Antibacterial mechanism of ε -Poly-lysine against Listeria monocytogenes and its application on cheese

Bacteriophage biocontrol of Shiga toxigenic Escherichia coli (STEC) O145 biofilms on stainless steel reduces the contamination of beef

Shiga toxigenic Escherichia coli (STEC) can form biofilms and frequently cause serious foodborne illnesses. A strain of STEC O145:H25 (EC19990166) known to be a strong biofilm former was used to evaluate the efficacy of bacteriophage AZO145A against biofilms formed on stainless steel (SS) coupons. Exposure of STEC O145:H25 to phage AZO145A (10¹⁰ PFU/mL) for 2 h resulted in a 4.0 log₁₀ reduction (P < 0.01) of planktonic cells grown in M9 broth at 24 °C for 24 h, while reductions were 2.0 log₁₀ CFU/mL if these cells were grown for 48 h or 72 h prior to phage treatment. STEC O145 biofilms formed on SS coupons for 24, 48 and 72 h were reduced (P < 0.01) 2.9, 1.9 and 1.9 log₁₀ CFU/coupon by phages. STEC O145 cells in biofilms were readily transferred from the surface of the SS coupon to beef (3.6 log₁₀ CFU/coupon) even with as little as 10 s of contact with the meat surface. However, transfer of STEC O145 cells from biofilms that formed on SS coupons for 48 h to beef was reduced (P < 0.01) by 3.1 log₁₀ CFU by phage (2 × 10¹⁰ PFU/mL) at 24 °C. Scanning electron microscopy revealed that bacterial cells within indentations on the surface of SS coupons were reduced by phage. These results suggest that bacteriophage AZO145A could be effective in reducing the viability of biofilm-adherent STEC O145 on stainless steel in food industry environments.

The Antibacterial Activity and Mechanism of Action of Luteolin Against Trueperella pyogenes

Purpose

This research aimed to investigate the antibacterial activity and potential mechanism of luteolin against T. pyogenes.

Materials and Methods

The broth microdilution method was used to determine the minimum inhibitory concentrations (MICs) of luteolin against various T. pyogenes strains. The potential mechanism of action of luteolin was elucidated through testing and analysing the luteolin-induced alterations of T. pyogenes in several aspects, including cell wall, cell membrane, protein expression, nucleic acid content, topoisomerase activity and energy metabolism.

Results

The MIC values of luteolin against various T. pyogenes isolates and ATCC19411 were 78 µg/mL. The increased cell membrane permeability, destruction of cell wall integrity and TEM images after exposure to luteolin showed that the cell wall and membrane were damaged. The content of total protein and nucleic acid in T. pyogenes decreased significantly after treatment with luteolin (1/2 MIC) for 12, 24, and 36 h. Moreover, a hypochromic effect was observed in the absorption spectrum of luteolin when deoxyribonucleic acid (DNA) was added. In addition, after treatment with luteolin, a decrease in nicked or relaxed DNA content, which was catalysed by T. pyogenes-isolated DNA topoisomerase, was observed. In addition, the adenosine triphosphate (ATP) content in cells and the activity of succinate dehydrogenase (SDH) both decreased when T. pyogenes was exposed to different concentrations (1/4 MIC, 1/2 MIC, 1 MIC, 2 MIC) of luteolin for 1 h.

Conclusion

Luteolin showed distinct antibacterial activity against T. pyogenes by multiple actions, which mainly include destroying the integrity of the cell wall and cell membrane, influencing the expression of proteins, inhibiting nucleic acid synthesis, and interfering with energy metabolism.

Preparation and characterization of corn starch/PVA/glycerol composite films incorporated with ε-polylysine as a novel antimicrobial packaging material

Corn starch/polyvinyl alcohol (PVA)/glycerol composite films incorporated with ε-polylysine were prepared, and their properties were investigated. The Fourier-transform infrared (FTIR) spectroscopy indicated that the interactions happened between the amino group of ε-polylysine and hydroxyl group starch/PVA composite films. X-ray diffraction (XRD) analysis showed that the addition of ε-polylysine decreased the intensity of all crystal peaks. Thermogravimetric (TGA) analysis suggested that ε-polylysine improved the thermal stability of composite films. Scanning electron microscopic (SEM) analysis showed that the upper surface of composite films incorporated with ε-polylysine presented more compact and flat surface. The antimicrobial activity of the composite film progressively increased with the increasing of ε-polylysine concentration (P < 0.05). The tensile strength, elongation at break and water absorption significantly increased, whereas water solubility decreased with the increasing of ε-polylysine concentration (P < 0.05). Therefore, the corn starch/PVA/glycerol composite films incorporated with ε-polylysine had good mechanical, physical and antimicrobial properties and could have potential application as a novel antimicrobial packaging material.

Transcriptional changes involved in inhibition of biofilm formation by ε-polylysine in Salmonella Typhimurium

The pathogenicity of Salmonella Typhimurium, a foodborne pathogen, is mainly attributed to its ability to form biofilm on food contact surfaces. ε-polylysine, a polymer of positively charged lysine, is reported to inhibit biofilm formation of both gram-positive and gram-negative bacteria. To elucidate the mechanism underlying ε-polylysine-mediated inhibition of biofilm formation, the transcriptional profiles of ε-polylysine-treated and untreated Salmonella Typhimurium cells were comparatively analysed. The genome-wide DNA microarray analysis was performed using Salmonella Typhimurium incubated with 0.001% ε-polylysine in 0.1% Bacto Soytone at 30 °C for 2 h. The expression levels of genes involved in curli amyloid fibres and cellulose production, quorum sensing, and flagellar motility were downregulated, whereas those of genes associated with colanic acid synthesis were upregulated after treatment with ε-polylysine. The microarray results were validated by quantitative real-time polymerase chain reaction (qRT-PCR). Furthermore, treatment with ε-polylysine decreased the production of colanic acid in Salmonella Typhimurium. The findings of this study improved our understanding of the mechanisms underlying ε-polylysine-mediated biofilm inhibition and may contribute to the development of new disinfectants to control biofilm during food manufacturing and storage.

Structural Changes and Antibacterial Activity of Epsilon-poly-l-lysine in Response to pH and Phase Transition and Their Mechanisms

ε-Poly-l-lysine (ε-PL) consists of 25-35 lysine residues which are linked by an isopeptide bond formed by dehydration condensation of α-carboxyl and ε-amino groups and has good antibacterial activity and broad-spectrum inhibition range. However, there is no clear conclusion about the structure and antibacterial mechanism of ε-PL in aqueous solution. Herein, a high purity of ε-PL was prepared using Amberlite IRC-50 ion-exchange resin. Membrane filtration and dynamic light scattering were used to study the variations of ε-PL aggregation in aqueous solution with pH value. The conformational changes and antibacterial activities of ε-PL and carbamoylated ε-PL in different water environments were studied with circular dichroism (CD) and inhibition zone. The structural changes during the spray-drying process were determined by Fourier transform infrared spectroscopy. The results indicated that the side chain amino charge played a decisive role in the ε-PL conformation and aggregation. ε-PL exhibited the properties of a β-sheet during spray drying from acidic liquids to solids. The cation enhanced the antibacterial activity of ε-PL but did not play a key role. Instead, the backbone of ε-PL might determine the mechanism of ε-PL antibacterial.

Antimicrobial, microscopic and spectroscopic properties of cellulose paper coated with chitosan sol-gel solution formulated by epsilon-poly-l-lysine and its application in active food packaging

Cellulose paper-chitosan (CC) double-layer films containing epsilon-poly-l-lysine (ε-PL) (0.5 and 1% w/v) were developed. FTIR analysis showed a strong association between the ε-PL and CC film. Antimicrobial activity against Listeria monocytogenes was investigated both in vitro and in the chicken breast meat. The CC films without ε-PL showed no antimicrobial activity, while the addition of ε-PL induced significant (p < 0.05) effects. During the 28 days of storage at 4 ^°C, no significant difference was found on the anti-listeria activity of films. When storage temperature was raised from 4 to 22 ^°C, the antimicrobial activity was reduced. Films containing 1% ε-PL exhibited 1.5 log₁₀ CFU/g reduction in L. monocytogenes population during 12 days storage of meat at 4 ^°C, while no significant reduction was found in CC films with 0.5% ε-PL (p > 0.05). This study revealed an antimicrobial activity for CC films impregnated with ε-PL, to control foodborne pathogens in meat.

Antimicrobial Susceptibility and Antibacterial Mechanism of Limonene against Listeria monocytogenes

Limonene is a monoterpenoid compound, which is founded in a lot of plants’ essential oils with good antibacterial activity against food-borne pathogens, but it has an ambiguous antimicrobial susceptibility and mechanism against Listeria monocytogenes (L. monocytogenes). In this study, the antimicrobial susceptibility of Limonene to L. monocytogenes was studied, and some new sights regarding its antibacterial mechanism were further explored. Scanning electron microscopy (SEM) verified that limonene caused the destruction of the cell integrity and wall structure of L. monocytogenes. The increase in conductivity and the leakage of intracellular biomacromolecules (nucleic acids and proteins) confirmed that limonene had an obvious effect on cell membrane permeability. The results of Propidium Iodide (PI) fluorescence staining were consistent with the results of the conductivity measurements. This indicated that limonene treatment caused damage to the L. monocytogenes cell membrane. Furthermore, the decrease in ATP content, ATPase (Na⁺K⁺-ATPase, Ca²⁺-ATPase) activity and respiratory chain complex activity indicated that limonene could hinder ATP synthesis by inhibiting the activity of the respiratory complex and ATPase. Finally, differential expression of proteins in the respiratory chain confirmed that limonene affected respiration and energy metabolism by inhibiting the function of the respiratory chain complex.

Multipathway Antibacterial Mechanism of a Nanoparticle-Supported Artemisinin Promoted by Nitrogen Plasma Treatment

Artemisinin has excellent antimalarial, antiparasitic, and antibacterial activities; however, the poor water solubility of artemisinin crystal limits their application in antibiosis. Herein, artemisinin crystal was first composited with silica nanoparticles (SNPs) to form an artemisinin@silica nanoparticle (A@SNP). After treating with nitrogen plasma, the aqueous solubility of plasma-treated A@SNP (A@SNP-p) approaches 42.26%, which is possibly attributed to the exposure of hydrophilic groups such as -OH groups on the SNPs during the plasma process. Compared with the pristine A@SNP, the antibacterial activity of A@SNP-p against both Gram-positive and Gram-negative strains is further enhanced, and its bactericidal rate against both strains exceeded 6 log CFU/mL (>99.9999%), which is contributed by the increased water solubility of the A@SNP-p. A possible multipathway antibacterial mechanism of A@SNP was proposed and preliminarily proved by the changes of intracellular materials of bacteria and the inhibition of bacterial metabolism processes, including the HMP pathway in Gram-negative strain and EMP pathway in Gram-positive strain, after treating with A@SNP-p. These findings from the present work will provide a new view for fabricating artemisinin-based materials as antibiotics.

Antibacterial activity of the noni fruit extract against Listeria monocytogenes and its applicability as a natural sanitizer for the washing of fresh-cut produce

This study was conducted to investigate the antibacterial activity of the noni fruit extract (NFE) against Listeria monocytogenes (ATCC, 19111 and 19115) and assess its applicability for the washing of fresh-cut produce. Based on the results of the disc diffusion test, L. monocytogenes (ATCC, 19111 and 19115) was susceptible to the activity of NFE than other pathogens studied. Additionally, results of the time-kill assay indicated that NFE at a concentration of 0.5-0.7% effectively killed L. monocytogenes within 7 h. Furthermore, analysis of the intracellular components such as nucleic acids and proteins released from the bacterial cells and their SEM imaging revealed that NFE could increase the membrane permeability of cells resulting in their death. Compared to their unwashed samples, washing of romaine lettuce, spinach, and kale with 0.5% NFE gave a reduction of 1.47, 2.28, and 3.38 log CFU/g, respectively against L. monocytogenes (ATCC, 19111 and 19115), which is significantly different to that of NaOCl. A significant correlation was observed between the antibacterial effect induced due to NFE washing with the surface roughness of the fresh-cut produce than its surface hydrophobicity. Moreover, washing with NFE was not found to affect the color of the samples. These results indicated that NFE demonstrates good antibacterial activity against L. monocytogenes and can be used as a natural sanitizer to ensure the microbiological safety of fresh-cut produce.

Inactivation mechanism of E. coli O157:H7 under ultrasonic sterilization

Ultrasonic sterilization (US), as a promising non-thermal sterilization method, exhibits unique superiorities than traditional sterilization methods. In this study, the inactivation mechanism of E. coli O157:H7 under US was investigated in cucumber and bitter gourd vegetable juices. Results revealed that the US treatment showed good antibacterial ability in countering E. coli O157:H7. Through determinations of conductivity and β-galactosidase activity, significant augmentation in membrane permeability of the bacteria was confirmed after the US treatment. The morphologies of the US treated E. coli O157:H7 demonstrated that the integrity of the cell membrane was disrupted by US treatment. SDS-PAGE and LSCM data further proved the disruptive action of US, leading to the leakage of proteins and DNA through the breakage on cell membrane. The decrease of metabolic-related enzyme activity was verified through investigation of bacterial metabolism. The antibacterial mechanism analysis indicated that the US can generate free radicals which resulted in the rise of intracellular oxidative stress, attenuation of energy metabolism and inhibition of hexose monophosphate pathway. As the application verification, the US treatment can cause the deprivation of E. coli O157:H7 cell viability in vegetable juices without obvious impact on the sensory quality.

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.

Antibacterial Activity and Mode of Action of Dihydromyricetin from Ampelopsis grossedentata Leaves against Food-Borne Bacteria

Dihydromyricetin (DMY) has recently attracted increased interest due to its considerable health-promoting activities but there are few reports on its antibacterial activity and mechanism. In this paper, the activity and mechanisms of DMY from Ampelopsis grossedentata leaves against food-borne bacteria are investigated. Moreover, the effects of pH, thermal-processing, and metal ions on the antibacterial activity of DMY are also evaluated. The results show that DMY exhibits ideal antibacterial activity on five types of food-borne bacteria (Staphylococcus aureus, Bacillus subtilis, Escherichia coli, Salmonella paratyphi, and Pseudomonas aeruginosa). The activities of DMY against bacteria are extremely sensitive to pH, thermal-processing, and metal ions. The morphology of the tested bacteria is changed and damaged more seriously with the exposure time of DMY. Furthermore, the results of the oxidative respiratory metabolism assay and the integrity of the cell membrane and wall tests revealed that the death of bacteria caused by DMY might be due to lysis of the cell wall, leakage of intracellular ingredients, and inhibition of the tricarboxylic acid cycle (TCA) pathway.

Antibacterial properties of nanofibers containing chrysanthemum essential oil and their application as beef packaging

This study was aimed to develop novel antibacterial packaging materials in order to reduce the microbial contamination of food surface. Chrysanthemum essential oil (CHEO) was successfully incorporated into chitosan nanofibers (CS/NF) through electrospinning which was demonstrated by SEM and AFM analysis. The antibacterial mechanism of CHEO against Listeria monocytogenes (L. monocytogenes) was explored as well. The cell membrane permeability of L. monocytogenes appeared to be increased by CHEO. In addition, respiratory metabolism of L. monocytogenes was inhibited by CHEO through the inhibition of the Embden-Meyerhof-Parnas (EMP) pathway. The presence of CHEO had a negative effect on the activity of hexokinase, phosphofructokinase and pyruvate kinase in L. monocytogenes cells. Release efficiency study indicated that the CHEO could be released slowly from CHEO/CS/NF to achieve long-lasting antibacterial effect. The antibacterial application of the CHEO nanofibers against L. monocytogenes was tested on beef, with an inhibition rate of 99.91%, 99.97%, and 99.95% at the temperature of 4 °C, 12 °C and 25 °C, respectively, after 7 days of storage. Beef parameters like thiobarbituric acid reactive substances (TBARS), pH values, and texture at different storage temperatures (4 °C, 12 °C and 25 °C) were evaluated as well. Due to the presence of antioxidant components in CHEO released from CHEO/CS/NF, the TBARS value in treated beef was 0.135 MDA/kg lower (P < 0.05) than the untreated sample at 4 °C after 12 days. PH value assay indicated that PH value of beef sample packed with CHEO/CS/NF (6.43) was lower than unpacked sample (7.05) at 4 °C after 10 days of storage. These obtained results all illustrated the fact that CHEO/CS/NF could prolong the shelf-life of beef, suggesting a potential application in food packaging.