can be loaded into nanoliposomes. Obtaining a suitable nanoliposome stabiliser may be the key to improving their EE%. In this study, three polyphenols were screened as stabilisers of nanoliposomes with high nisin EE%, with curcumin nanoliposomes (Cu-NLs) exhibiting the best performance (EE% = 95.94%). Characterizations of particle size, PDI and zeta potential indicate that the Cu-NLs had good uniformity and stability. TEM found that nisin accumulated at the edges of the Cu-NLs' phospholipid layer. DSC and FT-IR revealed that curcumin was involved in the formation of the phospholipid layer and altered its structure. FT-IR and molecular docking simulations indicate that the interactions between curcumin and nisin are mainly hydrogen bonding and hydrophobic. In whole milk, Cu-NLs effectively protected nisin activity. This study provides an effective strategy for improving the EE% of nanoliposomes loaded with nisin and other bacteriocins.

A novel bacteriocin against methicillin-resistant Staphylococcus aureus, purified from Lactiplantibacillus plantarum ZFM9

A novel bacteriocin, plantaricin ZFM9, was purified from Lactiplantibacillus plantarum ZFM9 using a combination of ammonium sulfate precipitation, XAD-2 macroporous resin, Sephadex G-50, Sephadex LH-20, and reversed-phase high performance liquid chromatography. The molecular mass of plantaricin ZFM9 was 1151.606 Da, and the purity was 98.3%. Plantaricin ZFM9 has thermal stability (95.6% retention at 120 °C for 30 min), pH stability (pH ≤ 5), and sensitivity to the pepsin, trypsin, papain, and proteinase K. Plantaricin ZFM9 exhibited broad-spectrum antimicrobial activity and notably inhibit methicillin-resistant Staphylococcus aureus D48 (MRSA). According to the results of electron microscopy and fluorescence leakage assay, it was found that plantaricin ZFM9 caused damage to the cells membrane and leakage of the contents of S. aureus D48. In addition, Lipid II was not the anti-MRSA target of plantaricin ZFM9. This study underscores the potential of plantaricin ZFM9 for applications in the food field and biopharmaceuticals against MRSA infection.

AcrR1, a novel TetR/AcrR family repressor, mediates acid and antibiotic resistance and nisin biosynthesis in Lactococcus lactis F44

Lactococcus lactis, widely used in the manufacture of dairy products, encounters various environmental stresses both in natural habitats and during industrial processes. It has evolved intricate machinery of stress sensing and defense to survive harsh stress conditions. Here, we identified a novel TetR/AcrR family transcription regulator, designated AcrR1, to be a repressor for acid and antibiotic tolerance that was derepressed in the presence of vancomycin or under acid stress. The survival rates of acrR1 deletion strain ΔAcrR1 under acid and vancomycin stresses were about 28.7-fold (pH 3.0, HCl), 8.57-fold (pH 4.0, lactic acid) and 2.73-fold (300 ng/mL vancomycin) greater than that of original strain F44. We also demonstrated that ΔAcrR1 was better able to maintain intracellular pH homeostasis and had a lower affinity to vancomycin. No evident effects of AcrR1 deletion on the growth and morphology of strain F44 were observed. Subsequently, we characterized that the transcription level of genes associated with amino acids biosynthesis, carbohydrate transport and metabolism, multidrug resistance, and DNA repair proteins significantly upregulated in ΔAcrR1 using transcriptome analysis and quantitative reverse transcription-PCR assays. Additionally, AcrR1 could repress the transcription of the nisin post-translational modification gene, nisC, leading to a 16.3% increase in nisin yield after AcrR1 deletion. Our results not only refined the knowledge of the regulatory mechanism of TetR/AcrR family regulator in L. lactis, but presented a potential strategy to enhance industrial production of nisin.

Optimization of culture conditions for endophytic bacteria in mangrove plants and isolation and identification of bacteriocin

Introduction: The antibacterial protein PAG14 was extracted from a metabolite of Bacillus G14 isolated from mangrove plants. Methods: In this study, Pseudomonas aeruginosa, Pasteurell multocide, Enterobacter aerogenes, and Enterococcus faecalis were used as indicator bacteria to screen endophytes that exhibited antibacterial activity. The endophyte culture conditions were optimized to enhance productivity. Subsequently, the culture supernatant was salted using ammonium sulfate, followed by purification using dextran gel chromatography and ion exchange column techniques. Finally, the structures of antibacterial proteins were identified using mass spectrometry. Results and Discussion: The optimal culture conditions for Bacillus G14 were 2% mannitol, 0.5% fish peptone, 0.05% KH2PO4 + 0.05% K2HPO4 + 0.025% MnSO4·H2O. The antibacterial substances exhibited stability within the temperature range of 30-40°C and pH range of 5.0-7.0, while displaying sensitivity toward enzymes. The antibacterial activity decreased as the duration of UV irradiation increased. The antibacterial protein PAG14, isolated from the culture broth of Bacillus G14 through purification using dextran gel and ion-exchange columns, was identified as a class III bacteriocin using LC-MS/MS, similar to Lysozyme C. These findings serve as a theoretical foundation for the investigation and application of bacteriocins in food products.

Bioactive peptides in dry-cured ham: A comprehensive review of preparation methods, metabolic stability, safety, health benefits, and regulatory frameworks

Dry-cured hams contain abundant bioactive peptides with significant potential for the development of functional foods. However, the limited bioavailability of food-derived bioactive peptides has hindered their utilization in health food development. Moreover, there is insufficient regulatory information regarding bioactive peptides and related products globally. This review summarizes diverse bioactive peptides derived from dry-cured ham and by-products originating from various countries and regions. The bioactivity, preparation techniques, bioavailability, and metabolic stability of these bioactive peptides are described, as well as the legal and regulatory frameworks in various countries. The primary objectives of this review are to dig deeper into the functionality of dry-cured ham and provide theoretical support for the commercialization of bioactive peptides from food sources, especially the dry-cured ham.

Antimicrobial Compounds in Wine

Ipsum vinum est potestas et possession (wine itself is power and possession). Wine is a complex system that triggers multisensory cognitive stimuli. Wine and its consumption are thoroughly intertwined with the development of human society. The beverage was appreciated in many ancient mythologies and plays an essential part in Christianity and rituals to this day. Wine has been said to enlighten and inspire artists and has even been prohibited by law and some religions, but has nevertheless played a role in human civilizations since the beginning. Winemaking is also a prospering and economically important industry and a longtime symbol of status and luxury. In winemaking, the formation of the final product is influenced by several factors that contribute to the chemical and sensory complexity often associated with quality vintages. Factors such as terroir, climatic conditions, variety of the grape, all aspects of the winemaking process to the smallest details, including metabolic processes carried out by yeast and malolactic bacteria, and the conditions for the maturation and storage of the final product, up to, and even beyond the point of deciding to open the bottle and enjoy the wine. In conjunction with the empiric and scientific process of winemaking, different molecules with antibacterial activity can be identified in wine during the production process, and several of them are clearly present in the final product. Some of these antibacterial components are phytochemicals, such as flavonoids and phenolic compounds, that may be delivered to the final product (wine) as a part of the grape, a variety of potential additive compounds, or from the oak barrels or clay amphoras used during the maturation process. Others are produced by yeasts and malolactic bacteria and play a role not only in the moderation of the fermentation process but contributing to the microbiological safety and beneficial properties spectra of the final product. Lactic acid bacteria, responsible for conducting malolactic fermentation, contribute to the final balance of the wine but are also directly involved in the production of different compounds exhibiting antibacterial activity. Some examples of these compounds include bacteriocins (antibacterial peptides), diacetyl, organic acids, reuterin, hydrogen peroxide, and carbon dioxide. Major aspects of these different beneficial metabolites are the subject of discussion in this review with the aim of highlighting their beneficial functions.

Co-encapsulation of Shirazi thyme (Zataria multiflora) essential oil and nisin using caffeic acid grafted chitosan nanogel and the effect of this nanogel as a bio-preservative in Iranian white cheese

The current study aims to co-encapsulate Shirazi thyme (Zataria multiflora) essential oil (ZEO) and nisin into chitosan nanogel as an antimicrobial and antioxidant agent to enhance the shelf-life of cheese. Chitosan-caffeic acid (CS-CA) nanogel was produced to co-encapsulate Zataria multiflora essential oil and nisin. This nanogel was characterized by dynamic light scattering (DLS), Fourier Transform Infrared (FTIR) spectroscopic analysis, X-ray diffraction (XRD) analysis, and scanning electron microscopy (SEM) images. The effect of free (TFZN) and encapsulated ZEO-nisin in chitosan nanogel (TCZN) on the chemical and microbiological properties of Iranian white cheese was assessed. The particle size, polydispersity index value (PDI), zeta potential, antioxidant activity, and encapsulation efficiency of the optimal chitosan-ZEO-nisin nanogel were 421.6 nm, 0.343, 34.0 mV, 71.06%-82.69%, and 41.3 ± 0.5%, 0.79 ± 0.06 mg/mL. respectively. FTIR and XRD approved ZEO and nisin entrapment within chitosan nanogel. The chitosan nanogel showed a highly porous surface with an irregular shape. The bioactive compounds of ZEO and nisin decreased the pH changes in cheese. On the 60th day of storage, the acidity of treated samples was significantly lower than that of control. Although the lowest anisidine index value was observed in samples treated with sodium nitrate (NaNO3) (TS), there was no significant difference between this sample and TCZN. The lowest microbial population was observed in TCZN and TS. After 60 days of ripening, Coliforms were not detected in the culture medium of TCZN and TS. The results can contribute to the development of a natural preservative with the potential for application in the dairy industry.

Effect of Soy Protein Products on Growth and Metabolism of Bacillus subtilis, Streptococcus lactis, and Streptomyces clavuligerus

Microbial nitrogen sources are promising, and soy protein as a plant-based nitrogen source has absolute advantages in creating microbial culture medium in terms of renewability, eco-friendliness, and greater safety. Soy protein is rich in variety due to different extraction technologies and significantly different in the cell growth and metabolism of microorganisms as nitrogen source. Therefore, different soy proteins (soy meal powder, SMP; soy peptone, SP; soy protein concentrate, SPC; soy protein isolate, SPI; and soy protein hydrolysate, SPH) were used as nitrogen sources to culture Bacillus subtilis, Streptococcus lactis, and Streptomyces clavuligerus to evaluate the suitable soy nitrogen sources of the above strains. The results showed that B. subtilis had the highest bacteria density in SMP medium; S. lactis had the highest bacteria density in SPI medium; and S. clavuligerus had the highest PMV in SPI medium. Nattokinase activity was the highest in SP medium; the bacteriostatic effect of nisin was the best in SPI medium; and the clavulanic acid concentration was the highest in SMP medium. Based on analyzing the correlation between the nutritional composition and growth metabolism of the strains, the results indicated that the protein content and amino acid composition were the key factors influencing the cell growth and metabolism of the strains. These findings present a new, high-value application opportunity for soybean protein.

Production of nisin from Lactococcus lactis in acid-whey with nutrient supplementation

The production of Nisin, an FDA-approved food preservative, was attempted by Lactococcus lactis subsp. lactis ATCC® 11454 using the underutilized milk industry effluent, acid-whey, as a substrate. Nisin production was further improved by studying the effect of supplementation of nutrients and non-nutritional parameters. The addition of yeast extract (6% w/v) as nitrogen source and sucrose (4% w/v) as carbon source were found to be suitable nutrients for the maximum nisin production. The changes in the medium pH due to lactic acid accumulation during batch fermentation and its influence on the production of nisin were analyzed in the optimized whey medium (OWM). The production characteristics in OWM were further compared with the nisin production in MRS media. The influence of nisin as an inducer for its own production was also studied and found that the addition of nisin at 0.22 mg/ml promote the nisin production. The analysis of consumption of various metal ions present in the OWM during the nisin production was also analyzed, and found that the copper ions are the most consumed ion. The highest nisin yield of 2.6 × 105 AU/mL was obtained with OWM.

Comprehensive Review on the Biocontrol of Listeria monocytogenes in Food Products

Listeria monocytogenes is a foodborne pathogen that causes listeriosis, a group of human illnesses that appear more frequently in countries with better-developed food supply systems. This review discusses the efficacy of actual biocontrol methods combined with the main types of food involved in illnesses. Comments on bacteriophages, lactic acid bacteria, bacteriocins, essential oils, and endolysins and derivatives, as main biological antilisterial agents, are made bearing in mind that, using them, food processors can intervene to protect consumers. Both commercially available antilisterial products and solutions presented in scientific papers for mitigating the risk of contamination are emphasized. Potential combinations between different types of antilisterial agents are highlighted for their synergic effects (bacteriocins and essential oils, phages and bacteriocins, lactic acid bacteria with natural or synthetic preservatives, etc.). The possibility to use various antilisterial biological agents in active packaging is also presented to reveal the diversity of means that food processors may adopt to assure the safety of their products. Integrating biocontrol solutions into food processing practices can proactively prevent outbreaks and reduce the occurrences of L. monocytogenes-related illnesses.

Production of bacterial cellulose (BC)/nisin composite with enhanced antibacterial and mechanical properties through co-cultivation of Komagataeibacter xylinum and Lactococcus lactis subsp. lactis

By employing co-cultivation technique on Komagataeibacter xylinum and Lactococcus lactis subsp. lactis, bacterial cellulose (BC)/nisin films with improved antibacterial activity and mechanical properties were successfully produced. The findings demonstrated that increased nisin production is associated with an upregulation of gene expression. Furthermore, results from Scanning electronic microscopy (SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and Thermogravimetric analysis (TG) confirmed the integration of nisin within BC. While being biocompatible with human cells, the BC/nisin composites exhibited antimicrobial activity. Moreover, mechanical property analyses showed a noticeable improvement in Young's modulus, tensile strength, and elongation at break by 161, 271, and 195 %, respectively. Additionally, the nisin content in fermentation broth was improved by 170 % after co-culture, accompanied by an 8 % increase in pH as well as 10 % decrease in lactate concentration. Real-time reverse transcription PCR analysis revealed an upregulation of 11 nisin-related genes after co-cultivation, with the highest increase in nisA (5.76-fold). To our knowledge, this is the first study which demonstrates that an increase in secondary metabolites after co-culturing is modulated by gene expression. This research offers a cost-effective approach for BC composite production and presents a technique to enhance metabolite concentration through the regulation of relevant genes.

Phylogenetic classification of natural product biosynthetic gene clusters based on regulatory mechanisms

The natural products (NPs) biosynthetic gene clusters (BGCs) represent the adapting biochemical toolkit for microorganisms to thrive different microenvironments. Despite their high diversity, particularly at the genomic level, detecting them in a shake-flask is challenging and remains the primary obstacle limiting our access to valuable chemicals. Studying the molecular mechanisms that regulate BGC expression is crucial to design of artificial conditions that derive on their expression. Here, we propose a phylogenetic analysis of regulatory elements linked to biosynthesis gene clusters, to classify BGCs to regulatory mechanisms based on protein domain information. We utilized Hidden Markov Models from the Pfam database to retrieve regulatory elements, such as histidine kinases and transcription factors, from BGCs in the MIBiG database, focusing on actinobacterial strains from three distinct environments: oligotrophic basins, rainforests, and marine environments. Despite the environmental variations, our isolated microorganisms share similar regulatory mechanisms, suggesting the potential to activate new BGCs using activators known to affect previously characterized BGCs.

Antibacterial Effect of Melanoidins Derived From Xylose and Phenylalanine Against Bacillus cereus and Clostridium perfringens

Melanoidins produced from the combination of D-xylose and L-phenylalanine have been reported to exhibit strong antibacterial effects. This study investigated the influence of environmental factors, such as temperatures (10, 15, 20, 25, 30, 35, 40, and 45°C), pH (5.5, 6.0, 6.5, 7.0, 7.5, and 8.0), and water activity (aw: 0.99, 0.96, and 0.93), on the antibacterial effect of the melanoidins produced from the combination of D-xylose with L-phenylalanine against Bacillus cereus and Clostridium perfringens in culture media. Furthermore, freeze-dried powdered melanoidin was used to determine the minimum concentration for growth inhibition, to compare the antibacterial effect of the melanoidin with conventional food preservatives. The liquid melanoidins significantly inhibited the growth of B. cereus (up to 4 log CFU/mL at the maximum) and C. perfringens (up to 6.5 log CFU/mL at the maximum) regardless of the incubation temperatures. However, the remarkable difference between the presence and absence of the melanoidins was demonstrated in the range of 20-35°C as 4 log-cycle lower in B. cereus and 2 log-cycle lower in C. perfringens than those without the melanoidins. The antibacterial effect of the melanoidin on B. cereus was not influenced by pH from 5.5 to 7.0, which exhibited 2-3 log-cycle lower viable counts than those without the melanoidin. Only one log-cycle difference between with and without the melanoidin was shown in C. perfringens growth under the pH range of 5.5-7.0. Although there was no significant difference in the growth of B. cereus between three aw conditions, the melanoidin exhibited a significant antibacterial effect at aw 0.99 demonstrating 4 log-cycle lower viable numbers than those without the melanoidin. Minimum inhibitory concentration of the melanoidin powder for B. cereus and C. perfringens was 7 mg/mL and 15 mg/mL, respectively, regardless of the kind of foods. Furthermore, the melanoidin exhibited comparable antibacterial effect on B. cereus and C. perfringens to potassium sorbate and sodium benzoate under the same concentration as the minimum inhibitory concentration of the melanoidin, demonstrating 2 log-cycle reduction during 3 days of incubation period at 25°C. The results presented here suggest that the xylose- and phenylalanine-based melanoidin demonstrates the possibility to be an alternative food preservative.

Bacteriocin-Producing Enterococcus faecium OV3-6 as a Bio-Preservative Agent to Produce Fermented Houttuynia cordata Thunb. Beverages: A Preliminary Study

Microbial contamination affects the quality of the fermented Houttuynia cordata Thunb. (H. cordata) beverage (FHB). The present study aimed to assess the bio-preservative property of Enterococcus faecium OV3-6 (E. faecium OV3-6) during the production of FHB. The antimicrobial activity against Escherichia coli, Salmonella, Bacillus cereus, and Staphylococcus aureus and the survival of E. faecium OV3-6 were studied. Then, FHB fermentation was performed with different preservatives (non-preservative, E. faecium OV3-6, cell-free supernatant of E. faecium OV3-6, and nisin) with and without representative pathogens. The maximum antimicrobial activity against S. aureus and B. cereus was observed after 18 h of cultivation in an MRS medium. E. faecium OV3-6 was used as a starter to produce the FHB, and the strain survived up to 48 h in the fermented beverage. E. faecium OV3-6 and its cell-free supernatant inhibited the growth of E. coli, Salmonella, B. cereus, and S. aureus in the stimulated FHB. The non-preservatives and nisin-containing FHB showed inhibition against Gram-positive pathogens. The FHB treated with E. faecium OV3-6 was rich in lactic acid bacteria, and the product was at an acceptable level of pH (less than 4.3). Certain limitations were identified in the study, such as lack of nutritional, metabolomics analysis, and safety and consumer acceptability of FHB. The results suggested that E. faecium OV3-6 could be used as a bio-preservative to produce fermented plant beverages (FPBs).

Phenotypic and genomic analyses of bacteriocin-producing probiotic Enterococcus faecium EFEL8600 isolated from Korean soy-meju

Enterococcus faecium is a prevalent species found in fermented soybean products, known for its contributions to flavor development and inhibition of pathogenic microorganisms during fermentation. This study aims to provide comprehensive phenotypic and genomic evidence supporting the probiotic characteristics of E. faecium EFEL8600, a bacteriocin-producing strain isolated from Korean soy-meju. Phenotypic analysis revealed that EFEL8600 produced a peptide with inhibitory activity against Listeria monocytogenes, estimated to be 4.6 kDa, corresponding to the size of enterocins P or Q. Furthermore, EFEL8600 exhibited probiotic traits, such as resilience in gastrointestinal conditions, antioxidant and anti-inflammatory activities, and protection of the intestinal barrier. Safety assessments demonstrated no hemolytic and bile salt deconjugation activities. Genomic analysis revealed the presence of several genes associated with probiotic characteristics and bacteriocin production, while few deleterious genes with a low likelihood of expression or transferring were detected. Overall, this study highlights E. faecium EFEL8600 as a potent anti-listeria probiotic strain suitable for use as a starter culture in soymilk fermentation, providing potential health benefits to consumers.

In Vitro Synergistic Antibacterial and Anti-Inflammatory Effects of Nisin and Lactic Acid in Yogurt against Helicobacter pylori and Human Gastric Cells

Helicobacter pylori is a bacterium that naturally thrives in acidic environments and has the potential to induce various gastrointestinal disorders in humans. The antibiotic therapy utilized for treating H. pylori can lead to undesired side effects, such as dysbiosis in the gut microbiota. The objective of our study was to explore the potential antibacterial effects of nisin and lactic acid (LA) in yogurt against H. pylori. Additionally, we investigated the anti-inflammatory effects of nisin and LA in human gastric (AGS) cells infected with H. pylori. Nisin and LA combination showed the strongest inhibitory activity, with confirmed synergy at 0.375 fractional inhibitory concentration index. Also, post-fermented yogurt with incorporation of nisin exhibited antibacterial effect against H. pylori. The combination of nisin and LA resulted in a significant reduction of mRNA levels of bacterial toxins of H. pylori and pro-inflammatory cytokines in AGS cells infected with H. pylori. Furthermore, this also increased bacterial membrane damage, which led to DNA and protein leakage in H. pylori. Overall, the combination of nisin and LA shows promise as an alternative therapy for H. pylori infection. Additionally, the incorporation of nisin into foods containing LA presents a potential application. Further studies, including animal research, are needed to validate these findings and explore clinical applications.

The Circular Bacteriocin enterocin NKR-5-3B has an Improved Stability Profile over Nisin

Bacteriocins are a large family of bacterial peptides that have antimicrobial activity and potential applications as clinical antibiotics or food preservatives. Circular bacteriocins are a unique class of these biomolecules distinguished by a seamless circular topology, and are widely assumed to be ultra-stable based on this constraining structural feature. However, without quantitative studies of their susceptibility to defined thermal, chemical, and enzymatic conditions, their stability characteristics remain poorly understood, limiting their translational development. Here, we produced the circular bacteriocin enterocin NKR-5-3B (Ent53B) in mg/L quantities using a heterologous Lactococcus expression system, and characterized its thermal stability by NMR, chemical stability by circular dichroism and analytical HPLC, and enzymatic stability by analytical HPLC. We demonstrate that Ent53B is ultra-stable, resistant to temperatures approaching boiling, acidic (pH 2.6) and alkaline (pH 9.0) conditions, the chaotropic agent 6M urea, and following incubation with a range of proteases (i.e., trypsin, chymotrypsin, pepsin, and papain), conditions under which most peptides and proteins degrade. Ent53B is stable across a broader range of pH conditions and proteases than nisin, the most widely used bacteriocin in food manufacturing. Antimicrobial assays showed that differences in stability correlated with differences in bactericidal activity. Overall, this study provides quantitative support for circular bacteriocins being an ultra-stable class of peptide molecules, suggesting easier handling and distribution options available to them in practical applications as antimicrobial agents.

Combination Treatment for Inhibition of the Growth of Staphylococcus aureus with Recombinant SAP8 Endolysin and Nisin

Staphylococcus aureus, a pathogenic species of genus Staphylococcus involved in foodborne illness always remain among the top priorities of the world major concerns. In the present study, we have used recombinant SAP8 endolysin from the bacteriophage SAP8 and commercial nisin to inhibit the viability of pathogenic S. aureus KCTC 3881 cells; however, the approach was not identified as cost-effective. A gradual decrease in the viable S. aureus KCTC 3881 cell counts was observed with an increase in the concentrations of recombinant SAP8 endolysin and nisin. However, combined treatment with recombinant SAP8 endolysin and nisin decreased the viable S. aureus KCTC 3881 cell counts in a significant manner. The combination of 0.01 µM of recombinant SAP8 endolysin with 9 IU/mL and 18 IU/mL of nisin demonstrated a promising decrease in the viable cell counts of the strain. Under the scanning electron microscope, the combination treatment with 0.01 µM of recombinant SAP8 endolysin and 18 IU/mL of nisin showed complete cellular destruction of S. aureus KCTC 3881. We propose that a combination of recombinant SAP8 endolysin and nisin could be a strong alternative to antibiotics to control the growth of S. aureus including MRSA.

LssR plays a positive regulatory role in acid and nisin tolerance response of Lactococcus lactis

In Lactococcus lactis, different regulation mechanisms can be activated to overcome the effects of adverse environmental stresses. Here, a TetR family regulator LssR was demonstrated as a positive regulator in the activation of the mechanisms involved in acid and nisin tolerance of L. lactis. The deletion of lssR led to the reduction of tolerance of L. lactis NZ9000 to nisin and acid stress, and the survival rates of NZ9000 under nisin and acid stress were roughly 20-fold, 10-fold (pH 3.0, hydrochloric acid), and 8.9-fold (pH 4.0, lactic acid) of the lssR mutant NZΔlssR, respectively. Moreover, the lssR mutant NZΔlssR also displayed a lower intracellular pH stability and a changed cell surface morphology. Subsequently, transcriptome analysis revealed that genes related to the arginine deiminase pathway, the surface polysaccharides biosynthesis, carbohydrates transport and metabolism, multidrug resistance, cell repair proteins and chaperones were predominantly down transcribed in NZΔlssR. The transcript levels of the arginine deiminase pathway and the surface polysaccharides biosynthesis-associated genes under acid and nisin stresses were compared between the wild type NZ9000 and NZΔlssR using real-time fluorescence quantitative PCR. It revealed that the arginine deiminase pathway genes (arcD1C1C2T) and the surface polysaccharides biosynthesis genes (cgT, gmhB, gmhA, hddA, tagH and tarS) were proposed to be the main regulatory mechanisms of LssR in response to the acid and nisin stresses. Overall, the important role of LssR in the acid and nisin stresses response was demonstrated and the putative regulation mechanism of LssR was revealed.

Designing a new alginate-fibrinogen biomaterial composite hydrogel for wound healing

Wound healing is a complex process and rapid healing necessitates a proper micro-environment. Therefore, design and fabrication of an efficacious wound dressing is an impressive innovation in the field of wound healing. The fabricated wound dressing in this scenario was designed using a combination of the appropriate coagulating and anti-bacterial materials like fibrinogen (as coagulating agent), nisin (as anti-bacterial agent), ethylenediaminetetraacetic acid (as anti-bacterial agent), and alginate (as wound healing agent). Biophysical characterization showed that the interaction of fibrinogen and alginate was associated with minor changes in the secondary structure of the protein. Conformational studies showed that the protein was structurally stable at 42 °C, is the maximum temperature of the infected wound. The properties of the hydrogel such as swelling, mechanical resistance, nisin release, antibacterial activity, cytotoxicity, gel porosity, and blood coagulation were assessed. The results showed a slow release for the nisin during 48 h. Antibacterial studies showed an inhibitory effect on the growth of Gram-negative and Gram-positive bacteria. The hydrogel was also capable to absorb a considerable amount of water and provide oxygenation as well as incorporation of the drug into its structure due to its sufficient porosity. Scanning electron microscopy showed pore sizes of about 14–198 µm in the hydrogel. Cell viability studies indicated high biocompatibility of the hydrogel. Blood coagulation test also confirmed the effectiveness of the synthesized hydrogel in accelerating the process of blood clot formation. In vivo studies showed higher rates of wound healing, re-epithelialization, and collagen deposition. According to the findings from in vitro as well as in vivo studies, the designed hydrogel can be considered as a novel attractive wound dressing after further prerequisite assessments.

A novel co-cultivation strategy to generate low-crystallinity bacterial cellulose and increase nisin yields

In this study, a co-culturing Enterobacter sp. and Lactococcus lactis strategy was developed to alter bacterial cellulose (BC) properties and increase nisin yields. We generated high nisin yields (6260 IU/mL) by altering inoculum ratios and inoculation times in a novel co-culture system. Critically, these were 85% higher than L. lactis monocultures. By monitoring fermentation broth pH and lactic acid yields, the pH was higher and lactic acid yields lower during co-culture conditions when compared with L. lactis monocultures, suggesting that co-culturing was more suitable for L. lactis nisin production. We also determined BC film yields and properties (BC, BC-N, and BC-N after nisin release). BC yields produced by co-culturing were not very different from Enterobacter sp. monocultures, but crystallinity was significantly altered. Collectively, our co-culture system adequately and economically modified BC fibers by interfering with self-assembly and crystallization processes during BC synthesis, with significantly improved nisin yields.

Probiotic Effects and Metabolic Products of Enterococcus faecalis LD33 with Respiration Capacity

Respiration metabolism could improve the long-term survival of lactic acid bacteria (LAB); however, its effect on potential probiotic traits of LAB was not reported. The difference made by Enterococcus faecalis LD33 that was cultured under respiration-permissive and fermentation conditions, such as the biomass, metabolites, antimicrobial activity, tolerance to acid and bile salt, adhesion capabilities, and the ability to inhibit the proliferation of cancer cells were studied. Under a respiration-permissive condition, the final biomass of the culture was about twice as compared to that of fermentation condition. When the metabolites were measured, glucose was exhausted within 8 h. Two-folds of acetic acid, triple of both acetoin and diacetyl, and less than half of lactic acid, were accumulated under the respiratory-permissive condition. No discrimination of growth inhibition on Salmonella enterica serovar Typhimurium ATCC 14028 and Shigella sonnei ATCC 25931 was observed when Enterococcus faecalis LD33 was cultured under both conditions; however, under respiration-permissive condition, the strain presented significant antimicrobial activities to Listeria monocytogenes ATCC19111 and Staphylococcus aureus ATCC6538P. Enterococcus faecalis LD33 displayed relatively strong bile salt tolerance and adherence capability but weaker acid tolerance when undergoing respiration metabolism. There was no significant difference in the anti-cancer effect of the viable bacterial cells on both growth modes; however, the supernatant showed a higher inhibition effect on HT-29 cells than the live bacteria, and there was no significant difference between the supernatant and the 5-Fluorouracil (7 μg/mL). Consequently, the Enterococcus faecalis LD33 undergoing respiration metabolism could bring higher biomass, more flavor metabolites, and better antimicrobial and anti-cancer activities. This study extends our knowledge of respiratory metabolism in LAB and its impact on probiotic traits. E. faecalis LD33 qualifies as a suitable strain against foodborne pathogens, cancer therapy, and eventual application in the food and pharmaceutical industries.

Recombinant production of the lantibiotic nisin using Corynebacterium glutamicum in a two-step process

BACKGROUND

The bacteriocin nisin is naturally produced by Lactococcus lactis as an inactive prepeptide that is modified posttranslationally resulting in five (methyl-)lanthionine rings characteristic for class Ia bacteriocins. Export and proteolytic cleavage of the leader peptide results in release of active nisin. By targeting the universal peptidoglycan precursor lipid II, nisin has a broad target spectrum including important human pathogens such as Listeria monocytogenes and methicillin-resistant Staphylococcus aureus strains. Industrial nisin production is currently performed using natural producer strains resulting in rather low product purity and limiting its application to preservation of dairy food products.

RESULTS

We established heterologous nisin production using the biotechnological workhorse organism Corynebacterium glutamicum in a two-step process. We demonstrate successful biosynthesis and export of fully modified prenisin and its activation to mature nisin by a purified, soluble variant of the nisin protease NisP (sNisP) produced in Escherichia coli. Active nisin was detected by a L. lactis sensor strain with strictly nisin-dependent expression of the fluorescent protein mCherry. Following activation by sNisP, supernatants of the recombinant C. glutamicum producer strain cultivated in standard batch fermentations contained at least 1.25 mg/l active nisin.

CONCLUSIONS

We demonstrate successful implementation of a two-step process for recombinant production of active nisin with C. glutamicum. This extends the spectrum of bioactive compounds that may be produced using C. glutamicum to a bacteriocin harboring complex posttranslational modifications. Our results provide a basis for further studies to optimize product yields, transfer production to sustainable substrates and purification of pharmaceutical grade nisin.

Fabrication of zein-modified starch nanoparticle complexes via microfluidic chip and encapsulation of nisin

A microfluidic chip is a micro-reactor that precisely manipulates and controls fluids. Zein is a group of prolamines extracted from corn that can form self-assembled nanoparticles in water or a low concentration of ethanol in a microfluidic chip. However, the zein nanoparticles have stability issues, especially in a neutral pH environment due to the proximity of the isoelectric point. This study was designed 1) to evaluate the effect of octenyl succinic anhydride (OSA) modified starch on the stability of zein nanoparticles formed using a microfluidic chip and 2) to apply the zein-OSA starch for encapsulation of nisin and evaluate its anti-microbial activity in a model food matrix. A T-junction configuration of the microfluidic chip was used to fabricate the zein nanoparticles using 1% or 2% zein solution and 0–10% (w/w) of OSA starch solution. The stability of the nanoparticles in various ionic strength environments was assessed. Encapsulation efficiency and anti-microbial activity of nisin in the zein nanoparticles against Listeria monocytogenes in a fresh cheese were measured. As the concentration of OSA starch increased from 0 to 10%, effective diameter increased from 117.8 ± 14.5 to 198.7 ± 13.9 nm without affecting polydispersity indexes and zeta-potential changed toward that of the modified starch indicating the zein surface coverage by the OSA starch. The zein-OSA starch nanoparticle complexes were more stable at various sodium chloride concentrations than the zein nanoparticles without OSA starch. The encapsulation efficiency of nisin was positively correlated with the OSA starch concentration. The anti-microbial activity of nisin in the fresh cheese also increased until 3-days of storage as the concentration of the OSA starch increased, which presented both a potential and challenge toward applications.

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Microfluidic chip formed zein nanoparticles with OSA-modified starch.

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Zein nanoparticle size and stability were affected by zein and modified starch concentration.

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Nisin was encapsulated in the zein nanoparticles via microfluidic chip.

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Anti-microbial activity of nisin was improved by the encapsulation.

Production of nisin and lactic acid from the starch of sweet potato by simultaneous saccharification and fermentation with two stage pH adjustment

Nisin is an antimicrobial peptide which is widely used as preservative, while lactic acid is a natural organic acid applied in the food industry. The aim of this work was to study the process for nisin and lactic acid production from starch of sweet potato with simultaneous saccharification and fermentation (SSF) by Lactococcus lactis subsp. Lactis with two stage pH adjustment. The factors impacting the nisin and lactic acid production including starch concentration, glucosidase concentration, CaCO₃ and Tween-80 were studied. The nisin titre reached a high of 2516.41 IU/mL, while the lactic acid reached a high of 37.06 g/L when the optimal conditions were 40 g/L starch, 100 U glucosidase/g starch, 2.5% CaCO₃ and 1 mL/L Tween-80. The lactic acid and nisin were separated by a two stage pH adjustment at last. The SSF of starch from sweet potato coupled with a two stage pH adjustment is a promising method to produce nisin and lactic acid.

Adaptive Evolution of Lactococcus Lactis to Thermal and Oxidative Stress Increases Biomass and Nisin Production

High values of agitation and temperature lead to stressful conditions in the fermentations of Lactococcus lactis due to its aero-tolerant and mesophilic nature. Here, the adaptive laboratory evolution (ALE) technique was applied to increase biomass and nisin production yields by enhancing L. lactis subsp. lactis robustness at higher growth temperature and aeration rates. In two separate ALE experiments, after 162 serial transfers, optimum agitation and growth temperature of L. lactis were shifted from 40 rpm and 30 °C to 200 rpm and 37 °C, respectively. Oxidative and acid resistance were enhanced in the evolved strain. Whole-genome sequencing revealed the emergence of five single-nucleotide polymorphisms in the genome of the evolved strain in jag, DnaB, ArgR, cation transporter genes, and one putative protein. The evolved strain of L. lactis in this study has more industrial desirable features and improved nisin production capability and can act more efficiently in nisin production in stressful conditions.

Epigallocatechin gallate and Lactobacillus plantarum culture supernatants exert bactericidal activity and reduce biofilm formation in Clostridium perfringens

Clostridium perfringens forms biofilms and spores that are a source of food contamination. In this study, the antibacterial activities of Lactobacillus plantarum culture supernatants (LP-S), LP-S fractions, and the plant-derived compound epigallocatechin gallate (EG) were evaluated. Specifically, their effects on the viability and biofilm-forming ability of C. perfringens were assessed. Moreover, the expression of quorum sensing-regulated genes associated with the pathogenesis of this microorganism and that of genes involved in biofilm formation was also investigated. The results showed that both EG and the LP-S exerted bactericidal activity against all C. perfringens strains tested. The minimal bactericidal concentration (MBC) of EG was 75 µg/mL for all strains but ranged from 61 to 121 µg of total protein per mL for LP-S. EG exerted only minor effects on biofilm formation, whereas LP-S, particularly its 10 and 30 K fractions, significantly reduced the biofilm-forming ability of all the strains. The antibiofilm activity of LP-S was lost following preincubation with proteases, suggesting that it was mediated by a proteinaceous molecule. The treatment of C. perfringens with either EG or LP-S did not change the transcript levels of two CpAL (C. perfringens quorum-sensing Agr-like system)-related genes, agrB and agrD, which are known to be involved in the regulation of biofilms, suggesting that LP-S exerted its biofilm inhibitory activity downstream of CpAL signaling. In summary, we demonstrated the bactericidal activity of EG and LP-S against C. perfringens and antibiofilm activity of LP-S at a subinhibitory dose. Our results suggested that these compounds can be further explored for food safety applications to control agents such as C. perfringens.

ComX improves acid tolerance by regulating the expression of late competence proteins in Lactococcus lactis F44

ComX can improve bacterial competence by modulating global gene expression. Although competence induction may also be a protective mechanism under stress, this has not been investigated in detail. Here, we demonstrated that ComX improved the acid tolerance and nisin yield of Lactococcus lactis, which is an important gram-positive bacterium increasingly used in modern biotechnological applications. We found that overexpression of comX could improve the survival rate up to 36.5% at pH 4.0, compared with only 5.4% and 1.1% with the wild-type and comX knockout strains, respectively. Moreover, quantitative real-time PCR results indicated that comX overexpression stimulated the expression of late competence genes synergistically with exposure to acid stress. Finally, electrophoretic mobility shift assay demonstrated the binding of purified ComX to the cin-box in the promoters of these genes. Taken together, our results reveal a regulation mechanism by which ComX and acid stress can synergistically modulate the expression of late competence genes to enhance cells' acid tolerance and nisin yield.

Inhibitory Activity of Natural Synergetic Antimicrobial Consortia Against Salmonella enterica on Broiler Chicken Carcasses

The currently most utilized antimicrobial agent in poultry processing facilities is peracetic acid, a chemical increasingly recognized as hazardous to human health. We evaluated the efficacy of mixtures of natural antimicrobial compounds, namely reuterin, microcin J25, and lactic acid, for reducing the viability of Salmonella enterica and total aerobes on broiler chicken carcasses. The compounds were compared singly and in combination with water and 0.1% peracetic acid. The minimum inhibitory concentrations of reuterin, lactic acid, and microcin J25 against S. enterica serovar Enteritidis were respectively 2 mM, 0.31%, and 0.03 μM. In vitro, the combinations of reuterin + lactic acid and reuterin + microcin J25 were synergic, making these compounds effective at four times lower concentrations than those used alone. Salmonella viable counts fell to zero within 10 min of contact with reuterin + lactic acid at 10 times the concentrations used in combination, compared to 18 h in the case of reuterin + microcin J25. Sprayed onto chilled chicken carcasses, this reuterin + lactic acid mixture reduced Salmonella spp. counts by 2.02 Log CFU/g, whereas reuterin + microcin J25 and peracetic acid reduced them by respectively 0.83 and 1.13 Log CFU/g. The synergy of reuterin with lactic acid or microcin J25 as inhibitors of bacterial growth was significant. Applied as post-chill spray, these mixtures could contribute to food safety by decreasing Salmonella counts on chicken carcasses.

Efficient Production of Nisin A from Low-Value Dairy Side Streams Using a Nonengineered Dairy Lactococcus lactis Strain with Low Lactate Dehydrogenase Activity

Nisin is commonly used as a biopreservative in foods. For industrial production, nisin-producing Lactococcus lactis strains are usually grown to high cell densities to achieve the highest possible nisin titer. However, accumulation of lactic acid eventually halts production, even in pH-controlled fermentations. Here, we describe a nisin-producing L. lactis strain Ge001, which was obtained after transferring the nisin gene cluster from L. lactis ATCC 11454, by conjugation, into the natural mutant L. lactis RD1M5, with low lactate dehydrogenase activity. The ability of Ge001 to produce nisin was tested using dairy waste as the fermentation substrate. To accommodate redox cofactor regeneration, respiration conditions were used, and to alleviate oxidative stress and to reduce adsorption of nisin onto the producing cells, we found it to be beneficial to add 1 mM Mn²⁺ and 100 mM Ca²⁺, respectively. A high titer of 12 084 IU/mL nisin could be reached, which is comparable to the highest titers reported using expensive, rich media. Summing up, we here present a 100% natural, robust, and sustainable approach for producing food-grade nisin and acetoin from readily available dairy waste.

Radiotherapy and the gut microbiome: facts and fiction

An ever-growing body of evidence has linked the gut microbiome with both the effectiveness and the toxicity of cancer therapies. Radiotherapy is an effective way to treat tumors, although large variations exist among patients in tumor radio-responsiveness and in the incidence and severity of radiotherapy-induced side effects. Relatively little is known about whether and how the microbiome regulates the response to radiotherapy. Gut microbiota may be an important player in modulating “hot” versus “cold” tumor microenvironment, ultimately affecting treatment efficacy. The interaction of the gut microbiome and radiotherapy is a bidirectional function, in that radiotherapy can disrupt the microbiome and those disruptions can influence the effectiveness of the anticancer treatments. Limited data have shown that interactions between the radiation and the microbiome can have positive effects on oncotherapy. On the other hand, exposure to ionizing radiation leads to changes in the gut microbiome that contribute to radiation enteropathy. The gut microbiome can influence radiation-induced gastrointestinal mucositis through two mechanisms including translocation and dysbiosis. We propose that the gut microbiome can be modified to maximize the response to treatment and minimize adverse effects through the use of personalized probiotics, prebiotics, or fecal microbial transplantation. 16S rRNA sequencing is the most commonly used approach to investigate distribution and diversity of gut microbiome between individuals though it only identifies bacteria level other than strain level. The functional gut microbiome can be studied using methods involving metagenomics, metatranscriptomics, metaproteomics, as well as metabolomics. Multiple ‘-omic’ approaches can be applied simultaneously to the same sample to obtain integrated results. That said, challenges and remaining unknowns in the future that persist at this time include the mechanisms by which the gut microbiome affects radiosensitivity, interactions between the gut microbiome and combination treatments, the role of the gut microbiome with regard to predictive and prognostic biomarkers, the need for multi “-omic” approach for in-depth exploration of functional changes and their effects on host-microbiome interactions, and interactions between gut microbiome, microbial metabolites and immune microenvironment.

Bio-synthesis of food additives and colorants-a growing trend in future food

Food additives and colorants are extensively used in the food industry to improve food quality and safety during processing, storage and packing. Sourcing of these molecules is predominately through three means: extraction from natural sources, chemical synthesis, and bio-production, with the first two being the most utilized. However, growing demands for sustainability, safety and "natural" products have renewed interest in using bio-based production methods. Likewise, the move to more cultured foods and meat alternatives requires the production of new additives and colorants. The production of bio-based food additives and colorants is an interdisciplinary research endeavor and represents a growing trend in future food. To highlight the potential of microbial hosts for food additive and colorant production, we focus on current advances for example molecules based on their utilization stage and bio-production yield as follows: (I) approved and industrially produced with high titers; (II) approved and produced with decent titers (in the g/L range), but requiring further engineering to reduce production costs; (III) approved and produced with very early stage titers (in the mg/L range); and (IV) new/potential candidates that have not been approved but can be sourced through microbes. Promising approaches, as well as current challenges and future directions will also be thoroughly discussed for the bioproduction of these food additives and colorants.

Harnessing biomolecules for bioinspired dental biomaterials

Dental clinicians have relied for centuries on traditional dental materials (polymers, ceramics, metals, and composites) to restore oral health and function to patients. Clinical outcomes for many crucial dental therapies remain poor despite many decades of intense research on these materials. Recent attention has been paid to biomolecules as a chassis for engineered preventive, restorative, and regenerative approaches in dentistry. Indeed, biomolecules represent a uniquely versatile and precise tool to enable the design and development of bioinspired multifunctional dental materials to spur advancements in dentistry. In this review, we survey the range of biomolecules that have been used across dental biomaterials. Our particular focus is on the key biological activity imparted by each biomolecule toward prevention of dental and oral diseases as well as restoration of oral health. Additional emphasis is placed on the structure-function relationships between biomolecules and their biological activity, the unique challenges of each clinical condition, limitations of conventional therapies, and the advantages of each class of biomolecule for said challenge. Biomaterials for bone regeneration are not reviewed as numerous existing reviews on the topic have been recently published. We conclude our narrative review with an outlook on the future of biomolecules in dental biomaterials and potential avenues of innovation for biomaterial-based patient oral care.

NisI Maturation and Its Influence on Nisin Resistance in Lactococcus lactis

NisI confers immunity against nisin, with high substrate specificity to prevent a suicidal effect in nisin-producing Lactococcus lactis strains. However, the NisI maturation process as well as its influence on nisin resistance has not been characterized. Here, we report the roles of lipoprotein signal peptidase II (Lsp) and prolipoprotein diacylglyceryl transferase (Lgt) in NisI maturation and nisin resistance of L. lactis F44. We found that the resistance of nisin of an Lsp-deficient mutant remarkably decreased, while no significant differences in growth were observed. We demonstrated that Lsp could cleave signal peptide of NisI precursor in vitro Moreover, diacylglyceryl modification of NisI catalyzed by Lgt played a decisive role in attachment of NisI on the cell envelope, while it exhibited no effects on cleavage of the signal peptides of NisI precursor. The dissociation constant (K_D ) for the interaction between nisin and NisI exhibited a 2.8-fold increase compared with that between nisin and pre-NisI with signal peptide by surface plasmon resonance (SPR) analysis, providing evidence that Lsp-catalyzed signal peptide cleavage was critical for the immune activity of NisI. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.IMPORTANCE Nisin, a safe and natural antimicrobial peptide, has a long and impressive history as a food preservative and is also considered a novel candidate to alleviate the increasingly serious threat of antibiotic resistance. Nisin is produced by certain L. lactis strains. The nisin immunity protein NisI, a membrane-bound lipoprotein, is expressed by nisin producers to avoid suicidal action. Here, we report the roles of Lsp and Lgt in NisI maturation and nisin resistance of L. lactis F44. The results verified the importance of Lsp to NisI-conferred immunity and Lgt to localization. Our study revealed the process of NisI maturation in L. lactis and presented a potential strategy to enhance industrial nisin production.

Phthalyl starch nanoparticles as prebiotics enhanced nisin production in Lactococcus lactis through the induction of mild stress in probiotics

AIM OF THE STUDY

Effect of internalized phthalyl starch nanoparticles (PSNs) on the antimicrobial ability of Lactococcus lactis (LL) KCTC 2013.

METHODS AND RESULTS

Phthalyl starch nanoparticles were prepared by self-assembly of phthalyl starch and the amount of the hydrophobic phthalic moieties were characterized by nuclear magnetic resonance: PSN1 (DS: 14·3 mol.%), PSN2 (DS: 17·8 mol.%) and PSN3 (DS: 30·4 mol.%). The sizes of PSN1, PSN2 and PSN3 measured by dynamic light scattering were 364·7, 248·4 and 213·4 nm, respectively, and the surface charges of PSNs measured by electrophoretic light scattering were negative charges and PSNs were spherical in shape according to scanning electron microscope. It was found that when PSNs were treated with LL, the PSNs were internalized into LL through nanoparticle size-, energy- and glucose transporter-dependent mechanisms. The internalization was confirmed by confocal laser scanning microscopy and fluorescence-activated cell sorting. Nisin was isolated and identified by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Also, more nisin was produced from PSNs-treated LL than untreated- or starch-treated LL. Co-culture assay and agar diffusion test were performed to test the antimicrobial ability. Antimicrobial ability against Gram-negative Escherichia coli k88, Salmonella gallinarum and Gram-positive Listeria monocytogenes of LL treated with PSNs was higher than that of untreated or starch-treated group. Finally, it was found that the expression level of stress response genes dnaK, dnaJ and groES was significantly higher in PSNs-treated groups compared with starch-treated group or LL alone.

CONCLUSION

The internalization of PSNs into LL enhanced the production of nisin through mild intracellular stimulation, resulting in enhanced antimicrobial ability.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows the promising potential of PSNs as new prebiotics for increasing the production of nisin, thus demonstrating a new method for the biological production of such antimicrobial peptides.

Nisin M: a Bioengineered Nisin A Variant That Retains Full Induction Capacity but Has Significantly Reduced Antimicrobial Activity

Nisin A is a potent antimicrobial with potential as an alternative to traditional antibiotics, and a number of genetically modified variants have been created that target clinically relevant pathogens. In addition to antimicrobial activity, nisin autoregulates its own production via a signal transduction pathway, a property that has been exploited in a protein expression system termed the nisin-controlled gene expression (NICE) system. Although NICE has become one of the most popular protein expression systems, one drawback is that the inducer peptide, nisin A, also has inhibitory activity. It has already been demonstrated that the N-terminal region of nisin A contributes to antimicrobial activity and signal transduction properties; therefore, we conducted bioengineering of nisin at positions Pro9 and Gly10 within ring B to produce a bank of variants that could potentially be used as alternative induction peptides. One variant, designated nisin M, has threonines at positions 9 and 10 and retains induction capacity comparable to that of wild-type nisin A, while most of the antimicrobial activity is abolished. Further analysis confirmed that nisin M produces a mix of peptides as a result of different degrees of dehydration of the two threonines. We show that nisin M exhibits potential as a more suitable alternative to nisin A for the expression of proteins that may be difficult to express or for production of proteins in strains that are sensitive to wild-type nisin. Moreover, it may address the increasing demand by industry for optimization of peptide fermentations to increase yields or production rates.IMPORTANCE This study describes the generation of a nisin variant with superior characteristics for use in the NICE protein expression system. The variant, termed nisin M, retains an induction capacity comparable to that of wild-type nisin A but exhibits significantly reduced antimicrobial activity and can therefore be used at concentrations that are normally toxic to the expression host.

d-Methionine and d-Phenylalanine Improve Lactococcus lactis F44 Acid Resistance and Nisin Yield by Governing Cell Wall Remodeling

Lactococcus lactis encounters various environmental challenges, especially acid stress, during its growth. The cell wall can maintain the integrity and shape of the cell under environmental stress, and d-amino acids play an important role in cell wall synthesis. Here, by analyzing the effects of 19 different d-amino acids on the physiology of L. lactis F44, we found that exogenously supplied d-methionine and d-phenylalanine increased the nisin yield by 93.22% and 101.29%, respectively, as well as significantly increasing the acid resistance of L. lactis F44. The composition of the cell wall in L. lactis F44 with exogenously supplied d-Met or d-Phe was further investigated via a vancomycin fluorescence experiment and a liquid chromatography-mass spectrometry assay, which demonstrated that d-Met could be incorporated into the fifth position of peptidoglycan (PG) muropeptides and d-Phe could be added to the fourth and fifth positions. Moreover, overexpression of the PG synthesis gene murF further enhanced the levels of d-Met and d-Phe involved in PG and increased the survival rate under acid stress and the nisin yield of the strain. This study reveals that the exogenous supply of d-Met or d-Phe can change the composition of the cell wall and influence acid tolerance as well as nisin yield in L. lactis IMPORTANCE As d-amino acids play an important role in cell wall synthesis, we analyzed the effects of 19 different d-amino acids on L. lactis F44, demonstrating that d-Met and d-Phe can participate in peptidoglycan (PG) synthesis and improve the acid resistance and nisin yield of this strain. murF overexpression further increased the levels of d-Met and d-Phe incorporated into PG and contributed to the acid resistance of the strain. These findings suggest that d-Met and d-Phe can be incorporated into PG to improve the acid resistance and nisin yield of L. lactis, and this study provides new ideas for the enhancement of nisin production.

Isolation and application of bacteriophages alone or in combination with nisin against planktonic and biofilm cells of Staphylococcus aureus

Staphylococcus aureus is a notorious foodborne pathogen since it has ability to produce variety of toxins including heat-stable enterotoxin, form biofilm, and acquire resistance to antibiotics. Biocontrol of foodborne pathogens by lytic bacteriophages garners increasing interest from both researchers and food industry. In the present study, 29 phages against S. aureus were successfully isolated from chicken, pork, and fish. Characterization of the isolates revealed that phage SA46-CTH2 belonging to Podoviridae family had a number of features suitable for food industry applications such as wide host range, short latent period, large burst size, high stress tolerance, and a genome free of virulence genes. Furthermore, phage SA46-CTH2 alone or in combination with nisin exhibited great efficacy in reducing planktonic and biofilm cells of S. aureus at various conditions tested. The combination of phage SA46-CTH2 and nisin was also found to be able to inhibit the regrowth of S. aureus at both 37 and 24 °C.Key points• A total of 29 S. aureus phages were successfully isolated from fish, pork, and chicken products. • Phage SA46-CTH2 was characterized by host range, morphology, and genome sequencing. • SA46-CTH2 significantly reduced both planktonic and biofilm cells of S. aureus. • Combination of SA46-CTH2 and nisin inhibited the regrowth of S. aureus.

Antimicrobials for food and feed; a bacteriocin perspective

Bacteriocins are natural antimicrobials that have been consumed via fermented foods for millennia and have been the focus of renewed efforts to identify novel bacteriocins, and their producing microorganisms, for use as food biopreservatives and other applications. Bioengineering bacteriocins or combining bacteriocins with multiple modes of action (hurdle approach) can enhance their preservative effect and reduces the incidence of antimicrobial resistance. In addition to their role as food biopreservatives, bacteriocins are gaining credibility as health modulators, due to their ability to regulate the gut microbiota, which is strongly associated with human wellbeing. Indeed the strengthening link between the gut microbiota and obesity make bacteriocins ideal alternatives to Animal Growth Promoters (AGP) in animal feed also. Here we review recent advances in bacteriocin research that will contribute to the development of functional foods and feeds as a consequence of roles in food biopreservation and human/animal health.