Antibacterial activity and mechanism of lactobionic acid against Pseudomonas fluorescens and Methicillin-resistant Staphylococcus aureus and its application on whole milk

The Antibacterial Efficacy and Mechanism of Tea Polyphenol Against Drug-Resistant Aeromonas veronii TH0426 In Vitro

The emergence of Motile Aeromonas Septicemia (MAS) caused by Aeromonas veronii in sturgeon farming has become a significant concern due to its high mortality impact on the aquaculture industry. The threat posed by MAS highlights the urgent need for effective control measures to combat bacterial infections in sturgeon populations. Tea polyphenol (TP) has demonstrated promising antibacterial properties against livestock and poultry bacterial infections. However, its antibacterial efficacy and mechanism in bacterial diseases of aquatic animals remain largely unexplored. This study aimed to investigate the in vitro antibacterial effect and mechanism of TP on fish-borne drug-resistant A. veronii TH0426 by assessing the impact of TP on TH0426 cell growth, antibiofilm activity, morphology, as well as measuring electrical conductivity, DNA extravasation, lactate dehydrogenase (LDH) activity, protein, and DNA contents. Results demonstrated that the minimum inhibitory concentration and the minimum bactericidal concentration of TP on TH0426 were 1024 and 2048 μg/mL, respectively. After a 4 h treatment, the growth of TH0426 was completely inhibited at the concentration of 1024 and 2048 μg/mL of TP. Meanwhile, TP exhibited a significant antibiofilm activity. Both scanning electron microscope and transmission electron microscope analyses revealed disrupted cell membrane structure, irregular cell morphology, and loss of intracellular contents following TP treatment. Moreover, increased cell membrane permeability induced by TP led to intracellular ion and DNA leakage, resulting in elevated electrical conductivity and DNA extravasation. Furthermore, TP decreased LDH activity, protein concentration and content, DNA fluorescence intensity, and density in a time-dependent manner, indicating inhibition of protein metabolism and DNA synthesis. In conclusion, TP exhibits potent antibacterial properties by inhibiting biofilm formation, disrupting cell membrane integrity, and interfering with protein metabolism and DNA synthesis in drug-resistant A. veronii TH0426 in vitro.

Antibacterial Activity and Mechanism of Lauric Acid Against Staphylococcus aureus and Its Application in Infectious Cooked Chicken

Staphylococcus aureus contamination and prevention has always been a major concern for food industry. This work investigated the antibacterial activity and mechanisms of lauric acid (LA) against S. aureus. Results revealed 156 μg/mL was the minimum inhibitory concentration (MIC) for LA and it retarded growth rate of S. aureus. The inhibitory effect was enhanced with LA concentration. After being treated with 2 MIC LA for 24 h, the number of S. aureus decreased by 3.56 log colony-forming unit (CFU)/mL. Scanning electron microscopy profiling revealed that LA resulted in altered morphology of S. aureus cells. In addition, propidium iodide staining of flow cytometry suggested that LA treatment disrupted the cell membrane integrity. Changes in 8-anilino-1-naphthalenesulfonic acid fluorescence indicated a depolarization change in cell membrane fluidity. For practical applications, LA also displayed an antimicrobial potential in cooked chicken food model system, with 1.25-5 g/L of LA prolonging shelf life by 2 days at 4°C. Moreover, it had no adverse effect on pH values, color in cooked chicken meat, and even reduced lipid oxidation. To sum up, LA has great antimicrobial properties and is a candidate preservative for cooked meat food.

Inhibitory Effects of Lactobionic Acid on Biofilm Formation and Virulence of Staphylococcus aureus

Staphylococcus aureus biofilm is a common bio-contaminant source that leads to food cross-contamination and foodborne disease outbreaks. Hence, there is a need for searching novel antibiofilm agents with potential anti-virulence properties to control S. aureus contamination and infections in food systems. In this study, the antibiofilm effects of lactobionic acid (LBA) against S. aureus and its influence on virulence were explored. The minimum inhibition concentration of LBA on S. aureus was 8 mg/mL. Viable count and crystal violet assays revealed that LBA inhibited and inactivated S. aureus biofilms. Microscopic observations further confirmed the antibiofilm activity of LBA on S. aureus that disrupted the biofilm architecture and inactivated the viable cells in biofilms. Moreover, LBA decreased the release of extracellular DNA (eDNA) and extracellular polysaccharide (EPS) in S. aureus biofilms. LBA suppressed biofilm formation by intervening metabolic activity and reduced virulence secretion by repressing the hemolytic activity of S. aureus. Furthermore, LBA altered the expressions of biofilm- and virulence-related genes in S. aureus, further confirming that LBA suppressed biofilm formation and reduced the virulence secretion of S. aureus. The results suggest that LBA might be useful in preventing and controlling biofilm formation and the virulence of S. aureus to ensure food safety.

Antibacterial activity and mechanism of cell-free supernatants of Lacticaseibacillus paracasei against Propionibacterium acnes

Propionibacterium acnes (P. acnes) is an anaerobic and gram-positive bacterium involved in the pathogenesis and inflammation of acne vulgaris. This study particularly focuses on the antimicrobial effect of Lacticaseibacillus paracasei LPH01 against P. acnes, a bacterium that causes acne vulgaris. Fifty-seven Lactobacillus strains were tested for their ability to inhibit P. acnes growth employing the Oxford Cup and double dilution methods. The cell-free supernatant (CFS) of L. paracasei LPH01 demonstrated a strong inhibitory effect, with an inhibition zone diameter of 24.65 ± 0.27 mm and a minimum inhibitory concentration of 12.5 mg/mL. Among the CFS, the fraction over 10 kDa (CFS-10) revealed the best antibacterial effect. Confocal laser scanning microscopes and flow cytometry showed that CFS-10 could reduce cell metabolic activity and cell viability and destroy the integrity and permeability of the cell membrane. A scanning electron microscope revealed that bacterial cells exhibited obvious morphological and ultrastructural changes, which further confirmed the damage of CFS-10 to the cell membrane and cell wall. Findings demonstrated that CFS-10 inhibited the conversion of triglycerides, decreased the production of free fatty acids, and down-regulated the extracellular expression of the lipase gene. This study provides a theoretical basis for the metabolite of L. paracasei LPH01 as a potential antibiotic alternative in cosmeceutical skincare products.

Adopting the "Missile boats-Aircraft carrier" strategy via human-contact friendly oxidized starch to achieve rapid-sustainably antibacterial paperboards

Polysaccharide-based antibacterial agents have received tremendous attention for the facile fabrication, low toxicity, and high compatibility with carbohydrate polymers. However, the antimicrobial mechanism, activity, and cytotoxicity for human-contact paperboards of oxidized starch (OST) with high carboxyl content, has not been explored. Herein, OST-27- 75 with 27- 75 wt% carboxyl contents were fabricated by H2O2 and coated on paperboards. Strikingly, OST-55 coating layer (16 g/m2) did not exfoliate from paperboard and possessed the rapid-sustainable antibacterial performance against Staphylococcus aureus and Escherichia coli. The soluble and insoluble components of OST-55 (OST55-S: OST55-IS mass ratio = 1: 2.1) presented different antimicrobial features and herein they were characterized by GC-MS, FT-IR, H-NMR, XRD, bacteriostatic activities, biofilm formation inhibition and intracellular constituent leakage to survey the antibacterial mechanism. The results revealed OST55-S displayed an amorphous structure and possessed superior antibacterial activity against S. aureus (MIC = 4 mg/mL) and E. coli (MIC = 8 mg/mL). Distinctively, OST55-S could rapidly ionize [H+] like "missile boats" from small molecule saccharides, while OST55-IS polyelectrolyte could continuously and slowly release for [H+] like an "aircraft carrier" to inhibit biofilm formation and disrupt cell structure. Eventually, the "Missile boats-Aircraft carrier" strategy provided a green methodology to fabricate polymeric antibacterial agents and expanded the use of cellulose-based materials.

Drug-loaded lipid nanoparticles improve the removal rates of the Staphylococcus aureus biofilm

Biofilms of the foodborne pathogen Staphylococcus aureus show improved resistance to antibiotics and are difficult to eliminate. To enhance antibacteria and biofilm dispersion via extracellular matrix diffusion, a new lipid nanoparticle was prepared, which employed a mixture of phospholipids and a 0.8% surfactin shell. In the lipid nanoparticle, 31.56 μg mL-1 of erythromycin was encapsulated. The lipid nanoparticle size was approximately 52 nm and the zeta-potential was -67 mV, which was measured using a Marvin laser particle size analyzer. In addition, lipid nanoparticles significantly dispersed the biofilms of S. aureus W1, CICC22942, and CICC 10788 on the surface of stainless steel, reducing the total viable count of bacteria in the biofilms by 103 CFU mL-1 . In addition, the lipid nanoparticle can remove polysaccharides and protein components from the biofilm matrix. The results of laser confocal microscopy showed that the lipid nanoparticles effectively killed residual bacteria in the biofilms. Thus, to thoroughly eliminate biofilms on material surfaces in food factories to avoid repeated contamination, drug-lipid nanoparticles present a suitable method to achieve this.

Chitosan inhibits Penicillium expansum possibly by binding to DNA and triggering apoptosis

Chitosan is a natural polysaccharide that is abundant, biocompatible and exhibits effective antifungal activity against various pathogenic fungi. However, the potential intracellular targets of chitosan in pathogenic fungi and the way of activity of chitosan are far from well known. The present work demonstrated that chitosan could inhibit Penicillium expansum, the principal causal agent of postharvest blue mold decay on apple fruits, by binding to DNA and triggering apoptosis. UV-visible spectroscopy, fluorescence spectroscopy and electrophoretic mobility assay proved the interaction between chitosan and DNA, while atomic force microscope (AFM) observation revealed the binding morphology of chitosan to DNA. Chitosan could inhibit in vitro DNA replication, and cell cycle analysis employing flow cytometry demonstrated that cell cycle was retarded by chitosan treatment. Furthermore, the reactive oxygen species (ROS) assay and membrane potential analysis showed that apoptosis was induced in P. expansum cells after exposure to chitosan. In conclusion, our results confirmed that chitosan interacts with DNA and induces apoptosis. These findings are expected to provide a feasible theoretical basis and practical direction for the promoting and implementing of chitosan in plant protection and further illuminate the possible antifungal mechanisms of chitosan against fungal pathogens.

New Polyene Macrolide Compounds from Mangrove-Derived Strain Streptomyces hiroshimensis GXIMD 06359: Isolation, Antifungal Activity, and Mechanism against Talaromyces marneffei

Mangrove-derived actinomycetes represent a rich source of novel bioactive natural products in drug discovery. In this study, four new polyene macrolide antibiotics antifungalmycin B-E (1-4), along with seven known analogs (5-11), were isolated from the fermentation broth of the mangrove strain Streptomyces hiroshimensis GXIMD 06359. All compounds from this strain were purified using semi-preparative HPLC and Sephadex LH-20 gel filtration while following an antifungal activity-guided fractionation. Their structures were elucidated through spectroscopic techniques including UV, HR-ESI-MS, and NMR. These compounds exhibited broad-spectrum antifungal activity against Talaromyces marneffei with minimum inhibitory concentration (MIC) values being in the range of 2-128 μg/mL except compound 2. This is the first report of polyene derivatives produced by S. hiroshimensis as bioactive compounds against T. marneffei. In vitro studies showed that compound 1 exerted a significantly stronger antifungal activity against T. marneffei than other new compounds, and the antifungal mechanism of compound 1 may be related to the disrupted cell membrane, which causes mitochondrial dysfunction, resulting in leakage of intracellular biological components, and subsequently, cell death. Taken together, this study provides a basis for compound 1 preventing and controlling talaromycosis.

Purification, bioactivity and application of maltobionic acid in active films

The objective of this study was to purify sodium maltobionate using Zymomonas mobilis cells immobilized in situ on flexible polyurethane (PU) and convert it into maltobionic acid for further evaluation of bioactivity (iron chelating ability, antibacterial potential and cytoprotection) and incorporation into films based on cassava starch, chitosan, and cellulose acetate. Sodium maltobionate exhibited a purity of 98.1% and demonstrated an iron chelating ability of approximately 50% at concentrations ranging from 15 to 20 mg mL-1. Maltobionic acid displayed minimal inhibitory concentrations (MIC) of 8.5, 10.5, 8.0, and 8.0 mg mL-1 for Salmonella enterica serovar Choleraesuis, Escherichia coli, Staphylococcus aureus, and Listeria monocytogenes, respectively. Maltobionic acid did not exhibit cytotoxicity in HEK-293 cells at concentrations up to 500 µg mL-1. Films incorporating 7.5% maltobionic acid into cassava starch and chitosan demonstrated inhibition of microbial growth, with halo sizes ranging from 15.67 to 22.33 mm. These films had a thickness of 0.17 and 0.13 mm, water solubility of 62.68% and 78.85%, and oil solubility of 6.23% and 11.91%, respectively. The cellulose acetate film exhibited a non-uniform visual appearance due to the low solubility of maltobionic acid in acetone. Mechanical and optical properties were enhanced with the addition of maltobionic acid to chitosan and cassava films. The chitosan film with 7.5% maltobionic acid demonstrated higher tensile strength (30.3 MPa) and elongation at break (9.0%). In contrast, the cassava starch film exhibited a high elastic modulus (1.7). Overall, maltobionic acid, with its antibacterial activity, holds promise for applications in active films suitable for food packaging.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03879-3.

Antibacterial Activity and Mechanism of Self-Assembly Spermidine-Capped Carbon Dots against Staphylococcus aureus

This paper investigated the antibacterial mechanism of spermidine-capped carbon dots (S-PCDs) against Staphylococcus aureus. The results showed that there were a large number of amino groups on the surface of S-PCDs and they had a high positive charge (+47.06 mV), which could be adsorbed on the negatively charged bacterial surface through electrostatic interaction and changed the permeability of the bacterial cell membrane. The extracellular protein and nucleic acid contents of S. aureus treated with S-PCDs were 5.4 and 1.2 times higher than those of the control group, respectively. The surface folds and defects of the bacterial cell membrane, and the leakage of cell contents were observed using SEM and TEM. The expression of metabolic oxidation regulatory genes dmpI, narJ and narK was upregulated and the intracellular ROS generation was induced, causing bacterial oxidative stress and eventually bacterial death. S-PCDs can effectively inhibit biofilm formation and had low cytotoxicity. The S-PCD treatment successfully inhibited microbial reproduction when pasteurized milk was stored at 25 °C and 4 °C. These results provide important insights into the antimicrobial mechanism of S-PCDs and lay the foundation for their application in the food field as a potentially novel bacteriostatic nanomaterial.

BCp12/PLA combination: A novel antibacterial agent targeting Mur family, DNA gyrase and DHFR

The combination of natural antimicrobial peptide BCp12/phenyllatic acid (BCp12/PLA) presents a more efficient antibacterial effect, but its antibacterial mechanism remains unclear. This study studied the synergistic antibacterial mechanism of BCp12 and PLA against S. aureus. The results demonstrated that the BCp12/PLA combination presented a synergistic antibacterial effect against S. aureus, with a fractional inhibitory concentration of 0.05. Furthermore, flow cytometry and scanning electron microscope analysis revealed that BCp12 and PLA synergistically promoted cell membrane disruption compared with the group treated only with one compound, inducing structural cell damage and cytoplasmic leakage. In addition, fluorescence spectroscopy analysis suggested that BCp12 and PLA synergistically influenced genomic DNA. BCp12 and PLA targeted enzymes related to peptidoglycan and DNA synthesis and interacted by hydrogen bonding and hydrophobic interactions with mur enzymes (murC, murD, murE, murF, and murG), dihydrofolate reductase, and DNA gyrase. Additionally, the combined treatment successfully inhibited microbial reproduction in the storage of pasteurized milk, indicating that the combination of BCp12 and PLA can be used as a new preservative strategy in food systems. Overall, this study could provide potential strategies for preventing and controlling foodborne pathogens.

Repurposing Salicylamides to Combat Phytopathogenic Bacteria and Induce Plant Defense Responses

Based on the research strategy of "drug repurposing", a series of derivatives and marketed drugs that containing salicylic acid skeleton were tested for their antibacterial activities against phytopathogens. Salicylic acid can not only regulate some important growth metabolism of plants, but also induce plant disease resistance. The bioassay results showed that the salicylamides exhibited excellent antibacterial activity. Especially, oxyclozanide showed the best antibacterial effect against Xanthomonas oryzae, Xanthomonas axonopodis pv. citri and Pectobacterium atroseptica with MICs of 0.78, 3.12 and 12.5 μg.mL-1, respectively. In vivo experiments with rice bacterial leaf blight had further demonstrated that oxyclozanide exhibited stronger antibacterial activity than the commercial bactericide, thiodiazole copper. Oxyclozanide could induce plant defense responses through the determination of salicylic acid content and the activities of defense-related enzymes including CAT, POD, and SOD in rice. The preliminarily antibacterial mechanism study indicated that oxyclozanide exhibited the antibacterial activity by disrupting cell integrity and reducing bacterial pathogenicity. Additionally, oxyclozanide could induce plant defense responses through the determination of salicylic acid content.

Antimicrobial activity enabled by chitosan-ε-polylysine-natamycin and its effect on microbial diversity of tomato scrambled egg paste

For a long time, food spoilage posed a severe impairment on food safety and public health. Although chemical preservatives are commonly used to inhibit spoilage/ pathogenic microbial growth, the disadvantages of a single target, potential toxicity and high dose of use limit the better use of preservatives. In this research, the combination of natural preservatives: Natamycin (Nat), ε-polylysine (ε-PL), and Chitosan (CS) could achieve an excellent antimicrobial effect including bacteria and fungi, and reduce the usage of a single preservative. Compound preservatives could destroy microbial morphology and damage the integrity of the cell wall/membrane by leakage of protein and alkaline phosphatase (AKP). Besides, high-throughput sequencing revealed that compound preservatives could decrease microbial diversity and richness, especially, Pseudomonas, Acinetobacter, Fusarium, and Aspergillus. Therefore, the combination of 1/8 × MIC CS, 1/4 × MIC ε-PL, and 1/2 × MIC Nat can achieve an excellent antibacterial effect, providing new ideas for food preservation.

Proteomic analysis of Staphylococcus aureus exposed to bacteriocin XJS01 and its bio-preservative effect on raw pork loins

Antibacterial mechanism of bacteriocins against foodborne S. aureus is still to be explored, particularly in proteomics, and a deep and comprehensive study on application of bacteriocins for preservation of raw pork is required. Here, proteomic mechanism of Lactobacillus salivarius bacteriocin XJS01 against foodborne S. aureus 2612:1606BL1486 (S. aureus_26) and its preservation effect on raw pork loins stored at 4 °C for 12 days was investigated. The results showed that 301 differentially abundant proteins (DAPs) were identified between XJS01-treated and -free groups (control group) using Tandem mass tag (TMT) quantitative proteomics technology, which were primarily involved in amino acids and carbohydrate metabolism, cytolysis, defense response, cell apoptosis, cell killing, adhesion, and oxygen utilization of S. aureus_26. Bacterial secretion system (SRP) and cationic antimicrobial peptide resistance may be key pathways to maintain protein secretion and counteract the deleterious effects on S. aureus_26 caused by XJS01. In addition, XJS01 could significantly improve the preservation of raw pork loins by the evaluation results of sensory and antibacterial activity on the meat surface. Overall, this study showed that XJS01 induced a complex organism response in S. aureus, and it could be potential pork preservative.

Growth patterns of Pseudomonas aeruginosa in milk fortified with chitosan and selenium nanoparticles during refrigerated storage

Pseudomonas spp are considered a common milk-associated psychotropic bacteria, leading to milk deterioration during storage; therefore, our study aimed to study the distribution of Pseudomonas aeruginosa in raw milk and its associated products then studying the growth behavior of P. aeruginosa in milk after employing chitosan nanoparticles (CsNPs 50, 25, and 15 mg/100ml) and selenium nanoparticles (SeNPs 0.5, 0.3 and 0.1 mg/100ml) as a trial to control the bacterial growth in milk during five days of cooling storage. Our study relies on the ion gelation method and green synthesis for the conversion of chitosan and selenium to nanosized particles respectively, we subsequently confirmed their shape using SEM and TEM. We employing Pseudomonas selective agar medium for monitoring the bacterial growth along the cooling storage. Our findings reported that high prevalence of Pseudomonas spp count in raw milk and kareish cheese and high incidence percent of P. aeruginosa in ice cream and yogurt respectively. Both synthesized nanoparticles exhibited antibacterial activity in a dose-dependent manner. Moreover, CsNPs50 could inhibit the P. aeruginosa survival growth to a mean average of 2.62 ± 1.18 log10cfu/ml in the fifth day of milk cooling storage; also, it was noted that the hexagonal particles SeNPs0.5 could inhibit 2.49 ± 11 log10cfu/ml in comparison to the control P. aeruginosa milk group exhibited growth survival rate 7.24 ± 2.57 log10cfu/ml under the same conditions. In conclusion, we suggest employing chitosan and selenium nanoparticles to improve milk safety and recommend future studies for the fate of nanoparticles in milk.

Antibacterial actions of Ag nanoparticles synthesized from Cimicifuga dahurica (Turcz.) Maxim. and their application in constructing a hydrogel spray for healing skin wounds

Cimicifuga dahurica (Turcz.) Maxim. is an edible natural food and a type of traditional herbal medicine with antipyretic and analgesic properties. In this study, we found that Cimicifuga dahurica (Turcz.) Maxim. extract (CME) has good skin wound healing qualities due to its antibacterial effects on both wound inflammation-related Gram positive (Staphylococcus aureus and Staphylococcus epidermidis) and Gram negative (Escherichia coli and Klebsiella pneumoniae) strains. Using CME as a reducing agent, CME-based Ag nanoparticles (CME-AgNPs) with an average particle size of 7 nm were synthesized. The minimum bactericidal concentration (MBC) of CME-AgNPs against the investigated bacterial species varied from 0.08 to 1.25 mg/mL, indicating much higher antibacterial activity than the pure CME. Additionally, a novel network-like thermosensitive hydrogel spray (CME-AgNPs-F127/F68) was developed and shown a skin wound healing rate of 98.40% in 14 days, demonstrating the spray's potential as a novel wound dressing that accelerates wound healing.

Sodium alginate emulsion loaded with linalool: Preparation, characterization and antibacterial mechanism against Shigella sonnei

This study aimed to prepare sodium alginate-linalool emulsion (SA-LE) to overcome the low solubility of linalool and explore its inhibitory activity against Shigella sonnei. The results indicated that linalool significantly reduced the interfacial tension between SA and oil phase (p < 0.05). Droplet sizes of fresh emulsions were uniform with sizes from 2.54 to 2.58 μm. The ζ-potential was between -23.94 and -25.03 mV, and the viscosity distribution was 973.62 to 981.03 mPa·s at pH 5-8 (near neutral pH) without significant difference. In addition, linalool could be effectively released from SA-LE in accordance with the Peppas-Sahlin model, mainly described by Fickian diffusion. In particular, SA-LE can inhibit S. sonnei with a minimum inhibitory concentration of 3 mL/L, which was lower than free linalool. The mechanism can be described as damaging the membrane structure and inhibiting respiratory metabolism accompanied by oxidative stress based on FESEM, SDH activity, ATP and ROS content. These results suggest that SA is an effective encapsulation strategy to enhance the stability of linalool and its inhibitory effect on S. sonnei at near neutral pH. Moreover, the prepared SA-LE has the potential to be developed as a natural antibacterial agent to address the growing food safety challenges.

Dissecting LuxS/AI-2 quorum sensing system-mediated phenyllactic acid production mechanisms of Lactiplantibacillus plantarum L3

The phenyllactic acid (PLA) produced by lactic acid bacteria (LAB) inhibits fungi and facilitates the quality control of fermented milk. A strain of Lactiplantibacillus plantarum L3 (L. plantarum L3) with high PLA production was screened in the pre-laboratory, but the mechanism of its PLA formation is unclear. The amount of autoinducer-2 (AI-2) increased with increasing culture time, as did cell density and PLA. The results in this study suggest that PLA production in L. plantarum L3 may be regulated by the LuxS/AI-2 Quorum Sensing (QS) system. Tandem mass tag (TMT) quantitative proteomics analysis showed that a total of 1291 differentially expressed proteins (DEPs) were quantified in the incubated for 24 h compared with the incubated for 2 h, of which 516 DEPs were up-regulated and 775 DEPs were down-regulated. Among them, S-ribosomal homocysteine lyase (luxS), aminotransferase (araT), and lactate dehydrogenase (ldh) are the key proteins for PLA formation. The DEPs were mainly involved in the QS pathway and the core pathway of PLA synthesis. Furanone effectively inhibited the production of L. plantarum L3 PLA. In addition, Western blot analysis demonstrated that luxS, araT, and ldh were the key proteins regulating PLA production. This study reveals the regulatory mechanism of PLA based on the LuxS/AI-2 QS system, which provides a theoretical basis for the efficient and large-scale production of PLA in industries in the future.

Multi-Enzymatic Synthesis of Lactobionic Acid Using Wood-Degrading Enzymes Produced by White Rot Fungi

Enzymes produced by white rot fungi are involved in the synthesis of secondary metabolites with valuable biotechnological properties. One of these metabolites is lactobionic acid (LBA). The aim of this study was to characterize a novel enzyme system consisting of a cellobiose dehydrogenase from Phlebia lindtneri (PlCDH), a laccase from Cerrena unicolor (CuLAC), a redox mediator (ABTS or DCPIP), and lactose as a substrate. We used quantitative (HPLC) and qualitative methods (TLC, FTIR) to characterise the obtained LBA. The free radical scavenging effect of the synthesised LBA was assessed with the DPPH method. Bactericidal properties were tested against Gram-negative and Gram-positive bacteria. We obtained LBA in all the systems tested; however, the study showed that the temperature of 50 °C with the addition of ABTS was the most advantageous condition for the synthesis of lactobionic acid. A mixture with 13 mM LBA synthesised at 50 °C with DCPIP showed the best antioxidant properties (40% higher compared with the commercial reagent). Furthermore, LBA had an inhibitory effect on all the bacteria tested, but the effect was better against Gram-negative bacteria with growth inhibition no lower than 70%. Summarizing the obtained data, lactobionic acid derived in a multienzymatic system is a compound with great biotechnological potential.

Antibacterial Activity and Mechanism of Madecassic Acid against Staphylococcus aureus

Antibacterial resistance has become one of the most serious problems threating global health. To overcome this urgent problem, many scientists have paid great attention to developing new antibacterial drugs from natural products. Hence, for exploring new antibacterial drugs from Chinese medicine, a series of experiments were carried out for verifying and elucidating the antibacterial activity and mechanisms of madecassic acid (MA), which is an active triterpenoid compound isolated from the traditional Chinese medicine, Centella asiatica. The antibacterial activity was investigated through measuring the diameter of the inhibition zone, the minimum inhibitory concentration (MIC), the growth curve, and the effect on the bacterial biofilm, respectively. Meanwhile, the antibacterial mechanism was also discussed from the aspects of cell wall integrity variation, cell membrane permeability, and the activities of related enzymes in the respiratory metabolic pathway before and after the intervention by MA. The results showed that MA had an inhibitory effect on eight kinds of pathogenic bacteria, and the MIC values for Staphylococcus aureus, Methicillin-resistant Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis, and Bacillus megaterium were 31.25, 62.5, 250, 125, 62.5, and 62.5 µg/mL, respectively. For instance, 31.25 µg/mL MA could inhibit the growth of Staphylococcus aureus within 28 h. The antibacterial mechanism experiments confirmed that MA could destroy the integrity of the cell membrane and cell wall of Staphylococcus aureus, causing the leakage of macromolecular substances, inhibiting the synthesis of soluble proteins, reducing the activities of succinate dehydrogenase and malate dehydrogenase, and interacting with DNA, leading to the relaxation and ring opening of supercoiled DNA. Besides, the activities of DNA topoisomerase I and II were both inhibited by MA, which led to the cell growth of Staphylococcus aureus being repressed. This study provides a theoretical basis and reference for the application of MA in the control and inhibition of food-borne Staphylococcus aureus.

Evaluation of the antimicrobial mechanism of biogenic selenium nanoparticles against Pseudomonas fluorescens

Selenium nanoparticles (SeNPs) can be biosynthesized by most Lactic acid bacteria thereby converting toxic sodium into SeNPs. However, few studies have reported the antimicrobial activity of biogenic SeNPs against Pseudomonas fluorescens which are the main species of psychrotrophic bacteria in raw milk. This study reported the synthesis and characterization of SeNPs from Lactobacillus casei ZK-AS 1.1482, and the antimicrobial mechanism against P. fluorescens ATCC 13525. The synthesized SeNPs were amorphous with sizes ranging from 52 to 103 nm. Fourier transform infrared spectroscopy (FT-IR) spectra showed the presence of proteins, polysaccharides, and lipids on the surface of particles, which evidently stabilized the SeNPs structure and morphology. Energy-dispersive X-ray (EDX) analysis revealed that the nanoparticles contained selenium. In addition, the minimal inhibitory concentration (MIC) of SeNPs against P. fluorescens ATCC 13525 was 0.1 mg ml^-1 and the biofilm inhibition rate was 43.52 ± 0.26%. SeNPs decreased the number of living bacteria observed by confocal laser scanning microscopy (CLSM). Meanwhile, after SeNPs treatment, the intracellular adenosine triphosphate (ATP) concentration and antioxidant enzyme activity decreased, the content of reactive oxygen species (ROS) and the malondialdehyde (MDA) content increased, and lipid peroxidation intensified. Real-time fluorescence quantitative PCR (RT-qPCR) assay showed that the expression of flgA, luxR, lapD, MCP, cheA, c-di-GMP, phoB, and pstC gene were down-regulated after SeNPs treatment. The rfbC and DegT/DnrJ/EryC1/StrS gene were significantly up-regulated, indicating that SeNPs could destroy the integrity of cell membrane and thus play an antimicrobial role. Biogenic SeNPs are expected to be developed as an efficient and novel antimicrobial agent for application in the food industry.

In Situ Sprayed Biotherapeutic Gel Containing Stable Microbial Communities for Efficient Anti-Infection Treatment

Systematic administration of antibiotics to treat infections often leads to the rapid evolution and spread of multidrug-resistant bacteria. Here, an in situ-formed biotherapeutic gel that controls multidrug-resistant bacterial infections and accelerates wound healing is reported. This biotherapeutic gel is constructed by incorporating stable microbial communities (kombucha) capable of producing antimicrobial substances and organic acids into thermosensitive Pluronic F127 (polyethylene-polypropylene glycol) solutions. Furthermore, it is found that the stable microbial communities-based biotherapeutic gel possesses a broad antimicrobial spectrum and strong antibacterial effects in diverse pathogenic bacteria-derived xenograft infection models, as well as in patient-derived multidrug-resistant bacterial xenograft infection models. The biotherapeutic gel system considerably outperforms the commercial broad-spectrum antibacterial gel (0.1% polyaminopropyl biguanide) in pathogen removal and infected wound healing. Collectively, this biotherapeutic strategy of exploiting stable symbiotic consortiums to repel pathogens provides a paradigm for developing efficient antibacterial biomaterials and overcomes the failure of antibiotics to treat multidrug-resistant bacterial infections.

Antibacterial activity and mechanism of flavonoids from Chimonanthus salicifolius S. Y. Hu. and its transcriptome analysis against Staphylococcus aureus

Introduction

Chimonanthus salicifolius S. Y. Hu. (FCS) possess many biological activities, but the antibacterial activity and underlying mechanisms of flavonoids from Chimonanthus salicifolius S. Y. Hu. (FCS) is still unknown.

Method

Maximum diameter of inhibition zone (DIZ), maximum diameter of inhibition zone (DIZ), the lowest minimum inhibition concentration (MIC), and the lowest minimum bactericide concentration (MBC) were used to detect the antibacterial activity. Meanwhile, related enzyme activities, the transcriptome analysis and quantitative RT-PCR were used to investigate the antibacterial activity mechanisms.

Results

The results showed that FCS (with a purity of 84.2 ± 2.0%) has potential effects on tested strains with the maximum diameter of inhibition zone (DIZ) was 15.93 ± 2.63 mm, the lowest minimum inhibition concentration (MIC) was 1.56 mg/ml and the lowest minimum bactericide concentration (MBC) was 6.25 mg/ml. In addition, the bacterial growth curve test, release of extracellular alkaline phosphatase (AKP), loss of intracellular components, DNA damage and transmission electron microscope (TEM) suggested that FCS could destroy the cell wall and membrane, cause the loss of intracellular substance, cause DNA damage and even lead to cell death. Moreover, the antibacterial mechanism of FCS against Staphylococcus aureus (S. aureus, Gram-positive bacteria) was further confirmed by the transcriptome analysis and quantitative RT-PCR at the molecular level for the first time. A total of 671 differentially expressed genes (DEGs) were identified after treated with FCS (1/2 MIC), with 338 and 333 genes showing up-regulation and down-regulation, respectively. The highlighted changes were those related to the biosynthesis of bacteria wall and membrane, DNA replication and repair, and energy metabolism.

Discussion

Overall, our research provides theoretical guidance for the application of FCS, which is expected to be potentially used as a natural antimicrobial agent in food safety.

Bioassay-Guided Isolation and Identification of Antibacterial Components against Escherichia coli from Industrial Hemp Leaves

Industrial hemp leaves have raised much interest in nutraceuticals and functional foods areas. To expand its application ranges, the antibacterial activities of industrial hemp leaf extract on Escherichia coli, Staphylococcus aureus, and Bacillus cereus were evaluated and the active components were screened. As a result, the industrial hemp leaf extract was found to have strong bacteriostatic effects on E. coli and S. aureus. Bioassay-guided fractionation and isolation from fractions active against E. coli were conducted. Two compounds, cannabidivarinic acid and cannabidiolic acid, were firstly recognized by analytical HPLC by comparing the retention times and UV spectra with standards and later isolated using preparative HPLC. Moreover, the antibacterial mechanisms of cannabidivarinic acid and cannabidiolic acid were investigated by testing the alkaline phosphatase activity, β-galactosidase activity, conductivity, proteins leakage, nucleic acid leakage, and scanning electron microscope observation. The results demonstrated that cannabidivarinic acid and cannabidiolic acid could destroy the cell wall and membrane of E. coli, resulting in the inhibition of enzyme activity and leakage of contents. They could damage the bacteria cell envelope as well. Presented results pointed out cannabidivarinic acid and cannabidiolic acid as promising natural bacteriostatic agents for the food, pharmaceutical, and cosmetic industry.

Antibacterial Activity and Mechanism of the Gallnut Water Extract against Vibrio parahaemolyticus

Vibrio parahaemolyticus is an important pathogen causing pandemic diseases in marine animals and brings about severe economic losses in aquaculture worldwide. The emergence of multi-drug-resistant V. parahaemolyticus and the prohibition of antibiotics both require the development of new therapeutic agents with alternative action. In this study, the effect of gallnut Galla chinensis water extract (GWE) on V. parahaemolyticus growth and virulence was investigated to determine the potential for its use in disease prevention and treatment in aquaculture. The minimum inhibitory concentration and minimum bactericidal concentration of GWE against V. parahaemolyticus were identified as 0.49 and 0.98 mg/mL, respectively. Membrane damage in V. parahaemolyticus was further verified through the increase of conductivity and leakage of nucleic acids and proteins. Moreover, GWE caused membrane invaginations and damage in V. parahaemolyticus as observed via scanning electron microscopy. After treatment with GWE, the biofilm formation and the activities of respiratory chain dehydrogenase, lactate dehydrogenase, and succinate dehydrogenase of V. parahaemolyticus were all significantly inhibited. These findings suggest that GWE has the potential to be developed as a supplemental agent to mitigate the infections caused by V. parahaemolyticus in aquaculture.

Discovery of New Anti-MRSA Agents Based on Phenoxyethanol and Its Mechanism

Methicillin-resistant Staphylococcus aureus (MRSA) poses a severe threat to public health and safety. The discovery and development of novel anti-MRSA drugs with a new mode of action are a challenge. In this study, a class of novel aryloxyethyl propiolates and their homologues as anti-MRSA agents have been designed and synthesized based on phenoxyethanol, of which compound II-39 showed high inhibitory activity against MRSA with an MIC of 0.78 μg/mL and an MBC of 3.13 μg/mL, which was better than that of vancomycin. Compound II-39 could destroy the cell wall and cell membrane, inhibited the formation of a biofilm, and bound to the DNA of MRSA through the electrostatic and groove interaction. Proteomic and metabolomic studies revealed that compound II-39 affected multiple intracellular metabolic pathways of MRSA. Notably, compound II-39 could effectively inhibit the expression of CrtPQMN proteins and block the biosynthesis of virulence factor (staphyloxanthin). Thus, aryloxyethyl propiolates and their homologues are promising anti-MRSA agents with multiple targets.

Antibacterial activity and action mechanism of flavonoids against phytopathogenic bacteria

As the most difficult to control in plant disease, phytopathogenic bacteria cause huge losses to agricultural products and economy worldwide. However, the commercially available bactericides are few and enhance pathogen resistance. To alleviate this situation, 50 flavonoids were evaluated for their antibacterial activities and mechanism of action against two intractable plant bacterial pathogens. The results of bioassays showed that most of the flavonoids exhibited moderate inhibitory effects against Xanthomonas oryzae (Xo) and Xanthomonas axonopodis pv citri (Xac). Remarkably, kaempferol showed excellent antibacterial activity against Xo in vitro (EC₅₀ = 15.91 μg/mL) and quercetin showed the best antibacterial activity against Xac in vitro (EC₅₀ = 14.83 μg/mL), which was better than thiodiazole copper (EC₅₀ values against Xo and Xac were 16.79 μg/mL, 59.13 μg/mL, respectively). Subsequently, in vivo antibacterial activity assay further demonstrated kaempferol exhibited a stronger control effect on bacterial infections than thiodiazole copper. Then, the preliminary antibacterial mechanism of kaempferol was investigated by ultrastructural observations, transcriptomic, qRT-PCR analysis and biochemical index determination. These results showed that kaempferol mainly exerted bacteriostatic effects at the molecular level by affecting bacterial energy metabolism, reducing pathogenicity, and leading to disruption of cellular integrity, leakage of contents and cell death eventually.

Understand the antibacterial behavior and mechanism of hydrothermal wastewater

Unlocking the antibacterial potential is an emerging strategy to valorizing the toxic wastewater from hydrothermal liquefaction (HTL). Here, we investigated the response and biological mechanism of antibacterial properties of HTL wastewater. Four different biowastes i.e. microalgae, cornstalk, cow manure and swine manure were used as the feedstock of HTL to create wastewater with diverse molecule spectrum, whereas ten strains i.e. five gram-positive strains and five gram-negative strains were employed to represent typical pathogenic microorganism. HTL wastewater exhibited antibacterial potential and obvious reduction on cell viability at high inclusion ratio, although the minimum inhibitory concentration (MIC) and cell response intensity varied depending on different HTL feedstocks and strain species. The decreased ATP generation and increased H₂O₂ accumulation in treated cells further confirmed the inhibition of HTL wastewater on the cell metabolism. The antibacterial mechanism of HTL wastewater was confirmed, including damage to biomolecules or membranes, depletion of crucial components, disruption of metabolic circuits and imbalance of creation of redox cofactor. The complex compounds in HTL wastewater were probably attributed to the multiple inhibition pathways and the relationship among those multiple pathways was speculated. The present study contributes to the mechanism analysis of complex compound mixture and bactericide characteristics of HTL wastewater.

Value-Added Utilization of Citrus Peels in Improving Functional Properties and Probiotic Viability of Acidophilus-bifidus-thermophilus (ABT)-Type Synbiotic Yoghurt during Cold Storage

Citrus peel, a fruit-processing waste, is a substantial source of naturally occurring health-promoting compounds, including polyphenols, and has great potential as a dietary supplement for enhancing the functional properties of food. The present work aimed to investigate the effects of sour orange (SO), sweet orange (SWO), and lemon (LO) peels on the typical physiochemical, antioxidant, antibacterial, and probiotic properties of synbiotic yoghurt fermented by acidophilus-bifidus-thermophilus (ABT)-type cultures during cold storage (0−28 days). High-performance liquid chromatography-diode array detection (HPLC-DAD) analysis showed that the total phenolic content in the SO peel were more than 2-fold higher than that in the SWO and LO peel. The predominant phenolic compounds were myricetin (2.10 mg/g dry weight) and o-coumaric acid (1.13 mg/g) in SO peel, benzoic acid (0.81 mg/g) and naringin (0.72 mg/g) in SWO peel, and benzoic acid (0.76 mg/g) and quercetin (0.36 mg/g) in LO peel. Only 0.5% (w/w) of citrus peel addition did not reduce the overall acceptance of ABT synbiotic yoghurt but led to increased acidity and decreased moisture during cold storage (14 and 28 days). Additionally, compared to control samples without citrus peel addition, supplementation with citrus peels improved the antioxidant property of the ABT synbiotic yoghurt. ABT milks with SO and SWO peel addition had significantly stronger DPPH radical scavenging activities than that with LO peel addition (p < 0.05). Antibacterial analysis of ABT synbiotic yoghurt with citrus peel addition showed that the diameters of inhibition zones against S. aureus, B. subtilis, and E. coli increased by 0.6−1.9 mm relative to the control groups, suggesting the enhancement of antibacterial activities by citrus peels. The viabilities of probiotic starter cultures (L. acidophilus, S. thermophilus, and Bifidobacterial sp.) were also enhanced by the incorporation of citrus peels in synbiotic yoghurt during cold storage. Hence, our results suggest that citrus peels, especially SO and SWO peels, could be recommended as a promising multifunctional additive for the development of probiotic and synbiotic yoghurt with enhanced antioxidant and antibacterial properties, as well as probiotic viability.

Antibacterial Effect of Eugenol on Shigella flexneri and Its Mechanism

Shigella flexneri (Sh. flexneri), which can be found in food and the environment, is a widespread food-borne pathogen that causes human diarrhea termed “shigellosis”. In this study, eugenol, a natural active substance, was investigated for its antibacterial activity against Sh. flexneri. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of eugenol against Sh. flexneri ATCC 12022 was 0.5 and 0.8 mg/mL. The growth curves and inhibitory effect in LB broth, PBS, vegetable juice, and minced pork showed that eugenol had a good activity against Sh. flexneri. Research findings indicated the superoxide dismutase activity of Sh. flexneri was inhibited after eugenol treatment, resulting in concentrations of intracellular reactive oxygen species and an increase in malondialdehyde. The flow cytometry analysis and field emission scanning electron microscopy results revealed obvious damage to cell membrane integrity and changes in the morphology of Sh. flexneri. In addition, the intracellular ATP concentration leaked from 0.5 μM to below 0.05 μM and the membrane potential showed a concentration-dependent depolarization after eugenol treatment. In summary, eugenol exerted strong antibacterial activity and has the potential to control Sh. flexneri in the food industry.

Isolation of Bioactive Compounds, Antibacterial Activity, and Action Mechanism of Spore Powder From Aspergillus niger xj

Aspergillus fungi can produce a wide range of secondary metabolites, and they have represented a potential resource of novel bioactive compounds. Bacterial plant diseases have a serious impact on the sustainable development of agriculture worldwide, so it is necessary to use natural antibacterial compounds in microorganisms to control plant pathogens. This study was conducted to investigate the bioactive compounds of Aspergillus niger xj, three plant pathogens (Agrobacterium tumefaciens T-37, Erwinia carotovora EC-1, and Ralstonia solanacearum RS-2) were used as indicator bacteria, according to the biological activity tracking, five compounds were isolated from A. niger xj spore powder, and characterization of compounds was done by NMR (1H-NMR and 13C-NMR) and EI-MS and was identified as ergosterol (1), β-sitosterol (2), 5-pentadecylresorcinol (3), 5-hydroxymethyl-2-furancarboxylic acid (4), and succinimide (5). Compounds 3 and 5 were isolated from A. niger xj for the first time. The minimum inhibitory concentration (MIC) of five compounds against three plant pathogens was evaluated, the results showed that compound 4 exhibited the strongest antibacterial activity against tested bacteria, and RS-2 was the most sensitive to compound 4, showing the lowest MIC of 15.56 μg/ml. We concluded that the mechanism of action of the compound 4 against RS-2 might be described as compound 4 acting on bacterial protein synthesis and intracellular metabolism according to the results of the scanning electron microscopy observation, permeability of cell membrane and SDS-PAGE. These results indicated that compound 4 has good potential to be as a biocontrol agent. In conclusion, the results from this study demonstrated that the compounds with antibacterial activity are of great significance of the prevention and control of plant phytopathogenic bacteria, and they may be applicable to exploring alternative approaches to integrated control of phytopathogens.

The antimicrobial and bioactive properties of lactobionic acid

Lactobionic acid (LBA) is a bioactive molecule that has generated keen interest in different industries. However, its future application in the food area is one of the most promising. Chemically, it is a polyhydroxy acid formed by the union of two molecules (galactose and gluconic acid) linked by an ether-bond, showing many interesting and unusual properties due to its structure and composition, although it is traditionally known in the food industry for its chelating, moisturizing, gelling, and antioxidant properties. There has been much research into the production of LBA, either by microbial fermentation or biocatalytic approaches such as enzymatic synthesis, but its use in foodstuffs, to produce new functional products and to evaluate its antimicrobial activity against food-borne pathogens, is a relatively new topic that has attracted the interest of the international research community recently. Furthermore, in spite of the potential of LBA, it has been approved only by the US Food and Drug Administration, and for its use as the salt form, but the publication of new comprehensive studies, able to agglutinate all the new food-related LBA research results, could disseminate knowledge about this compound and have an influence on its current regulation status. The aim of the present review is to describe the most recent advances and research on its antimicrobial potential, as well as summarizing the significant aspects that make LBA a promising bioactive compound for the food sector. © 2022 Society of Chemical Industry.

Antibacterial Mode of Eucommia ulmoides Male Flower Extract Against Staphylococcus aureus and Its Application as a Natural Preservative in Cooked Beef

The research was conducted to elucidate the antibacterial performance and mode of action of Eucommia ulmoides male flower extract (EUMFE) against Staphylococcus aureus and its application as a natural preservative in cooked beef. The antibacterial activity was evaluated by determining the diameter of inhibition zone (DIZ), minimum inhibitory concentration (MIC), and minimum bactericide concentration (MBC). The changes in membrane potential, contents of bacterial DNA and protein, integrity and permeability of the cell membrane, and cell morphology were analyzed to reveal the possible mode of action. The effect of EUMFE on the counts of S. aureus, pH, color, total volatile basic nitrogen (TVB-N), and thiobarbituric acid reactive substances (TBARS) of the cooked beef stored at 4°C for 9 days were studied. The results showed that the DIZ, MIC, and MBC of EUMFE against S. aureus were 12.58 ± 0.23 mm, 40 mg/mL, and 80 mg/mL, respectively. The mode of action of EUMFE against S. aureus included hyperpolarization of cell membrane, decrease in bacterial DNA and protein contents, destruction of cell membrane integrity, increase in cell membrane permeability, and damage of cell morphology. After treatments with EUMFE, the growth of S. aureus and lipid oxidation in cooked beef were significantly inhibited (P < 0.05). The pH and TVB-N values of cooked beef treated with EUMFE were significantly reduced as compared to control group (P < 0.05). The color of cooked beef samples containing EUMFE showed decreased L* and b* values, and increased a* and ΔE* values. Therefore, our findings showed that EUMFE had a good antibacterial effect on S. aureus, and provided a theoretical basis for the application of EUMFE as a natural preservative in the preservation of cooked beef.

Preparation of Edible Films with Lactobacillus plantarum and Lactobionic Acid Produced by Sweet Whey Fermentation

Cheese whey, one of the most abundant by-products of the dairy industry, causes economic losses and pollution problems. In this study, deproteinised sweet whey was fermented by Pseudomonas taetrolens LMG 2336 to produce a prebiotic compound (lactobionic acid, LBA). Endotoxins produced by these microorganisms were successfully removed using microfiltration techniques, allowing the fermented whey permeate to be used in the food industry. The fermented whey permeate was used to develop prebiotic edible films by adding two different concentrations of gelatine (0.45 and 0.9 g gelatine g−1 LBA; LBA45 and LBA90). Furthermore, Lactobacillus plantarum CECT 9567 was added as a probiotic microorganism (LP45 and LP90), creating films containing both a prebiotic and a probiotic. The mechanical properties, water solubility, light transmittance, colour, and microstructure of the films were fully characterised. Additionally, the LBA and probiotic concentration in LP45 and LP90 were monitored under storage conditions. The strength and water solubility of the films were affected by the presence of LBA, and though all these films were homogeneous, they were slightly opaque. In LP45 and LP90, the presence of LBA as a prebiotic improved the viability of L. plantarum during cold storage, compared to the control. Therefore, these films could be used in the food industry to coat different foodstuffs to obtain functional products.