Antibacterial effect of phenyllactic acid against Vibrio parahaemolyticus and its application on raw salmon fillets

Inactivation mechanism of phenyllactic acid against Bacillus cereus spores and its application in milk beverage

Phenyllactic acid (PLA) as a natural phenolic acid exhibits antibacterial activity against non-spore-forming bacteria, while the inhibitory effect against bacterial spore remained unknown. Herein, this study investigated the inactivation effect of PLA against Bacillus cereus spores. The results revealed that the minimum inhibitory concentration of PLA was 1.25 mg/mL. PLA inhibited the outgrowth of germinated spores into vegetative cells rather than germination of spores. PLA disrupted the spore coat, and damaged the permeability and integrity of inner membrane. Moreover, PLA disturbed the establishment of membrane potential due to the inhibition of oxidative metabolism. SEM observations further visualized the morphological changes and structural disruption caused by PLA. Besides, PLA caused the degradation of DNA of germinated spores. Finally, PLA was applied in milk beverage, and showed promising inhibitory effect against B. cereus spores. This finding could provide scientific basis for the application of PLA against spore-forming bacteria in food industry.

Antifungal mechanism of phenyllactic acid against Mucor investigated through proteomic analysis

The primary inhibitory targets of phenyllactic acid (PLA, including D-PLA and L-PLA) on Mucor were investigated using Mucor racemosus LD3.0026 isolated from naturally spoiled cherry, as an indicator fungi. The results demonstrated that the minimum inhibitory concentration (MIC) of PLA against Mucor was 12.5 mmol·L-1. Results showed that the growing cells at the tip of the Mucor were not visibly deformed, and there was no damage to the cell wall following PLA treatment; however, PLA damaged the cell membrane and internal structure. The results of isobaric tags for relative and absolute quantification (iTRAQ) indicated that the Mucor mitochondrial respiratory chain may be the target of PLA, potentially inhibiting the energy supply of Mucor. These results indicate that the antifungal mechanism of PLA against mold is independent of its molecular configuration. The growth of Mucor is suppressed by PLA, which destroys the organelle structure in the mycelium and inhibits energy metabolism.

Rapid Revealing of Quorum Sensing (QS)-Regulated PLA, Biofilm and Lysine Targets of Lactiplantibacillus plantarum L3

Quorum sensing (QS) can regulate the production of multiple functional factors in bacteria, but the process of identifying its regulatory targets is very complex and labor-intensive. In this study, an efficient and rapid method to find QS targets through prediction was used. The genome of Lactiplantibacillus plantarum (L. plantarum) L3 was sequenced and characterized, and then linked the L. plantarum L3 genome to the STRING database for QS system regulatory target prediction. A total of 3,167,484 base pairs (bps) were examined from the genome of L. plantarum L3, and 30 QS-related genes were discovered (including luxS). The STRING database prediction indicated that the 30 QS-related genes are mainly involved in the regulation of nine metabolic pathways. Furthermore, metE, metK, aroB, cysE, and birA1 were predicted to be regulatory targets of the LuxS/AI-2 QS system, and these five targets were validated based on quantitative real-time PCR and content determination. Successful elucidation of the LuxS/AI-2 QS system's key targets and regulation mechanism in L. plantarum L3 demonstrated the effectiveness of the new approach for predicting QS targets and provides a scientific basis for future work on improving regulation of functional factor production.

The antibacterial and inhibition effect of chitosan grafted gentisate acid derivatives against Pseudomonas fluorescens: Attacking multiple targets on structure, metabolism system, antioxidant system, and biofilm

This work aimed to investigate the antibacterial ability and potential mechanism of chitosan grafted gentisate acid derivatives (CS-g-GA) against Pseudomonas fluorescens. The results showed that CS-g-GA had a significant suppressive impact on the growth of Pseudomonas fluorescens, the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) were 0.64 mg/mL and 1.28 mg/mL, respectively. Results of scanning electron microscopy (SEM) and alkaline phosphatase (AKPase) confirmed that CS-g-GA destroyed the cell structure thereby causing the leakage of intracellular components. In addition, 1 × MIC of CS-g-GA could significantly inhibit the formation of biofilms, and 74.78 % mature biofilm and 86.21 % extracellular polysaccharide of Pseudomonas fluorescens were eradicated by CS-g-GA at 2 × MIC. The results on the respiratory energy metabolism system and antioxidant system demonstrated that CS-g-GA caused respiratory disturbance and energy limitation by influencing the key enzyme activities. It could also bind to DNA and affect genetic metabolism. From this, it could be seen that CS-g-GA had the potential to control foodborne contamination of Pseudomonas fluorescens by attacking multiple targets.

Volatile Fatty Acids Effective as Antibacterial Agents against Three Enteric Bacteria during Mesophilic Anaerobic Incubation

The antibacterial effects of a selection of volatile fatty acids (acetic, propionic, butyric, valeric, and caproic acids) relevant to anaerobic digestion were investigated at 1, 2 and 4 g/L. The antibacterial effects were characterised by the dynamics of Enterococcus faecalis NCTC 00775, Escherichia coli JCM 1649 and Klebsiella pneumoniae A17. Mesophilic anaerobic incubation to determine the minimum bactericidal concentration (MBC) and median lethal concentration of the VFAs was carried out in Luria Bertani broth at 37 °C for 48 h. Samples collected at times 0, 3, 6, 24 and 48 h were used to monitor bacterial kinetics and pH. VFAs at 4 g/L demonstrated the highest bactericidal effect (p < 0.05), while 1 g/L supported bacterial growth. The VFA cocktail was the most effective, while propionic acid was the least effective. Enterococcus faecalis NCTC 00775 was the most resistant strain with the VFAs MBC of 4 g/L, while Klebsiella pneumoniae A17 was the least resistant with the VFAs MBC of 2 g/L. Allowing a 48 h incubation period led to more log decline in the bacterial numbers compared to earlier times. The VFA cocktail, valeric, and caproic acids at 4 g/L achieved elimination of the three bacteria strains, with over 7 log10 decrease within 48 h.

Bactericidal efficacy of plasma-activated water against Vibrio parahaemolyticus on Litopenaeus vannamei

In this study, the antimicrobial mechanism of plasma-activated water (PAW) against Vibrio parahaemolyticus and the effectiveness of PAW in artificially contaminated Litopenaeus vannamei were investigated. The results demonstrated a significant reduction (p < 0.05) in viable counts of V. parahaemolyticus with increasing plasma discharge time (5, 10, 20, and 30 min) and PAW immersion time (3, 5, 10, 20, and 30 s). Specifically, the count of V. parahaemolyticus decreased by 2.1, 2.7, 3.3, and 4.4 log CFU/mL after exposed to PAW 5, PAW 10, PAW 20, and PAW 30 for 30 s, respectively. Significant cell surface wrinkling, accompanied by notable nucleic acid and protein leakage were observed after treatment with PAW. The permeability of the inner and outer cell membranes was significantly increased (p < 0.05), along with an increase in electrical conductivity (p < 0.05). The reactive oxygen species (ROS) within V. parahaemolyticus cells were significantly increased (p < 0.05), while superoxide dismutase (SOD) activity, and the relative expression of the ompW, emrD, and luxS genes were significantly decreased (p < 0.05). A reduction number of 1.3, 1.8, 2.1, and 2.2 log CFU/g of V. parahaemolyticus in artificially contaminated L. vannamei was obtained with PAW for 5 min. The study elucidated that PAW could destroy cell membranes, leading to cell death. The findings would strengthen strategies for V. parahaemolyticus control and provide a potential application of PAW for preserving aquatic products.

Insight into the inhibitory activity and mechanism of bovine cathelicidin BMAP 27 against Salmonella Typhimurium

This study synthesized an antimicrobial peptide based on the bovine cathelicidin BMAP 27 sequence. It was found to have a broad spectrum of antibacterial activity, with exceptionally high activity against Salmonella. However, the antibacterial mechanism of BMAP 27 against Salmonella remains unclear. The minimum inhibitory concentrations (MIC) and minimum bactericidal concentrations (MBC) of BMAP 27 against Salmonella enterica serovar Typhimurium were determined to be 2 μM and 4 μM, respectively. After treatment with 2 MIC of BMAP 27, the absorbance of DNA in centrifugal supernatant increased from 0.244 to 1.464, and that of protein rose from 0.174 to 0.774, respectively. BMAP 27 has compromised the cell membrane as observed through field emission scanning electron microscope (FESEM) and transmission electron microscopy (TEM), and confirmed by the propidium iodide (PI) test. The alkaline phosphatase (AKP) enzyme activity in the supernatant of the 2 MIC treatment group was 2.15 times higher than the control group, indicating extracellular membrane damage. BMAP 27 treatment increased intracellular ROS levels as tested by dichlorofluorescein diacetate (DCFH) staining. DNA interaction analysis revealed that BMAP 27 has a binding affinity towards DNA, causing its characteristic bands to disappear and peak intensity at 260 nm to reduce. Molecular docking identified its potential binding mode with DNA. The crystal violet biofilm staining results demonstrated that BMAP 27 inhibited S. Typhimurium biofilm formation by 43.1 % and cleared mature biofilms by 53.62 %. Confocal Laser scanning electron microscopy (CLSM) observed that BMAP 27 could kill bacteria within the biofilm and dislodge bacteria from the surface of glasses. Swimming tests identified that the motor capacity of S. Typhimurium was diminished by BMAP 27. By counting the total bacteria, BMAP 27 was revealed to exert bacteriostatic effects in chilled pork and orange juice, which might provide a basis for its application in the inhibition of Salmonella.

Transcriptomic analyses of Vibrio parahaemolyticus under the phenyllactic acid stress

Phenyllactic acid (PLA) generally recognized as a natural organic acid shows against Vibrio parahaemolyticus activity. In this study, V. parahaemolyticus ATCC17802 (Vp17802) was cultured under the stress of 1/2MIC PLA, and then the antibacterial mechanisms were explored via transcriptomics. The minimum inhibitory concentration (MIC) of PLA against Vp17802 was 3.2 mg/mL, and the time-kill analysis resulted that Vp17802 was inhibited. PLA was able to destroy the bacterial membrane, leading to the leakage of intracellular substances and decline of ATP levels. The RNA-sequencing analysis results indicated that 1616 significantly differentially expressed genes were identified, among which 190 were up-regulated and 1426 were down-regulated. Down-regulation of the icd2 gene in the TCA cycle mediates blockage of tyrosine metabolic, arginine biosynthesis, and oxidative phosphorylation, causing insufficient energy supply of Vp17802. Moreover, PLA could cause amino acids, metal ions, and phosphate transporters to be blocked, affecting the acquisition of nutrients. The treatment by PLA altered the expression of genes encoding functions involved in quorum sensing, flagellar assembly, and cell chemotaxis pathway, which may be interfering with the biofilm formation in Vp17802, reducing cell motility. Overall, 1.6 mg/mL PLA inhibited the growth of Vp17802 by disrupting to uptake of nutrients, cell metabolism, and the formation of biofilms. The results suggested a new direction for exploring the activity of PLA against Vp17802 and provided a theoretical basis for bacterial pathogen control in the food industry. KEY POINTS: •RNA sequencing was carried out to indicate the antibacterial mechanism of Vp17802. •The icd2 gene in the TCA cycle mediates blockage of metabolic of Vp17802. •The biofilm formation has interfered with 1.6 mg/mL PLA, which could reduce cell motility and virulence.

Targeted phytogenic compounds against Vibrio parahaemolyticus biofilms

As one of main culprit of seafood-associated human illness, Vibrio parahaemolyticus can readily accumulate on biotic or abiotic surfaces to form biofilms in the seafood processing environment. Biofilm formation on various surfaces can provide a protective barrier for viable bacterial cells that are resistant to most traditional bacteriostatic measures. This underscores the necessity and urgency of developing effective alternative strategies to control V. parahaemolyticus biofilms. Plants have always provided an extensive and infinite source of biologically active compounds for "green" antibiofilm agents. This review summarizes recent developments in promising multitargeted phytogenic compounds against V. parahaemolyticus biofilms. This review provides valuable insights into potential research targets that can be pursued further to identify potent natural antibiofilm agents in the food industry.

Antibacterial efficacy of phenyllactic acid against Pseudomonas lundensis and Brochothrix thermosphacta and its synergistic application on modified atmosphere/air-packaged fresh pork loins

Microbial contamination is a crucial problem that is difficult to solve for the meat industry. Therefore, this study explored the antibacterial efficacy of phenyllactic acid (PLA) against Pseudomonas lundensis (PL) and Brochothrix thermosphacta (BT) solely and in combination (PL + BT). It also provided insights into its synergistic preservation effect during inoculation in chilled (4 °C) fresh pork loins under air (AP) and modified atmosphere packaging (MAP). The minimum inhibitory concentration (MIC) of PLA was 10 mg/mL. Growth kinetics, scanning electron microscopy (SEM), zeta potential, and cell viability investigations showed that PLA treatment exhibited reduced bacterial growth, aided morphological alterations, and leakage in cell membrane integrity in vitro. Nonetheless, PLA and MAP (70 %N2/30 %CO2) showed an excellent synergistic antibacterial ability against spoilage indicators(total glucose, pH, TVB-N, and TBARS), bacterial counts than AP, without impairing organoleptic acceptability. These results demonstrate the broad antibacterial efficacy of PLA as a biopreservative for the meat industry.

A Novel Strategy for Whole-Cell Biotransformation Enabling Simultaneous l-Phenyllactic Acid Production and Coenzyme Regeneration

l-Phenyllactic acid (l-PLA) is a small molecular organic acid that exhibits a powerful capacity for inhibition against foodborne pathogens. In this work, we developed a new cost-effective and environmentally friendly process for the biosynthesis of l-PLA. This strategy designed a novel whole-cell biotransformation system employing two heterologous enzymes, namely, phenylalanine dehydrogenase (PheDH) and l-hydroxyisocaproate dehydrogenase (l-HicDH). The novelty of this strategy lies in the first-time utilization of these two enzymes, which not only enables cascade catalysis for the production of l-PLA but also facilitates the regeneration of the coenzymes (NAD+/NADH) using only two enzymes rather than introducing more heterologous enzymes to the system. Consequently, this strategy can effectively simplify the biosynthesis process of l-PLA and minimize production costs. The initial l-PLA yield using this process achieved 2.53 ± 0.07 g/L. Furthermore, through meticulous optimization of the parameters for inducible enzyme expression and l-PLA biosynthesis, the l-PLA yield was successfully increased to 4.68 ± 0.04 g/L with a yield rate of 64.54 ± 0.29%. Moreover, this novel strategy is versatile in the biosynthesis of other organic acids, which can be achieved by easily modulating the combinations of substrates and enzymes.

Comparative Study of Quercetin and Hyperoside: Antimicrobial Potential towards Food Spoilage Bacteria, Mode of Action and Molecular Docking

The antibacterial activities of quercetin and hyperoside were evaluated towards two major spoilage bacteria in fish, Pseudomonas aeruginosa (PA) and Shewanella putrefaciens (SP). Hyperoside showed a lower minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) towards both spoilage bacteria, PA and SP, than quercetin. Cell membrane morphology was affected when treated with hyperoside and quercetin. The release of content from the treated cells occurred, as ascertained by the release of potassium and magnesium ions and the increase in conductivity of the culture media. The morphology of cells was significantly changed, in which shrinkage and pores were obtained, when observed using SEM. Both compounds negatively affected the motility, both swimming and swarming, and the formation of extracellular polymeric substance (EPS), thus confirming antibiofilm activities. Agarose gel analysis revealed that both compounds could bind to or degrade the genomic DNA of both bacteria, thereby causing bacterial death. Molecular docking indicated that the compounds interacted with the minor groove of the DNA, favoring the adenine-thymine-rich regions. Thus, both quercetin and hyperoside could serve as potential antimicrobial agents to retard the spoilage of fish or perishable products.

Itaconic acid and dimethyl itaconate exert antibacterial activity in carbon-enriched environments through the TCA cycle

Itaconic acid (IA), a metabolite generated by the tricarboxylic acid (TCA) cycle in eukaryotic immune cells, and its derivative dimethyl itaconate (DI) exert antibacterial functions in intracellular environments. Previous studies suggested that IA and DI only inhibit bacterial growth in carbon-limited environments; however, whether IA and DI maintain antibacterial activity in carbon-enriched environments remains unknown. Here, IA and DI inhibited the bacteria with minimum inhibitory concentrations of 24.02 mM and 39.52 mM, respectively, in a carbon-enriched environment. The reduced bacterial pathogenicity was reflected in cell membrane integrity, motility, biofilm formation, AI-2/luxS, and virulence. Mechanistically, succinate dehydrogenase (SDH) activity and fumaric acid levels decreased in the IA and DI treatments, while isocitrate lyase (ICL) activity was upregulated. Inhibited TCA circulation was also observed through untargeted metabolomics. In addition, energy-related aspartate metabolism and lysine degradation were suppressed. In summary, these results indicated that IA and DI reduced bacterial pathogenicity while exerting antibacterial functions by inhibiting TCA circulation. This study enriches knowledge on the inhibition of bacteria by IA and DI in a carbon-mixed environment, suggesting an alternative method for treating bacterial infections by immune metabolites.

Mechanism of Inhibiting the Growth and Aflatoxin B1 Biosynthesis of Aspergillus flavus by Phenyllactic Acid

Phenyllactic acid (PLA), a promising food preservative, is safe and effective against a broad spectrum of food-borne pathogens. However, its mechanisms against toxigenic fungi are still poorly understood. In this study, we applied physicochemical, morphological, metabolomics, and transcriptomics analyses to investigate the activity and mechanism of PLA inhibition of a typical food-contaminating mold, Aspergillus flavus. The results showed that PLA effectively inhibited the growth of A. flavus spores and reduced aflatoxin B₁ (AFB₁) production by downregulating key genes associated with AFB₁ biosynthesis. Propidium iodide staining and transmission electron microscopy analysis demonstrated a dose-dependent disruption of the integrity and morphology of the A. flavus spore cell membrane by PLA. Multi-omics analyses showed that subinhibitory concentrations of PLA induced significant changes in A. flavus spores at the transcriptional and metabolic levels, as 980 genes and 30 metabolites were differentially expressed. Moreover, KEGG pathway enrichment analysis indicated PLA-induced cell membrane damage, energy-metabolism disruption, and central-dogma abnormality in A. flavus spores. The results provided new insights into the anti-A. flavus and -AFB₁ mechanisms of PLA.

Bactericidal Action of Shrimp Shell Chitooligosaccharide Conjugated with Epigallocatechin Gallate (COS-EGCG) against Listeria monocytogenes

The antibacterial effect of chitooligosaccharide conjugated with five different polyphenols, including catechin (COS-CAT), epigallocatechin gallate (COS-EGCG), gallic acid (COS-GAL), caffeic acid (COS-CAF), and ferulic acid (COS-FER), against Listeria monocytogenes was investigated. Among all the conjugates tested, COS-EGCG showed the highest inhibition toward Listeria monocytogenes, with a minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of 1024 and 1024 µg/mL, respectively. The COS-EGCG conjugate also had a bactericidal effect on the environmental and clinical strains of L. monocytogenes. The low concentration of COS-EGCG conjugate augmented the formation of biofilm and the growth of L. monocytogenes. Nevertheless, the inhibition of biofilm formation and bacterial growth was achieved when treated with the COS-EGCG conjugate at 2 × MIC for 48 h. In addition, the COS-EGCG conjugate at 2 × MIC had the potential to inactivate the pre-biofilm, and it reduced the production of the extracellular polysaccharides of L. monocytogenes. The COS-EGCG conjugate at the MIC/4 effectively impeded the motility (the swimming and swarming) of L. monocytogenes, with an 85.7-94.3% inhibition, while 100% inhibition was achieved with the MIC. Based on scanning electron microscopic (SEM) images, cell wall damage with numerous pores on the cell surface was observed. Such cell distortion resulted in protein leakage. As a result, COS-EGCG could penetrate into the cell and bind with the DNA backbone. Therefore, the COS-EGCG conjugate could be further developed as a natural antimicrobial agent for inhibiting or controlling L. monocytogenes.

Vibrio parahaemolyticus Isolates from Asian Green Mussel: Molecular Characteristics, Virulence and Their Inhibition by Chitooligosaccharide-Tea Polyphenol Conjugates

Fifty isolates of Vibrio parahaemolyticus were tested for pathogenicity, biofilm formation, motility, and antibiotic resistance. Antimicrobial activity of chitooligosaccharide (COS)-tea polyphenol conjugates against all isolates was also studied. Forty-three isolates were randomly selected from 520 isolates from Asian green mussel (Perna viridis) grown on CHROMagarTM Vibrio agar plate. Six isolates were acquired from stool specimens of diarrhea patients. One laboratory strain was V. parahaemolyticus PSU.SCB.16S.14. Among all isolates tested, 12% of V. parahaemolyticus carried the tdh+trh- gene and were positive toward Kanagawa phenomenon test. All of V. parahaemolyticus isolates could produce biofilm and showed relatively strong motile ability. When COS-catechin conjugate (COS-CAT) and COS-epigallocatechin-3-gallate conjugate (COS-EGCG) were examined for their inhibitory effect against V. parahaemolyticus, the former showed the higher bactericidal activity with the MBC value of 1.024 mg/mL against both pathogenic and non-pathogenic strains. Most of the representative Asian green mussel V. parahaemolyticus isolates exhibited high sensitivity to all antibiotics, whereas one isolate showed the intermediate resistance to cefuroxime. However, the representative clinical isolates were highly resistant to nine types of antibiotics and had multiple antibiotic resistance (MAR) index of 0.64. Thus, COS-CAT could be used as potential antimicrobial agent for controlling V. parahaemolyticus-causing disease in Asian green mussel.

Redefining methods for augmenting lactic acid bacteria robustness and phenyllactic acid biocatalysis: Integration valorizes simplicity

The production of phenyllactic acid (PLA) has been reported by several researchers, but so far, no mention has been made of augmented PLA production using an orchestrated assembly of simple techniques integrated to improve lactic acid bacteria (LAB) metabolism for the same. This review summarizes sequentially tailoring LAB growth and metabolism for augmented PLA catalysis through several strategies like monitoring LAB sustenance by choosing appropriate starter PLA-producing LAB strains isolated from natural environments, with desirably fastidious growth rates, properties like acidification, proteolysis, bacteriophage-resistance, aromatic/texturing-features, etc.; entrapping chosen LAB strains in novel cryogels and/or co-cultivating two/more LAB strains to improve their biotransformation potential and promote growth dependency/sustainability; adopting adaptive evolution methods designed to improve LAB strains under selection pressure inducing desired phenotypes tolerant to stress factors like heat, salt, acid, and solvent; monitoring physico-chemical LAB fermentation factors like temperature, pH, dissolved oxygen content, enzymes, and cofactors for PLA biosynthesis; and modulating purification/downstream processes to extract substantial PLA yields. This review paper serves as a comprehensive preliminary guide that can evoke a strategic experimental plan to produce industrial-scale PLA yields using simple techniques orchestrated together in the pursuit of conserving time, effort, and resources.

3-Phenyllactic acid generated in medicinal plant extracts fermented with plant-derived lactic acid bacteria inhibits the biofilm synthesis of Aggregatibacter actinomycetemcomitans

In the present study, the effect of PLA on a periodontic pathogen, Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans), the biofilm, and virulence-related genes was investigated. We confirmed that two lactic acid bacteria (LAB) strains isolated from plant sources, Lactiplantibacillus plantarum MSC-C2 and Pediococcus pentosaceus K40, secrete PLA into the de Man, Rogosa &amp; Sharpe (MRS) broth when supplemented with phenyl pyruvic acid (PPA) as a precursor to PLA. Moreover, PLA was generated in the fermentation broths of two medicinal plant extracts, Paeonia lactiflora Pall (PR) and Carthamus tinctorius (CT), when used by each LAB strain and each extract supplemented with PPA. We determined that the minimum inhibitory concentration (MIC) of PLA against A. actinomycetemcomitans was 20 mM. PLA significantly decreased biofilm formation and suppressed the transcription of pgA, ltxA, and cdtB genes, which encode the poly-N-acetylglucosamine (PGA) polysaccharide of biofilm matrix and exotoxins leukotoxin and cytolethal distending toxin (CDT), respectively. The PLA produced by the MSC-C2 and K40 strains was increased several times by the addition of PPA to the MRS broth. The anti-biofilm effect of the extracts from the fermentation broth was proportional to the increasing PLA concentration, while a cumulatively higher effect than that of PLA alone suggested a combinational effect of PLA and the other metabolites, such as lactic acid (LA). Among the two medicinal plants, PLA, produced after the addition of PPA, was higher in PR extract in case of both the LAB strains. PLA production by the MSC-C2 strain in the PR extract reached 4.8 ± 0.23 mM, which was obviously higher than that in the MRS broth (3.88 ± 0.12 mM) supplemented with 1 mg/ml PPA. The activity to inhibit biofilm formation in the fermented PR extract was clearly high. PLA formed in the fermented PR extract downregulated the dispersin B encoding the dspB gene together with pgA, ltxA, and cdtB. In conclusion, this study shows a promising activity of PLA against the A. actinomycetemcomitans biofilm and virulence genes. In addition, the combinational effect of PLA and the medicinal plant extract can be achieved by fermentation with a specific plant-derived LAB strain.

Juglone Inhibits Listeria monocytogenes ATCC 19115 by Targeting Cell Membrane and Protein

Foodborne disease caused by Listeria monocytogenes is a major global food safety problem. A potential solution is the antimicrobial development of the highly bioactive natural product juglone, yet few studies exist on its antibacterial mechanism against L. monocytogenes. Thus, we aimed to elucidate the antibacterial mechanism of action of juglone against L. monocytogenes by determining the resultant cell morphology, membrane permeability, membrane integrity, and proteome changes. The minimum inhibitory concentration of juglone against L. monocytogenes was 50 μg/mL, and L. monocytogenes treated with juglone had longer lag phases compared to controls. Juglone induced L. monocytogenes cell dysfunction, leakage of potassium ions, and membrane potential hyperpolarization. Confocal laser scanning microscopy and field-emission-gun scanning electron microscope assays revealed clear membrane damage due to juglone treatment. Fourier transform infrared analyses showed that L. monocytogenes responded to juglone by some conformational and compositional changes in the molecular makeup of the cell membrane. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis showed that juglone either destroyed proteins or inhibited proteins synthesis in L. monocytogenes. Therefore, our findings established juglone as a natural antibacterial agent with potential to control foodborne L. monocytogenes infections.

The Antimicrobial Effect of Melissa officinalis L. Essential Oil on Vibrio parahaemolyticus: Insights Based on the Cell Membrane and External Structure

The study was to evaluate the antimicrobial impacts on Melissa officinalis L. essential oil (MOEO) against Vibrio parahaemolyticus. The minimum inhibitory concentration (MIC) of MOEO on Vibrio parahaemolyticus was 1 μL⋅mL^–1. The kill-time curve exhibited that MOEO had good antimicrobial activity. The analysis of cellular ingredients leakage and cell viability illustrated that MOEO has destruction to the morphology of the cell membrane. The damage to the membrane integrity by MOEO has been confirmed by transmission and scanning electron microscopy, obvious morphological and ultrastructural changes were observed in the treated bacterial cells. The MOEO at 0.5 μL⋅mL^–1 can inhibit the biofilm formation, biofilm motility, and extracellular polysaccharide production. Meanwhile, the qPCR results exhibited MOEO inhibited the expression of virulence genes. The findings showed that MOEO exerted its antimicrobial effect mainly by destroying the membrane, which indicated its potential as a natural food preservative.