The antimicrobial effects and synergistic antibacterial mechanism of the combination of ε-Polylysine and nisin against Bacillus subtilis

The synergistic antibacterial effect and inhibition of biofilm formation of nisin in combination with terpenes against Listeria monocytogenes

This study was to investigate the synergistic antibacterial effect and inhibition of biofilm formation of nisin in combination with terpenes (carvacrol, cinnamaldehyde, citral, and thymol) against Listeria monocytogenes. The bactericidal ranking of terpenes combined with nisin was carvacrol > cinnamaldehyde, citral > thymol. The minimum inhibitory concentration assay (MIC) of nisin and carvacrol when used together were determined to be 0.1563 mg/ml + 0.0195 mg/ml (nisin at MIC/2 + carvacrol at MIC/16). The addition of nisin at MIC/2 + carvacrol at MIC/2 caused more decrease in membrane potential than carvacrol or nisin at MIC individually. The decrease rates of hlyA and plcA gene expressions caused by nisin at MIC/2 + carvacrol at MIC/2 were significantly higher than those caused by carvacrol or nisin at MIC individually (P < 0.05). Nisin combined with carvacrol showed the highest inhibition activity to formation of L. monocytogenes biofilm on stainless steel and lettuce. The inhibition effect of nisin at MIC/2 + carvacrol at MIC/16 was significantly higher than that of nisin at MIC/2 and carvacrol at MIC/16 (P < 0.05).

Physical properties and antibacterial activity of the composited films based on carboxymethyl cellulose and gelatin functionalized with ε-polylysine

Antibacterial composite films were produced from carboxymethyl cellulose-gelatin (CMC-Gel) blend with different concentration of ε-polylysine (ε-PL) and their physical and chemical properties were characterized. Compared with the control CMC-Gel film, the functionalized films had almost indistinguishable crystalline type, thickness, tensile strength, and elongation at break, however, poor water vapor barrier properties. The results showed that the ε-PL was well incorporated into CMC-Gel matrix by electrostatic interaction, as the changes of absorption peaks in the Fourier transform infrared spectrometer and the increase of glass transition temperature in differential scanning calorimeter. The films containing ε-PL showed excellent antibacterial activity against S. aureus, B. subtilis, E. coli and P. aeruginosa. In the composting experiment, the films become degraded on the seventh day, and further degraded with the growth of molds over time. The present results showed that the active films could be a potential material for food packaging.

Buforin I an alternative to conventional antibiotics: Evaluation of the antimicrobial properties, stability, and safety

Cationic antimicrobial peptides are being developed as a promising class of antimicrobial sub-stances. The introduction of a new antibiotic component requires a comprehensive study of its properties so that it can be relied upon to continue laboratory procedures and clinical trials on laboratory animals or human volunteers. Antimicrobial activity of buforin I was evaluated against 15 of the most important pathogenic bacterial and fungal strains. This was followed by assessing anti-biofilm activity, time-dependent inhibitory, thermal stability, plas-ma stability, hemolysis, and cytotoxic activities. The range of obtained MICs was between 4 and 16 μg/mL. The most resistant and most sensitive microbial strains were S. salivarius and C. perfringens, respectively. Buforin I not only inhibited biofilm formation, but also showed a high biofilm radiation activity. Buforin I was stable in human plasma and also at different temperatures including 40, 60, and 80 °C. Although no significant anti-cancer properties were observed for buforin I, the lack of cytotoxicity as well as the lack of hemolytic activity confirm its safety. The high therapeutic index indicated that buforin I has a considerable pharmaceutical potential and can be a reasonable candidate to replace antibiotics or administered in combination with antibiotics to increase the effectiveness as well as reduce the dose of antibiotics.

Bacitracin-Ag Nanoclusters as a Novel Antibacterial Agent Combats Shigella flexneri by Disrupting Cell Membrane and Inhibiting Biofilm Formation

A novel nanomaterial Bacitracin-Ag Nanoclusters (Bacitracin-AgNCs) was formed to achieve a better antibacterial effect on Shigella flexneri which poses a serious threat to human health. In the current study, X-ray photoelectron spectrometer (XPS), Fourier transform infrared (FTIR), field-emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HR-TEM) and thermal gravimetric analysis (TGA) were used to characterize the properties of composited Bacitracin-AgNCs. Furthermore, the inhibitory effects of Bacitracin-AgNCs against S. flexneri were explored, and the inhibition mechanism was discussed in terms of its aspects of cell membrane ravage, ATPase activity decline and biofilm inhibition. The results reveal that the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of Bacitracin-AgNCs against S. flexneri were 0.03 mg/mL and 4 mg/mL. Bacitracin-AgNCs may cause irreversible impairment to cells and greatly change the cell morphology. The cell membrane integrity of S. flexneri was destroyed with changes in the characteristics of membrane permeability and intracellular substances leakage. Moreover, our study further proved that Bacitracin-AgNCs significantly inhibited the formation of S. flexneri biofilms and reduced the number of viable bacteria in biofilm. These findings provide a potential method for the exploitation of organic composite nanomaterials as a novel antimicrobial agent and its application in the food industry.

Visualizing Teixobactin Supramolecular Assemblies and Cell Wall Damage in B. Subtilis Using CryoEM

The antibiotic teixobactin targets bacterial cell walls. Previous research has proposed that the active form of teixobactin is a nano-/micron-sized supramolecular assembly. Here, we use cryogenic transmission electron microscopy to show that at 1 mg/mL, teixobactin forms sheet-like assemblies that selectively act upon the cell wall. At 4 μg/mL, teixobactin is active, and aggregates are formed either transiently or sparingly at the cell surface.

Fabrication and Characterization of Composite Biofilm of Konjac Glucomannan/Sodium Lignosulfonate/ε-Polylysine with Reinforced Mechanical Strength and Antibacterial Ability

In order to enforce the mechanical strength and antibacterial ability of biofilm and explore the underlying mechanism, sodium lignosulfonate (SL) and ε-polylysine (ε-PL) were introduced to fabricate the composite film of konjac glucomannan (KGM)/SL/ε-PL in the present study. According to our previous method, 1% (w/v) of KGM was the optimal concentration for the film preparation method, on the basis of which the amount of SL and ε-PL were screened by mechanical properties enforcement of film. The structure, mechanical performance and thermal stability of the film were characterized by SEM, FTIR, TGA and tensile strength tests. The optimized composite film was comprised of KGM 1% (w/v), SL 0.2% (w/v), and ε-PL 0.375% (w/v). The tensile strength (105.97 ± 4.58 MPa, p < 0.05) and elongation at break (95.71 ± 5.02%, p < 0.05) of the KGM/SL/ε-PL composite film was greatly improved compared with that of KGM. Meanwhile, the thermal stability and antibacterial property of film were also enhanced by the presence of SL and ε-PL. In co-culturation mode, the KGM/SL/ε-PL composite film showed good inhibitory effect on Escherichia coli (22.50 ± 0.31 mm, p < 0.05) and Staphylococcus aureus (19.69 ± 0.36 mm, p < 0.05) by determining the inhibition zone diameter. It was revealed that KGM/SL/ε-PL composite film shows enhanced mechanical strength and reliable antibacterial activities and it could be a potential candidate in the field of food packaging.

Juglone Inactivates Pseudomonas aeruginosa through Cell Membrane Damage, Biofilm Blockage, and Inhibition of Gene Expression

Due to the strong drug resistance of Pseudomonas aeruginosa (P. aeruginosa), the inhibition effects of conventional disinfectants and antibiotics are not obvious. Juglone extracted from discarded walnut husk, as a kind of plant-derived antimicrobial agent, has the advantages of naturalness, high efficiency, and low residue, with a potential role in the inhibition of P. aeruginosa. This study elucidated the inhibitory effect of juglone on the growth of plankton and the formation of P. aeruginosa biofilm. The results showed that juglone (35 μg/mL) had an irreversible inhibitory effect on P. aeruginosa colony formation (about 10⁷ CFU/mL). The integrity and permeability of the cell membrane were effectively destroyed, accompanied by disorder of the membrane permeability, mass leakage of the cytoplasm, and ATP consumption. Further studies manifested that juglone could induce the abnormal accumulation of ROS in cells and block the formation of the cell membrane. In addition, RT-qPCR showed that juglone could effectively block the expression of five virulence genes and two genes involved in the production of extracellular polymers, thereby reducing the toxicity and infection of P. aeruginosa and preventing the production of extracellular polymers. This study can provide support for the innovation of antibacterial technology toward P. aeruginosa in food.

A Synergistic New Approach Toward Enhanced Antibacterial Efficacy via Antimicrobial Peptide Immobilization on a Nitric Oxide-Releasing Surface

Despite technological advancement, nosocomial infections are prevalent due to the rise of antibiotic resistance. A combinatorial approach with multimechanistic antibacterial activity is desired for an effective antibacterial medical device surface strategy. In this study, an antimicrobial peptide, nisin, is immobilized onto biomimetic nitric oxide (NO)-releasing medical-grade silicone rubber (SR) via mussel-inspired polydopamine (PDA) as a bonding agent to reduce the risk of infection. Immobilization of nisin on NO-releasing SR (SR-SNAP-Nisin) and the surface characteristics were characterized by Fourier transform infrared spectroscopy and scanning electron microscopy with energy-dispersive X-ray spectroscopy and contact angle measurements. The NO release profile (7 days) and diffusion of SNAP from SR-SNAP-Nisin were quantified using chemiluminescence-based nitric oxide analyzers and UV-vis spectroscopy, respectively. Nisin quantification showed a greater affinity of nisin immobilization toward SNAP-doped SR. Matrix-assisted laser desorption/ionization mass spectrometry analysis on surface nisin leaching for 120 h under physiological conditions demonstrated the stability of nisin immobilization on PDA coatings. SR-SNAP-Nisin shows versatile in vitro anti-infection efficacy against Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus in the planktonic and adhered states. Furthermore, the combination of NO and nisin has a superior ability to impair biofilm formation on polymer surfaces. SR-SNAP-Nisin leachates did not elicit cytotoxicity toward mouse fibroblast cells and human umbilical vein endothelial cells, indicating the biocompatibility of the material in vitro. The preventative and therapeutic potential of SR-SNAP-Nisin dictated by two bioactive agents may offer a promising antibacterial surface strategy.

Fabrication of Bacterial Cellulose-Based Dressings for Promoting Infected Wound Healing

Bacterial cellulose (BC) holds several unique properties such as high water retention capability, flexibility, biocompatibility, and high absorption capacity. All these features make it a potential material for wound healing applications. However, it lacks antibacterial properties, which hampers its applications for infectious wound healings. This study reported BC-based dressings containing ε-polylysine (ε-PL), cross-linked by a biocompatible and mussel-inspired polydopamine (PDA) for promoting infectious wound healing. BC membranes were coated with PDA by a simple self-polymerization process, followed by treating with different contents of ε-PL. The resulted membranes showed strong antibacterial properties against tested bacteria by both in vitro and in vivo evaluations. The membranes also exhibited hemocompatibility and cytocompatibility by in vitro investigations. Moreover, the functionalized membranes promoted infected wound healing using Sprague-Dawley rats as a model animal. A complete wound healing was observed in the group treated with functionalized membranes, while wounds were still open for control and pure BC groups in the same duration. Histological investigations indicated that the thickness of newborn skin was greater and smoother in the groups treated with modified membranes in comparison to neat BC or control groups. These results revealed that the functionalized membranes have great potential as a dressing material for infected wounds in future clinical applications.

One-pot synthesis of magnetic self-assembled carrageenan-ε-polylysine composites: A reusable and effective antibacterial agent against Alicyclobacillus acidoterrestris

Epsilon-polylysine (EPL) is a cationic polymer with broad antibacterial activity. Base on this property, anionic carrageenan (CG) was self-assembled with EPL and introduced into the one-pot coprecipitation process to fabricate the magnetic Fe₃O₄-CG-EPL composites. To demonstrate the successful synthesis of composites, characterization techniques including XRD, FT-IR, TEM, XPS and VSM were employed. The prepared composites exhibited effective antibacterial activity against Alicyclobacillus acidoterrestris with the minimum inhibitory concentration of 10.0 mg mL^-1. Investigations into deciphering the antibacterial mechanism demonstrated that the presence of Fe₃O₄-CG-EPL caused irreversible damage to the cell membrane and serious leakage of intracellular protein, resulting in the inactivation of bacteria. The aim of this work is to develop a new effective control method for A. acidoterrestris in food field.

Hyaluronan/Poly-L-lysine/Berberine Nanogels for Impaired Wound Healing

Physiological wound healing process can be delayed in the presence of certain pathologies, such as diabetes or cancer. In this perspective, the aim of this study was to design a new nanogel platform of hyaluronan, poly-L-lysine and berberine suitable for wound treatment. Two different nanogel formulations were selected after a first formulation screening. They were prepared by adding dropwise 2 mg/mL hyaluronan aqueous solution (200 or 700 kDa) to 1.25 mg/mL poly-L-lysine aqueous solution. Blank nanogels formulated with 200 kDa HA resulted stable after freeze-drying with dimensions, polydispersity index and zeta potential of 263.6 ± 13.1 nm, 0.323 ± 0.029 and 32.7 ± 3.5 mV, respectively. Both blank and berberine-loaded nanogels showed rounded-shape structures. Loaded nanogels released nearly 50% of loaded berberine within 45 min, whereas the remaining 50% was released up to 24 h in vitro. Both, blank and berberine-loaded nanogels were able to completely close the fibroblasts gap in 42 h.

Evaluation of Antimicrobial Activity of Buforin I and Nisin and Synergistic Effect of the Combination of them as a Novel Antimicrobial Preservative

One of the most effective methods for increasing the antimicrobial activity of a substance is to combine it with one or more other antimicrobial agents. The aim of the present study was to evaluate the antimicrobial effect of buforin I and nisin alone and investigate the synergistic action of these compounds against the most important food spoilage microorganisms in clouding B. subtilis, S. epidermidis, L. innocua, E. coli, S. Enteritidis, A. oryzae, R. glutinis and G. candidum. The results of MIC and MBC/MFC examinations showed that buforin I had higher antimicrobial activity than nisin on all the microbial strains used in this study (p≤0.5). E.coli was the most resistant to both antimicrobial agents, while Listeria innocua and Staphylococcus epidermidis were the most sensitive to nisin and buforin I, respectively. The results of synergistic interaction between buforin I and nisin indicated that the combination of buforin I and nisin on B. subtilis, S. epidermidis and A. oryzae showed synergistic effect, while it had no effect on S. Enteritidis and Geotrichum candidum. The combination of buforin I and nisin showed partial synergistic effect on Listeria innocua, Escherichia coli, Rhodotorula glutinis. Assessment of viability of the microorganisms under the antimicrobial agents alone and in combination with each other at MICs and FICs indicated that use of these antimicrobial agents in combination enhances antimicrobial activity at lower concentrations of both agents. The present study investigated the antimicrobial properties of buforin I against food spoilage microorganisms for the first time and suggests that its use alone or in combination with nisin may provide a clear horizon for the application of antimicrobial peptides as natural preservatives. Thus, the combination of antimicrobial peptides and traditional antimicrobial food preservative could be a promising option for the prevention of contamination, spoilage, and infestation of food and beverage products.

Effects of ε-polylysine and rosemary extract on the quality of large yellow croaker (Pseudosciaena crocea) stored on ice at 4 ± 1°C

To evaluate the quality changes in large yellow croaker (Pseudosciaena crocea) with ε-polylysine and rosemary extract stored on ice at 4 ± 1°C. About 0.1% ε-polylysine (PL) and 0.2% rosemary extract (RE) were individually or in combination with each other were treated with samples. Samples treated with deionized water were regarded as control check (CK) group. Physicochemical (texture profile analysis (TPA), pH, total volatile basis nitrogen (TVB-N), thiobarbituric acid (TBA)), endogenous enzyme activity (cathepsin B and D), proteolytic degradation (Trichloroacetic Acid (TCA)-soluble peptides and Sodium Salt-polyacrylamide gel electrophoresis (SDS-PAGE)), microbiological (Total viable count (TVC), Shewanella bacteria count, Pseudomonas bacteria count, Psychrophilic bacteria count) and sensory evaluation were conducted during the whole storage. As a result, PL + RE could delay the increase in pH, TVB-N, TBA value, and improve the texture attributes compared with the CK group. In addition, PL + RE could inhibit cathepsin B and D activities, protein degradation, and microbial growth effectively. Moreover, the shelf life of samples could be prolonged at least 4 days when compared with the CK group according to the quality index method (QIM) and physicochemical assay, indicating that the PL + RE treatment could maintain the quality of large yellow croaker more effectively. PRACTICAL APPLICATIONS: The preservation of fish is becoming increasingly important in aquatic products. According to the fence theory, a combination of biopreservatives with different functions could be used to maintain the freshness synergistically. Furthermore, this research indicates that the combination of ε-polylysine and rosemary extract, a promising method for the preservation of aquatic products, could slow down the deterioration of large yellow croaker and prolong its shelf life.

Effects of cell physiological structure on the fermentation broth viscosity during poly-γ-glutamic acid production by Bacillus subtilis GXA-28

The high viscosity of fermentation broth limited the further improvement of PGA titer. Our previous studies indicated that adding KCl to the medium could decrease the fermentation broth viscosity and improve the PGA titer. In order to clarify the reason, effects of cell physiological structure on the fermentation broth viscosity were investigated. Results from cell morphology observation showed that the reduction of cell aggregation caused by the weakened cross-linking between PGA and cells might be an important reason for the decrease in the fermentation broth viscosity. Besides, when 201.2 mM KCl was added to the medium, the zeta potential of cell surface decreased from - 70.48 ± 3.35 mV to - 81 ± 2.46 mV. The cell membrane integrity was reduced and permeability was enhanced. Furthermore, the percentage of lauric acid C12:0 in cell membrane increased by 12.36%, but palmitic acid C16:0 and stearic acid C18:0 decreased by 6.83% and 5.64%, respectively, which improved the fluidity of cell membrane. The above changes in cell membrane further affect the cross-linking between PGA and cells, thereby playing an important role in reducing the fermentation broth viscosity. This study provided some novel information for understanding the decrease of PGA fermentation broth viscosity by KCl.

Highly Absorbent Antibacterial and Biofilm-Disrupting Hydrogels from Cellulose for Wound Dressing Applications

In this study, a carboxyl-modified cellulosic hydrogel was developed as the base material for wound dressings. ε-poly-l-lysine, a natural polyamide, was then covalently linked to the hydrogel through a bioconjugation reaction, which was confirmed by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR). The antibacterial efficacy of the hydrogel was tested against two model bacteria, Staphylococcus aureus and Pseudomonas aeruginosa, two of the most commonly found bacteria in wound infections. Bacterial viability and biofilm formation after exposure of bacteria to the hydrogels were used as efficacy indicators. Live/Dead assay was used to measure the number of compromised bacteria using a confocal laser scanning microscope. The results show that the antibacterial hydrogel was able to kill approximately 99% of the exposed bacteria after 3 h of exposure. In addition, NIH/3T3 fibroblasts were used to study the biocompatibility of the developed hydrogels. Water-soluble tetrazolium salt (WST)-1 assay was used to measure the metabolic activity of the cells and Live/Dead assay was used to measure the viability of the cells after 24, 48, and 72 h. The developed antibacterial hydrogels are light weight, have a high water-uptake capacity, and show high biocompatibility with the model mammalian cells, which make them a promising candidate to be used for wound dressing applications.

Ferulic Acid Inactivates Shigella flexneri through Cell Membrane Destructieon, Biofilm Retardation, and Altered Gene Expression

Antibiotic resistance and capacity for biofilm formation of Shigella flexneri render previous prevention and control strategies minimally effective. Ferulic acid (FA) has been demonstrated to be useful due to its application in foods as an alternative natural preservative. However, information regarding the S. flexneri phenotype and molecular responses to FA exposure is limited. The present study investigated the effects of FA on S. flexneri planktonic growth and biofilm formation. The results demonstrated that the cell membrane of S. flexneri in planktonic growth mode exhibited irreversible destruction after FA exposure, as characterized by decreased cell viability, leakage of cytoplasmic constituents, accelerated adenosine triphosphate (ATP) consumption, cell membrane depolarization, and cellular morphological changes. FA significantly inhibited S. flexneri adhesion and biofilm formation at a working concentration (1/8 MIC) that almost did not inhibit planktonic growth. Transcriptomics profiling showed that the exposure to a subinhibitory concentration of FA dramatically altered gene expression in the S. flexneri biofilm, as a total of 169 differentially expressed genes (DEGs) were upregulated and 533 DEGs were downregulated, compared to the intact biofilm. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis showed that the DEGs were mainly involved in pathways of ribosomes, ABC transporters, and the citrate cycle. Furthermore, we show that FA altered the transcription of S. flexneri genes associated with adhesion, transcriptional regulation, and the synthesis and transport of extracellular polymeric substances that contribute to biofilm formation. These data provide novel insights into S. flexneri behavioral responses to FA exposure and suggest that FA could effectively constrain S. flexneri and its biofilm formation.

Mechanism of the antimicrobial activity of whey protein-ε-polylysine complexes against Escherichia coli and its application in sauced duck products

ε-Polylysine (ε-PL) is a natural and highly effective cationic antimicrobial, of which antibacterial activity is limited in food matrix because of ε-PL's charged amino groups that form complexes with food polyanions. Whey protein-ε-PL complexes delivery system was found to be able to solve the problem and keep the antibacterial activity. This study investigated the antibacterial activity of the complexes and its mechanism against Escherichia coli. The minimal inhibitory concentration of ε-PL was in the range 11.72-25.00 g/mL for the complexes containing different amount of ε-PL and was similar to that of free ε-PL. The results of scanning electron microscopy showed that the complexes could destroy the structure of E. coli cell membrane surface, leaving holes on the surface of the bacteria, leading to the death of the bacteria. The molecular dynamics simulation results showed that the mechanism of the antibacterial activity of the complexes was as follows: under electrostatic interaction, the complexes captured the phospholipid molecules of the bacterial membrane through the hydrogen bonds between the positively charged amino groups of ε-PL and the oxygen atom of the phosphate head groups of the membrane, which could create holes on the surface of the bacteria and lead to the death of the bacteria. The results of activity on real food systems showed that the complexes kept the number of E. coli within 5.8 log₁₀ CFU/g after 7 d storage in sauced duck products, while the positive control (ε-PL) was 6.5 log₁₀ CFU/g and negative control (sterile water) was 7.8 log₁₀ CFU/g. Overall, this study confirmed the antibacterial activity of the complexes and provided fundamental knowledge of its antibacterial activity mechanism.

Antibacterial activity and mechanism of linalool against Pseudomonas aeruginosa

The purpose of this study was to evaluate the antibacterial activity and mechanism of linalool against Pseudomonas aeruginosa. The determination of antibacterial activity was based on the minimum inhibitory concentration (MIC) and the minimum bactericide concentration (MBC). Further, the antibacterial mechanism was explored by a growth curve assay, scanning electron microscopy (SEM), cell membrane permeability, membrane potential and respiratory chain dehydrogenase determination. The MIC and the MBC of linalool were 431 μg/mL and 862 μg/mL, respectively. The growth curve assay showed that the growth of P. aeruginosa was inhibited. The results of SEM revealed that linalool disrupted the normal morphology of the cell. The release of nucleic acids as well as the decrease in the membrane potential proved that the membrane integrity of P. aeruginosa was destroyed. Moreover, the respiratory chain was damaged by respiratory chain dehydrogenase determination as the absorbance at 490 nm decreased. This research suggested that it was possible for linalool to become a preservative of food by destroying the cell membrane, resulting in cell death.

Amyloid-like protein aggregates combining antifouling with antibacterial activity

A new class of biopolymer coating based on amyloid-like protein aggregates is reported to combine both antifouling and antibacterial activity.

ε-Polylysine Inhibits Shewanella putrefaciens with Membrane Disruption and Cell Damage

ε-Polylysine (ε-PL) was studied for the growth inhibition of Shewanella putrefaciens (S. putrefaciens). The minimal inhibitory concentration (MIC) of ε-PL against S. putrefaciens was measured by the broth dilution method, while the membrane permeability and metabolism of S. putrefaciens were assessed after ε-PL treatment. Additionally, growth curves, the content of alkaline phosphatase (AKP), the electrical conductivity (EC), the UV absorbance and scanning electron microscope (SEM) data were used to study cellular morphology. The impact of ε-PL on cell metabolism was also investigated by different methods, such as enzyme activity (peroxidase [POD], catalase [CAT], succinodehydrogenase [SDH] and malic dehydrogenase [MDH]) and cell metabolic activity. The results showed that the MIC of ε-PL against S. putrefaciens was 1.0 mg/mL. When S. putrefaciens was treated with ε-PL, the growth of the bacteria was inhibited and the AKP content, electrical conductivity and UV absorbance were increased, which demonstrated that ε-PL could damage the cell structure. The enzyme activities of POD, CAT, SDH, and MDH in the bacterial solution with ε-PL were decreased compared to those in the ordinary bacterial solution. As the concentration of ε-PL was increased, the enzyme activity decreased further. The respiratory activity of S. putrefaciens was also inhibited by ε-PL. The results suggest that ε-PL acts on the cell membrane of S. putrefaciens, thereby increasing membrane permeability and inhibiting enzyme activity in relation to respiratory metabolism and cell metabolism. This leads to inhibition of cell growth, and eventually cell death.

Antimicrobial Activity and Proposed Action Mechanism of 3-Carene against Brochothrix thermosphacta and Pseudomonas fluorescens

3-Carene is an antimicrobial monoterpene that occurs naturally in a variety of plants and has an ambiguous antibacterial mechanism against food-borne germs. The antibacterial effects and action mechanism of 3-carene against Gram-positive Brochothrix thermosphacta ACCC 03870 and Gram-negative Pseudomonas fluorescens ATCC 13525 were studied. Scanning electron microscopy (SEM) examination and leakage of alkaline phosphatase (AKP) verified that 3-carene caused more obvious damage to the morphology and wall structure of B. thermosphacta than P. fluorescens. The release of potassium ions and proteins, the reduction in membrane potential (MP), and fluorescein diacetate (FDA) staining further confirmed that the loss of the barrier function of the cell membrane and the leakage of cytoplasmic contents were due to the 3-carene treatment. Furthermore, the disorder of succinate dehydrogenase (SDH), malate dehydrogenase (MDH), pyruvate kinase (PK), and ATP content indicated that 3-carene could lead to metabolic dysfunction and inhibit energy synthesis. In addition, the results from the fluorescence analysis revealed that 3-carene could probably bind to bacterial DNA and affect the conformation and structure of genomic DNA. These results revealed that 3-carene had strong antibacterial activity against B. thermosphacta and P. fluorescens via membrane damage, bacterial metabolic perturbations, and genomic DNA structure disruption, interfering in cellular functions and even causing cell death.

Synergistic effect of natural antifungal agents for postharvest diseases of blackberry fruits

BACKGROUND

Blackberry postharvest diseases are caused by fungal pathogens, and treatment of fruits with edible, natural products could reduce the postharvest losses and contribute to food sustainability. Based on the hypothesis that inhibition of fungal pathogens will significantly extend the shelf-life of food products, the effects of natural antifungal agents on fungal pathogens were tested.

RESULTS

Two pathogenic fungal isolates, Aspergillus japonicus and Gilbertella persicaria, from infected blackberry fruits were identified morphologically using scanning electron microscopy and confirmed by DNA sequence analysis. The inhibitory effects and synergistic action of natural antifungal agents against the two fungal isolates were investigated. The results obtained demonstrated that the natamycin, chitosan and ferulic acid exhibited significant antifungal activities against the tested strains based on the calculated minimum inhibitory concentration. The best antifungal activity was obtained using a combination of ferulic acid and natamycin, which generated a total synergistic effect on both tested strains with a fractional inhibitory concentration index of 0.281. Application of the selected agents on postharvest blackberry fruits reduced the rot ratio and weight loss and also increased fruit firmness. In addition, the shelf-life of fresh blackberry fruits was extended up to 12-15 days at 4 °C and 90 ± 5% relative humidity.

CONCLUSION

The combined utilization of ferulic acid and natamycin showed synergistic antifungal activity against two pathogenic fungal isolates, and extended the shelf life of fresh blackberry fruits up to 12-15 days. © 2018 Society of Chemical Industry.

Shelf life extension of mozzarella cheese contaminated with Penicillium spp. using the antifungal compound ɛ-polylysine

Molds are one of the most important spoilage organisms on cheese which can lead to economic loss as well as raising public health concerns due to the production of mycotoxins. This study investigates the use of ɛ-polylysine as natural antimicrobial to inhibit fungal growth. The minimal inhibitory concentrations and minimal fungicidal concentrations of ɛ-polylysine were determined against Penicillium roqueforti, Penicillium nordicum, and Penicillium solitum. Then, polylysine was tested as surface antimicrobial for the preservation of mozzarella slice cheese inoculated with these Penicillium spp. and stored in plastic trays during 25 days. The minimal inhibitory concentrations calculated for the three fungi tested were of 60 mg/l whereas the minimal fungicidal concentrations detected were of 125-10,000 mg/l. The shelf life observed for the control experiments was of 15 days, and just using the ɛ-polylysine at 0.00625, 0.0125, and 0.025% was evidenced a shelf life increment in comparison with the control of 1-3 days.

Antibacterial Effect of Black Pepper Petroleum Ether Extract against Listeria monocytogenes and Salmonella typhimurium

The aim of the present study was to evaluate the antibacterial effect of black pepper petroleum ether extract (BPPE) against Listeria monocytogenes ATCC 19115 and Salmonella typhimurium ATCC 14028. The results showed that the BPPE had a strong antimicrobial activity against L. monocytogenes and S. typhimurium, and 2-methylene-4,8,8-trimethyl-4-vinyl-bicyclo[5.2.0]nonane (9.36%) and caryophyllene oxide (4.85%) were identified as the two primary components of BPPE. The ability of cells to break down hyperoxide was decreased, and the activities of POD and CAT were inhibited. The activities of key metabolic enzymes shed some light on the biochemical mechanism of aglycon cell growth inhibition, indicating that the energetic metabolism of L. monocytogenes and S. typhimurium was markedly influenced by the BPPE. The contents of key organic acids varied significantly, resulting in remarkable abnormalities in the energetic metabolism of L. monocytogenes and S. typhimurium. Thus, the consecution of energetic metabolism was destroyed by the BPPE, which contributed to metabolic dysfunction, the suppression of gene transcription, and cell death.

Development of antibacterial ε-polylysine/chitosan hybrid films and the effect on citrus

In this study, the ε-polylysine (PL)/chitosan (CS) composite films with the PL concentration of 0%, 1%, 3%, 5% and 7% were prepared via casting method. The Fourier transform infrared (FTIR), scanning electron microscope (SEM), mechanical and antibacterial properties of PL/CS composite films, as well as the effect of PL/CS coatings on citrus were observed. FTIR spectra suggested that there was no strong hydrogen bond between CS and PL. SEM images indicated that the film surface was smooth and the addition of PL to chitosan lead to more holes. The tensile strength was deceasing and the elongation at break of the films was elevating with the increase of PL content. The antibacterial activity against Staphylococcus aureus and Escherichia coli of PL/CS composite films was enhanced with increasing of PL content. Furthermore, it was found that the PL/CS coatings inhibited efficiently the decline of total soluble solids (TSS), ascorbic acid content and reduced the disease incidence of citrus, compared with the control. Also the disease incidence of citrus was decreasing with the increase of PL content. However, the weight loss did not show significant differences between the coated citrus and control during the whole storage period.

Modified atmosphere packaging decreased Pseudomonas fragi cell metabolism and extracellular proteolytic activities on meat

Modified atmosphere packaging (MAP) is considered an effective method for extending the shelf life of meat. The use of optimal mixture of gases (CO₂ and N₂) in food packaging containers has been proved to effectively inhibit the growth of microorganisms in poultry meat. In general, a minimum CO₂ concentration range of 20%-30% is required for the inhibitory effect. The aim of this study was to investigate the mechanism by which MAP (CO₂/N₂ 30%/70%) inhibits Pseudomonas fragi, a dominant spoilage microorganism in aerobically stored chilled meat. The cell physiological changes were determined by measuring membrane integrity, membrane potential, ATP level, and extracellular proteolytic activity. The results showed that samples stored under MA retained cell membrane integrity, but lost significant membrane potential and ATP synthesis activity. Furthermore, the peptides issued from 2 structural proteins (myosin and actin) were mainly identified in air samples, indicating that these fragments result from bacterial proteolytic activity while MAP inhibited this activity. Overall, the study found that cell metabolism and extracellular protease activity decreased under MAP conditions. This study showed that MAP is an effective food preservation strategy and revealed mechanisms by which MAP inhibits spoilage.

Recent Advances in the Development of Antimicrobial Nanoparticles for Combating Resistant Pathogens

The rapid growth of harmful pathogens and their multidrug-resistance poses a severe challenge for health professionals and for the development of new healthcare products. Various strategies are exploited for the development of effective antimicrobial agents, and nanoparticles are a particularly promising class of materials in this respect. This review summarizes recent advances in antimicrobial metallic, polymeric, and lipid-based nanoparticles such as liposomes, solid lipid nanoparticles, and nanostructured lipid carriers. The latter materials in particular are engineered for antimicrobial agent delivery and act by encapsulation, receptor-based binding, and disruption of microbial adherence to cellular substrates. Potential strategies for the design of multifunctional antimicrobial nanocarriers, combining material chemistry and biological interface science, are also discussed.