Essential oils and nanoparticles for antimicrobial use
Currently, research is increasing on the use of plant-derived substances and essential oils against food pathogens, due to their antimicrobial effect. According to the current trend, chemical substances are used in the smallest possible amount, therefore essential oils can be suitable alternatives to chemical antimicrobial substances. Various beneficial effects can be achieved in combination with nanoparticles (e.g. controlled release of the active ingredient). Below is a short summary of the publications that have come to our attention on the subject.

Essential Oil against Pectobacterium carotovorum

Infected by Pectobacterium carotovorum subsp. carotovorum (Pcc), the quality of Chinese cabbage could severely decline. Using chemical bactericides to control Pcc could cause food safety problems. Thus, Chinese authors investigated the applicability of Polygonum orientale L. essential oil (POEO) against Pcc. Briefly, the results of the study demonstrate that POEO showed effective inhibitory activity against Pcc.

Recently, Polygonum orientale L., a medicinal plant, has generated increased research interest due to its various bioactivities. The insecticidal activity of its extract has been shown. However, there are no reports related to the antibacterial activity of POEO against Pcc.

Previous research indicates that certain essential oils have also shown antibacterial activity against Pcc. The current results show that POEO has a stronger inhibitory effect against Pcc. Almost no decay developed on Chinese cabbage stems treated with 20 mg/mL of POEO in the protective and curative assays.

Through GC-MS analysis, 29 different compounds were obtained and 23 compounds were identified in POEO extract. The activity of EOs depended on their chemical compounds. Among these compounds, phytol was more abundant than other compounds in POEO.

Inhibition of Aflatoxins Production by Natural Essential Oils

The aim of a study in 2022 was to evaluate the in vitro effect of four essential oils (EOs) (lavandins Grosso and Abrial, Origanum virens, and Rosmarinus officinalis) and four natural phenolic acids (PAs) (caffeic, chlorogenic, ferulic, and p-coumaric) on the growth and aflatoxins (B1, B2, G1, and G2) production by Aspergillus parasiticus.

In this experiment, EOs were the most effective substances to control the growth and aflatoxins production of A. parasiticus since they were able to completely inhibit the mold, with the exception of Rosmarinus officinalis.

A significant reduction of mycelial mass for all EOs at concentrations lower than MIC values was observed. In the present study, lavandin Abrial was the only essential oil (EO) that stimulated, at low concentrations, fungal growth and mycotoxin production of A. parasiticus. Other authors suggested that low fungicide doses create some stress conditions that may be responsible for the production of more secondary metabolites as a defense mechanism by the fungus. In this study, Origanum virens EO was the most effective for inhibiting growth of A. parasiticus.

Fungal growth and mycotoxin production may not be affected in the same way by the different compounds contained in essential oils. It was particularly remarkable that while essential rosemary oil (R. officinalis) did not inhibit fungal growth at MIC/MFC < 5 µL/mL, it showed the strongest inhibition of aflatoxin production.

Essential oils are usually rich in various compounds, comprising 20 to 60 active substances, and, in many cases, can be characterized by up to three major components at a relatively high concentration compared to other compounds present in trace amounts. The major components found in EO are often responsible for their biological properties. It is difficult to establish a relationship between EO composition and biological activity because of the synergistic actions between various components. It is indeed well-known that EOs have a higher antifungal activity than does a mixture of their major components, which suggests that the minor components are critical to the activity and may contribute to a synergistic effect.

Regarding the present work, A. parasiticus showed susceptibility to all PAs when exposed for 10 days although none of them was able to completely inhibit mold growth. However, despite the great inhibition of mycelial growth achieved at any of the concentrations below the MIC values (more than 70%), only the highest concentration significantly reduced aflatoxin production, while lower concentrations showed a variable effect, even stimulating the biosynthesis of aflatoxins. In summary, reduction of fungal growth was found to require lower concentrations of PAs than mycotoxin production. These results suggest that there is not such a direct relationship between PAs and growth inhibition or mycotoxin production.

Effects of cinnamon essential oil on oxidative damage and outer membrane protein genes of Salmonella enteritidis cells

In a study published in 2022, by measuring protein carbonylation levels as well as ROS (reactive oxygen species), MDA (malon-dialdehyde), SOD (superoxid-dismutase), CAT (catalase) and POD (peroxidase) content, the damage effect and mechanism of CEO on Salmonella enteritidis were evaluated and its damage mechanism from OmpA and OmpX, OmpW, and OmpF outer membrane gene levels was studied further. This work was expected to provide a theoretical basis for the development of new natural food preservatives and the prevention and control of Salmonella enteritidis.

CEO treatment could induce Salmonella enteritidis to produce a large amount of ROS, MDA and protein carbonylation levels, which could cause oxidative damage to cells and irreversible damage to cells. At the same time, CEO could inhibit the activity of bacterial protective enzymes SOD, CAT and POD, which weakened the self-protection and defense function of bacteria and created excess Salmonella enteritidis cells; in addition, the free radicals of Salmonella enteritidis could not be removed, causing serious damage to cells and thereby achieving a bacteriostasis effect. CEO treatment could increase the expression of OmpA, OmpX, OmpW, and OmpF genes to disrupt the normal metabolism of the bacteria and achieve a bacteriostatic effect.

Reduction of Bacterial Enteric Pathogens on Tomato Skin Surfaces

Plant-derived antimicrobials (PDAs) is a method to achieve pathogen reductions using a variety of means of application, including novel technologies such as nano-emulsion and edible film coatings. A recent research compared the utility of PDA-loaded nanoparticles constructed of the polimeric surfactant Pluronic F-127 (GNP) to decontaminate tomato skins from the pathogens Salmonella enterica and E. coli versus other sanitization treatments as a function of the sequence of contamination and sanitization treatments. Across the contamination and sanitization scenarios, GNP treatment provided the greatest pathogen reduction under differing conditions of pre-and/or post-harvest cross-contamination, and reduced hygiene-indicating microbes the most of all treatments on non-inoculated samples.

Bioactive zein/chitosan systems loaded with essential oils for food-packaging application

There has recently been increased interest in biodegradable and sustainable packaging within the food industry.

A corn protein, zein, possesses excellent film-forming properties because of its hydrophobic nature. It can be used for making edible films and for producing nanofibrous layers.

Combination with polysaccharides like chitosan offers promising prospects for the production of delivery systems for the controlled release of active substances.

The current trend is to minimize the content of chemical additives; thus essential oils are suitable alternatives to synthetic antimicrobials.

A study aimed to develop various zein/chitosan-based film-forming solutions, films, and coatings with antimicrobial substances to prepare active food packaging. Thymol and three essential oils (thyme, cinnamon, oregano) were applied as bioactive ingredients against bacteria, yeasts, and fungi. The incorporation of these natural active compounds led to a decrease in particle size in most film-forming solutions and a reduction of zeta potential compared to controls. Release of the bioactive compound into an aqueous environment was proved by antimicrobial test. A zein/chitosan-based coating with thymol was applied on fresh strawberries. Microbiological analysis over 10 days confirmed the efficient control of bacterial and fungal growth.

Zein/chitosan (7:1) systems are suitable as bioactive compound carriers to make barriers and to prevent moisture loss, ensuring microbial food quality and prolonging the shelf life of fruits. These systems can serve as sustainable active food packaging.

FSA study: green food contact materials

The Food Standards Agency (FSA) has called for more research into bio-based food contact materials (BBFCMs) in order to rule out concerns they could pose allergen risks.

The call comes following the completion of a review into the eco-friendly packaging options by scientific centre of excellence Fera. The review found that in many areas limited research had been undertaken into BBFCMs and that there was little information available on these new materials, for example the potential risk of allergens present in these bio-based materials transferring to food.

Some of the proteins used to produce packaging materials, edible films and coatings are known food allergens (milk and egg proteins, soya, corn, gluten) and therefore, it is important to understand if their allergenic potential remains in the final product.

In addition to the primary packaging, the allergenic potential of other components such as coatings and fillers need to be considered. Edible films are often coated with seed oils or plant essential oils such as rosemary, oregano, tea tree and others. Some of these are known to be able to elicit allergic reactions by oral or skin contact.

Currently, it has not been proven that bio-based packaging materials pose an allergenic risk to consumers, however, due to the increasing number of this type of packaging, it is necessary to investigate the possible risks and other unintended consequences of their use.

Polymeric nanofibers loaded with essential oils for biomedical and food-packaging applications

Nanofibers loaded with different essential oils can increase the shelf life of different foods while being used as active packaging materials. Various polymers are used to provide a suitable vehicle system to deliver essential oils. These polymers are used primarily in the form of nanomaterials with different dimensions (e.g., 0-D, 1-D, 2-D, and 3-D). However, the nanomaterials in the form of 1-D structures; for instance, the polymeric nanofibers, are vigorously used due to their large surface-to-volume ratio compared to other nanostructures.

A review article published in 2021 discusses the application of several essential oils, such as lavender, thyme, cinnamon, peppermint, tea tree, etc., encapsulating composite fibers, and mainly electrospun fibers. The ultimate goal is to build practical membranes for biomedical and food-packaging uses.

The studies discussed in this article have shown that the bioactivity of essential oils is preserved when combined with polymer solutions and transformed into nanofibers, especially by electrospinning. These nanofibers show a fibrous morphology, with a high surface-to-volume ratio, high porosity, and appropriate fiber diameters in the nanometer and submicron range. These properties are desirable in terms of the sustained release of the active ingredients from the packaging membrane onto the surface of the food. The nanofibers regulate the amount and release profile of the essential oils, thus reducing the cytotoxic effect of certain components of the essential oils on human cells.

Bacteria-killing, biodegradable food packaging material

Perishables such as fruit and meat are often at the mercy of harmful bacteria, especially when left for a long time outside the refrigerator. To salvage such food items, scientists have created a biodegradable packaging material able to release the bacteria and fungi-killing compounds only when needed, such as when there is a rise in humidity. The material is made from corn protein, starch, and other naturally derived substances, and is infused with a cocktail of natural antimicrobial compounds such as the oil from thyme, and citric acid. The new material was made through a process called electro-spinning - where the compounds are drawn into tubes using electric force and turned into fibers. These fibers released minuscule amounts of antimicrobial compounds that got rid of the bacteria. Scientists are currently working to optimize the material’s manufacturing process and looking into other types of biopolymers - beyond corn protein - to create different forms of sustainable packaging that would help to manage the triple threats of food safety, food waste, and unsustainable packaging.

Silver Nanoparticle on Biofilm Formation by Clostridium perfringens

A study published in 2022 determined the inhibitory effect of silver nanoparticles (AgNP) on the biofilm formation ability of Clostridium perfringens.

Biofilms are surface-related bacterial communities formed due to the adhesion of bacteria to surfaces and subsequent production of extracellular polymeric substances (EPs). This biofilm matrix, formed of polysaccharides, DNA, and proteins, protects the bacteria against body defense mechanisms, and the effect of antibiotics and disinfectants. The biofilm layer enables the microbe to cause different diseases. Thus, 65–80% of infections are estimated to be caused by biofilm-forming bacteria.

Biofilms produced by C. perfringens isolates contribute to developing gastrointestinal diarrheal diseases in humans and animals due to the protective effect of biofilms against antibiotic treatment.

82-94% of the tested 50 C. perfringens isolates showed resistance to the following agents: penicillin, cefotaxime, cefoxitin, ceftriaxone, clindamycin and chloramphenicol. All isolates were sensitive to vancomycin and 86% to metronidazole. 74% were sensitive to ampicillin, amoxicillin and ampicillin-sulbactam. 92% of the isolates were multiresistant.

Due to the smaller size and higher surface area to volume ratio, nanoparticles (NPs) have been investigated for their antibacterial and anti-biofilm effects. Silver nanoparticles (AgNPs) have a biocidal effect on various foodborne pathogens. AgNPs have been proven to have the potential to inhibit multidrug-resistant bacterial isolates, including Clostridial species.

The present study was the first to examine the AgNPs anti-biofilm activity against C. perfringens isolates.

In the study, the (45nm) AgNP particles inhibited biofilm formation at concentrations of 75 and 100 µg/mL, with higher AgNP concentrations the degree of inhibition ranged from 80.8 to 82.8%.

Antibacterial nanotexture for plastic packaging inspired by insect wings

Researchers have created an antibacterial nanotexture for rigid plastic packaging based on the structure of insect wings that can kill bacteria by pulling, stretching, or slicing them apart. According to the research team, when bacteria settle on the wing, the pattern of the nanopillars pulls the cells apart, in turn rupturing their membranes and killing them. Previous research has examined the nanomaterials’ potential to fight superbugs, such as those resistant to antibiotics. The research team claims that the nanotexture developed in the laboratory kills up to 70% of bacteria, reportedly retaining its effectiveness when transferred to plastic. The team suggests that the nanotexture could be used in packaging to reduce waste both during exports, as entire shipments can be rejected if bacterial growth is detected, and while on the shelf.

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