A Novel Class IIb Bacteriocin-Plantaricin EmF Effectively Inhibits Listeria monocytogenes and Extends the Shelf Life of Beef in Combination with Chitosan

An insight into structure-activity relationships in subclass IIb bacteriocins: Plantaricin EvF

This study reports the structure-activity relationships of a unique subclass IIb bacteriocin, plantaricin EvF, which consists of two peptide chains and possesses potent antimicrobial activity. Because the plantaricin Ev peptide chain lacks an α-helix structure, plantaricin EvF is unable to exert its antimicrobial activity through helix-helix interactions like typical subclass IIb bacteriocins. We have shown by various structural evaluation methods that plantaricin Ev can be stabilized by hydrogen bonding at amino acid residues R3, V12, and R13 to the N-terminal region of plantaricin F. This binding gives plantaricin EvF a special spade-shaped structure that exerts antimicrobial activity. In addition, the root-mean-square deviations (RMSDs) of the amino acid residues Y6, F8, and R13 of plantaricin Ev pre- and post-binding were 1.512, 1.723, and 1.369, respectively, indicating that they underwent large structural changes. The alanine scanning experiments demonstrated the important role of the above key amino acids in maintaining the structural integrity of plantaricin EvF. This study not only reveals the unique structural features of plantaricin EvF, but also provides an insight into the structure-activity relationships of subclass IIb bacteriocins.

Antibacterial Activity and Mechanism of Action of Osthole against Listeria monocytogenes

The purpose of this study was to investigate the antibacterial activity and mechanism of action of osthole against Listeria monocytogenes. The antibacterial activity of osthole was evaluated by determining the minimum inhibitory concentration (MIC) and growth curve. Cell morphology, membrane permeability, membrane integrity, bacterial physiology, and metabolism were explored using different methods to elucidate the mechanism of action of osthole. It was shown that the MIC of osthole against L. monocytogenes was 62.5 μg/mL and it inhibited the growth of L. monocytogenes effectively in a concentration-dependent manner. Scanning electron microscopy (SEM) images demonstrated morphology changes of L. monocytogenes, including rough surface, cell shrinkage, and rupture. It was found that extracellular conductivity and macromolecule content were increased significantly in the presence of osthole, indicating the disruption of cell membrane integrity and permeability. Laser confocal microscopy results supported the conclusion that osthole caused severe damage to the cell membrane. It was also noticed that osthole depleted intracellular adenosine triphosphate (ATP), inhibited Na+-K+-ATPase and Ca2+-Mg2+-ATPase activity, and promoted the accumulation of intracellular reactive oxygen species (ROS), leading to cell death. This study suggests that osthole is a promising antibacterial agent candidate against L. monocytogenes, and it shows potential in the prevention and control of foodborne pathogens.

Screening and Identification of Goat-Milk-Derived Lactic Acid Bacteria with Bacteriocin-like Activity and Probiotic Potentials

In the current study, we screened 46 isolates of lactic acid bacteria (LAB) derived from goat milk for bacteriocin producers that can inhibit common foodborne pathogens (Staphylococcus aureus, Listeria monocytogenes, and Bacillus cereus). The following three strains that showed antimicrobial activity against all indicators were identified: Enterococcus faecalis DH9003 and DH9012, and Lactococcus lactis DH9011. Their antimicrobial products exhibited typical bacteriocin characteristics, such as heat stability and proteinase nature. The bacteriostatic activity of concentrated bacteriocins produced by these LAB was observed at low concentrations (half-minimum inhibitory concentration [MIC50] and 4MIC50), whereas complete inhibition activity against Listeria monocytogenes was detected at high concentrations (16MIC50) of the two Enterococcus faecalis strains (DH9003 and DH9012). Furthermore, the probiotic potentials of the three strains were investigated and described. The results revealed that none of the strains had hemolytic activity, whereas all: were sensitive to ampicillin (50 mg/mL) and streptomycin sulfate (100 mg/mL); were resistant to bile, artificial simulated intestinal tract, and gastric juice at different pH levels (2.5, 3.0, 3.5); and had β-galactosidase activity. Furthermore, all strains exhibited an auto-aggregating phenotype, with self-aggregation ranging from 30% to 55%. DH9003 and DH9012 co-aggregated well with Listeria monocytogenes and Escherichia coli (52.6% and 63.2%, 68.5% and 57.6%, respectively), whereas DH9011 co-aggregated poorly with Listeria monocytogenes (15.6%) and did not co-aggregate with Escherichia coli. Furthermore, our results showed that all three isolates exhibited strong antibacterial activity, tolerance to bile and simulated gastrointestinal environments, adhesion capability, and safety. Finally, DH9003 was selected and used for gavage in rats. By observing the pathological characteristics of rat intestinal and liver tissue sections, DH9003 showed no harmful effects on the intestine and liver of rats, but rather resulted in a denser and longer intestinal mucosa, as well as improving the intestinal mucosa of rats. Considering their substantial prospective applications, we concluded that these three isolates are potential probiotic candidates.

Extending the Shelf Life of Raw Milk and Pasteurized Milk with Plantaricin FB-2

Raw milk and pasteurized milk are characterized by a short shelf life, and drinking expired raw milk and pasteurized milk causes illness. In the study, Plantaricin FB-2 (extracted from Lactiplantibacillus plantarum FB-2) was added to liquid milk. By evaluating the microbial growth, acidity changes, protein content, and sensory changes in raw milk and pasteurized milk during storage, it was found that when Plantaricin FB-2 was added at 0.4 g/kg, the shelf life of raw milk was extended by 3 days (7 days if not added). The shelf life of pasteurized milk with Plantaricin FB-2 was extended to 31 days (25 days in the control group), and the optimal amount was 0.3 g/kg. This confirmed that Plantaricin FB-2 can effectively prolong the shelf life of raw and pasteurized milk. This study provides valuable information for the application of bacteriocins produced by Lactiplantibacillus plantarum in raw milk and pasteurized milk to improve their shelf life.

Bridge-DNA synthesis triggered by an allosteric aptamer for the colorimetric detection of pathogenic bacteria

Rapid and sensitive quantification of pathogenic bacteria is highly desired for environmental health supervision and food safety control. Yet, the amplification and detection of bacteria with a concentration lower than 10² cfu mL^-1 remains a great challenge. Here, we combined an allosteric aptamer (AAP) with a gold nanoparticle (AuNP) for assembling a bridge-DNA synthesis system (named as AuNP-BDS) to amplify the bacterial signals. The AAP and its paired primer (PP) were covalently linked to two different AuNPs, respectively: one named as AAP-AuNP and the other PP-AuNP. Upon recognition of the antigen from the pathogenic bacteria, AAP alters its conformation to initiate DNA synthesis on the AuNP surface. The DNA products from AAP-AuNP and PP-AuNP form bridges to each other through base pairing, resulting in the aggregation and colorimetric response of the AuNPs. By using E. coli O157:H7 as an example, the AuNP-BDS could quantify pathogenic bacteria in water with a concentration as low as 10 cfu mL^-1 within 60 min and without any enrichment. The colorimetric response values of AuNP-BDS were found to be linearly related to the bacterial concentrations in the range of 10 to 10³ cfu mL^-1. Good practicability of the AuNP-BDS in quantifying E. coli O157:H7 from tap water, juices, and milks was demonstrated. The AuNP-BDS could be exploited to facilitate the rapid and sensitive quantification of pathogenic bacteria for food safety control.

Screening and Probiotic Potential Evaluation of Bacteriocin-Producing Lactiplantibacillus plantarum In Vitro

Probiotics are gaining attention due to their functions of regulating the intestinal barrier and promoting human health. The production of bacteriocins is one of the important factors for probiotics to exert beneficial properties. This study aimed to screen bacteriocin-producing Lactiplantibacillus plantarum and evaluate the probiotic potential in vitro. It was found that L. plantarum Q7, L. plantarum F3-2 and L. plantarum YRL45 could produce bacteriocins and inhibit common intestinal pathogens. These three strains had probiotic potential with tolerance to the gastrointestinal environmental and colonization in the gut, and exhibited various degrees of anti-inflammatory activity and tight junction function in the intestinal barrier. Particularly, L. plantarum YRL45 could significantly (p < 0.05) reduce the increase in nitric oxide (NO), prostaglandin E2 (PGE2), necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) induced by lipopolysaccharide (LPS), thereby easing inflammatory response. L. plantarum F3-2 could remarkably (p < 0.05) up-regulate the expression levels of ZO-1, Occludin and Claudin-1 in intestinal epithelial injured cells, which was conducive to protecting the intestinal barrier. These findings provided fundamental information about the probiotic properties of bacteriocin-producing L. plantarum, which suggested that L. plantarum Q7, L. plantarum F3-2 and L. plantarum YRL45 had the potential to be used as novel probiotic strains.