Antimicrobial mechanism of the major active essential oil compounds and their structure–activity relationship

Essential oil-nanoemulsion based edible coating: Innovative sustainable preservation method for fresh/fresh-cut fruits and vegetables

Utilizing plant-based sources for the preservation of fresh and fresh-cut fruits and vegetables offers a natural and chemical-free method. However, the inherent instability of plant bioactive compounds underscores the necessity for encapsulation techniques. Essential oil-based nanoemulsions (EO-NEs) stand out among food additives due to their distinctive antibacterial and antioxidant properties. This review delves into recent advancements in the application of EO-NEs as edible coatings for fresh and fresh-cut produce. It examines the efficacy of EO-NEs in enhancing the preservation of fruits and vegetables by harnessing their bioactive compounds for antibacterial, antifungal, and antioxidant activities. Additionally, the review accentuates the efficacy of EO-NEs in inhibiting biofilm formation on fruits and vegetables. It reveals that coatings derived from plant-source nanoemulsions exhibit exceptional mechanical, optical, and microstructural qualities, as well as superior water barrier properties. In contrast to conventional emulsions, nanocoatings facilitate the gradual and controlled release of antimicrobial and antioxidant compounds during food storage. This feature enhances bioactivity, extends shelf life, and enhances the nutritional profile of products. By preserving and protecting shelf stability, EO-NEs contribute to the maintenance of vegetable freshness. Nonetheless, ensuring their commercial viability necessitates additional research into the toxicity of EO-based nanoemulsions.

Enhancing Antibiotic Efficacy with Natural Compounds: Synergistic Activity of Tannic Acid and Nerol with Commercial Antibiotics against Pathogenic Bacteria

The search for synergies between natural products and commercial antibiotics is a promising strategy against bacterial resistance. This study determined the antimicrobial capacity of Nerol (NE) and Tannic Acid (TA) against 14 pathogenic bacteria, including ESKAPE pathogens. TA exhibited the lowest Minimum Inhibitory Concentrations (MICs) at 162.5 µg/mL against Pasteurella aerogenes and 187.5 µg/mL against Acinetobacter baumannii (WHO priority 1). NE showed its lowest MIC of 500 µg/mL against both Pasteurella aerogenes and Salmonella enterica. A total of 35 combinations of NE and 13 of TA with eight commercial antibiotics were analyzed. For NE, combinations with Streptomycin and Gentamicin were effective against Salmonella enterica, Bacillus subtilis, and Streptococcus agalactiae, with antibiotic MIC reductions between 75.0 and 87.5%. TA showed six synergies with Chloramphenicol, Ampicillin, Erythromycin, and Streptomycin against Acinetobacter baumannii, Streptococcus agalactiae, and Pasteurella aerogenes, with MIC reductions between 75.0 and 93.7%. Additionally, 31 additive effects with antibiotics for NE and 8 for TA were found. Kinetic studies on these synergies showed complete inhibition of bacterial growth, suggesting that natural products enhance antibiotics by facilitating their access to targets or preventing resistance. Given their safety profiles recognized by the EPA and FDA, these natural products could be promising candidates as antibiotic enhancers.

Antimicrobial and antibiofilm activity of specialized metabolites isolated from Centaurea hyalolepis

The discovery of plant-derived compounds that are able to combat antibiotic-resistant pathogens is an urgent demand. Over years, Centaurea hyalolepis attracted considerable attention because of its beneficial medical properties. Phytochemical analyses revealed that Centaurea plant species contain several metabolites, such as sesquiterpene lactones (STLs), essential oils, flavonoids, alkaloids, and lignans.The organic extract of C. hyalolepis plant, collected in Palestine, showed significant antimicrobial properties towards a panel of Gram-negative and Gram-positive bacterial strains when the Minimal Inhibitory Concentration (MIC) values were evaluated by broth microdilution assays. A bio-guided fractionation of the active extract via multiple steps of column and thin layer chromatography allowed us to obtain three main compounds. The isolated metabolites were identified as the STLs cnicin, 11β,13-dihydrosalonitenolide and salonitenolide by spectroscopic and spectrometric analyses. Cnicin conferred the strongest antimicrobial activity among the identified compounds. Moreover, the evaluation of its antibiofilm activity by biomass assays through crystal violet staining revealed almost 30% inhibition of biofilm formation in the case of A. baumannii ATCC 17878 strain. Furthermore, the quantification of carbohydrates and proteins present in the extracellular polymeric substance (EPS) revealed the ability of cnicin to significantly perturb biofilm structure. Based on these promising results, further investigations might open interesting perspectives to its applicability in biomedical field to counteract multidrug resistant infections.

Nanosizing of Lavender, Basil, and Clove Essential Oils into Microemulsions for Enhanced Antioxidant Potential and Antibacterial and Antibiofilm Activities

Plant essential oils (EOs) possess significant bioactivities (antibacterial and antioxidant) and can be substituted for potentially harmful synthetic preservatives in the food industry. However, limited water solubility, bioavailability, volatility, and stability limit their use. Therefore, the goal of this research was nanosizing lavender essential oil (LEO), basil essential oil (BEO), and clove essential oil (CEO) in a microemulsion (ME) to improve their physicochemical attributes and bioefficacy. Tween 80 and Transcutol P were utilized for construction of pseudoternary phase diagrams. It was observed that the concentration of EOs had a great impact on the physicochemical and biological properties of MEs. A spherical droplet of MEs with a diameter of less than 20 nm with a narrower size distribution (polydispersity index (PDI) = 0.10-0.27) and a ζ potential of -0.27 to -9.03 was observed. ME formulations were also evaluated for viscosity, conductivity, and the refractive index. Moreover, the impact of delivery systems on the antibacterial property of EOs was assessed by determining the zone of inhibition and minimum inhibitory concentration against two distinct pathogen classes (S. aureus and E. coli). Crystal violet assay was used to measure the growth and development of biofilms. According to bioefficacy assays, ME demonstrated more efficient antibacterial activity against microorganisms at concentrations lower than pure EOs. CEO ME had superior activity againstS. aureus and E. coli. Similarly, dose-dependent antioxidant capacity was noted for MEs. Consequently, nanosized EO formulations with improved physicochemical properties and enhanced bioactivities can be employed in the food processing sector as a preservation agent.

Inhibition of Salmonella Enteritidis by Essential Oil Components and the Effect of Storage on the Quality of Chicken

This research investigates the antibacterial potential of plant essential oil components including thymol, carvacrol, citral, cinnamaldehyde, limonene, and β-pinene against Salmonella Enteritidis (S. Enteritidis). Through the determination of minimum inhibitory concentration, three kinds of natural antibacterial agents with the best inhibitory effect on S. Enteritidis were determined, namely thymol (128 μg/mL), carvacrol (256 μg/mL), and cinnamaldehyde (128 μg/mL). Physical, chemical, microbial, and sensory characteristics were regularly monitored on days 0, 2, 4, and 6. The findings of this study reveal that both thymol at MIC of 128 μg/mL and carvacrol at MIC of 256 μg/mL not only maintained the sensory quality of chicken, but also decreased the pH, moisture content, and TVB-N value. Additionally, thymol, carvacrol and cinnamaldehyde successfully inhibited the formation of S. Enteritidis biofilm, thereby minimizing the number of S. Enteritidis and the total aerobic plate count in chicken. Hence, thymol, carvacrol, and cinnamaldehyde have more effective inhibitory activities against S. Enteritidis, which can effectively prevent the spoilage of chicken and reduce the loss of its functional components.

Antibiofilm Action of Plant Terpenes in Salmonella Strains: Potential Inhibitors of the Synthesis of Extracellular Polymeric Substances

Salmonella can form biofilms that contribute to its resistance in food processing environments. Biofilms are a dense population of cells that adhere to the surface, creating a matrix composed of extracellular polymeric substances (EPS) consisting mainly of polysaccharides, proteins, and eDNA. Remarkably, the secreted substances, including cellulose, curli, and colanic acid, act as protective barriers for Salmonella and contribute to its resistance and persistence when exposed to disinfectants. Conventional treatments are mostly ineffective in controlling this problem; therefore, exploring anti-biofilm molecules that minimize and eradicate Salmonella biofilms is required. The evidence indicated that terpenes effectively reduce biofilms and affect their three-dimensional structure due to the decrease in the content of EPS. Specifically, in the case of Salmonella, cellulose is an essential component in their biofilms, and its control could be through the inhibition of glycosyltransferase, the enzyme that synthesizes this polymer. The inhibition of polymeric substances secreted by Salmonella during biofilm development could be considered a target to reduce its resistance to disinfectants, and terpenes can be regarded as inhibitors of this process. However, more studies are needed to evaluate the effectiveness of these compounds against Salmonella enzymes that produce extracellular polymeric substances.

Inhibitory Activity of Proanthocyanidins Against Escherichia coli 1-Deoxy-D-Xylulose-5-Phosphate Reductoisomerase

1-Deoxy-D-xylulose 5-phosphate reductoisomerase (DXR) is a key enzyme in the methylerythritol phosphate pathway for terpenoid biosynthesis. Furthermore, it is an ideal target for the screening of novel antibiotics because it is present in causative organisms, but absent from humans. To identify more lipophilic DXR inhibitors from natural resources, we tested the DXR inhibitory activity of five proanthocyanidins in this study. The results indicated that all these compounds specifically restrained the activity of DXR, with procyanid B2 exhibiting a relatively low effect against DXR (IC50 ∼ 305 μM) and procyanid C1 displaying moderate activity (IC50 75.1 μM). The other three compounds cinnamtannin A2, cinnamtannin B1, and cinnamtannin D1 (IC50 ∼ 89.3, 105.0, and 97.8 μM, respectively) showed DXR inhibitory effects that were slightly weaker than that of procyanid C1. In addition, based on the initial characterization, the structure–activity relationship of this series of compounds against DXR is discussed.

Sustained-release modeling of clove essential oil in brine to improve the shelf life of Iranian white cheese by bioactive electrospun zein

In this study, the sustained-release of clove essential oil (CEO) loaded into the structure of electrospun zein was used as a biopreservative to extend the shelf life of Iranian white cheese. CEO was loaded at levels of 5, 7.5, and 10% w/w in the structure of electrospun nanofibers. In this study, a concentration of 35% w/v zein was used to produce electrospun fibers, and in the field emission scanning electron microscope (FESEM) it was observed that by increasing the loading of CEO from 5 to 10% w/w in the fiber structure, their diameter decreased from 517.96 ± 41.57 nm to 457.88 ± 32.45 nm. Although increasing the level of CEO reduced the diameter of the electrospun nanofibers, Young's modulus, tensile strength, and a higher level of CEO increased elongation at break of the films. The results of mechanical properties showed that by increasing the amount of CEO application in the structure of electrospun zein nanofibers from 5 to 10% w/w tensile strength from 8.18 ± 0.62 to 4.43 ± 0.86 MPa, and Young's modulus from 38.25 ± 2.81 to 27.25 ± 3.48 MPa decreased. Successful encapsulation of CEO in designed structures and the absence of adverse bonds between the encapsulant material (zein) and the core (CEO) were confirmed by the Fourier-transform infrared spectroscopy (FTIR) test. The in vitro sustained-release of the CEO in 8% w/v brine during 45 days of storage at 4 °C was modeled. The Fickian diffusion was the dominant release mechanism of the CEO and the Peppas-Sahlin model was the best model describing the essential oil release behavior. The electrospun films containing CEO were well able to suppress the growth of Listeria monocytogenes and Escherichia coli O157: H7 in samples of Iranian white cheese kept in 8% brine for 45 days at 4 °C. The samples treated with the electrospun film containing 7.5% w/w of CEO had the highest acceptability among different treatments.

Essential Oils and Their Application in Food Safety

Food industries are facing a great challenge due to contamination of food products with different microbes such as bacteria, fungi, viruses, parasites, etc. These microbes deteriorate food items by producing different toxins during pre- and postharvest processing. Mycotoxins are one of the most potent and well-studied toxic food contaminants of fungal origin, causing a severe health hazard to humans. The application of synthetic chemicals as food preservatives poses a real scourge in the present scenario due to their bio-incompatibility, non-biodegradability, and environmental non-sustainability. Therefore, plant-based antimicrobials, including essential oils, have developed cumulative interest as a potential alternative to synthetic preservatives because of their ecofriendly nature and generally recognized as safe status. However, the practical utilization of essential oils as an efficient antimicrobial in the food industry is challenging due to their volatile nature, less solubility, and high instability. The recent application of different delivery strategies viz. nanoencapsulation, active packaging, and polymer-based coating effectively addressed these challenges and improved the bioefficacy and controlled release of essential oils. This article provides an overview of essential oils for the preservation of stored foods against bacteria, fungi, and mycotoxins, along with the specialized mechanism of action and technological advancement by using different delivery systems for their effective application in food and agricultural industries smart green preservative.

Determination of the Activity of 1-Deoxy-D-Xylulose 5-Phosphate Synthase by Pre-column Derivatization-HPLC Using 1,2-Diamino-4,5-Methylenedioxybenzene as a Derivatizing Reagent

α-Ketoacids can be determined by HPLC through pre-column derivatization with 1,2-diamino-4,5-methylenedioxybenzene (DMB) as a derivatizing reagent. Using this method, the specific activity and the steady-state kinetic of 1-deoxy-D-xylulose-5-phosphate synthase (DXS) were measured. Firstly, DXS substrate pyruvate was derivatized with DMB in acidic solution; then the corresponding quinoxalinone was elucidated by LC-ESI-MS and quantified by HPLC-UV. The optimum derivatization conditions were as follows: aqueous medium at pH 1.0, reaction temperature 80 °C, reaction time 60 min, molar ratio of DMB to pyruvate 10:1. The HPLC was run with isocratic elution using the mixture of methanol and water (60:40, v/v) as a mobile phase. The detective limit and the linear correlation range of the method were 0.05 µM and 0.002-1.0 mM (R = 0.994), respectively. The relative standard deviation (RSD) of six determinations was 2.48%. The steady-state kinetic parameters of DXS for pyruvate determined with the method were identical to the reported data. The established method is a practical route for evaluation of DXS activity, especially in the research and development of DXS inhibitors.

Inhibitory Activity of Plant Essential Oils against E. coli 1-Deoxy-d-xylulose-5-phosphate reductoisomerase

The rate-limiting enzyme of the 2-methyl-d-erythritol-4-phosphate (MEP) terpenoid biosynthetic pathway, 1-deoxy-d-xylulose-5-phosphate reductoisomerase (DXR), provides the perfect target for screening new antibacterial substances. In this study, we tested the DXR inhibitory effect of 35 plant essential oils (EOs), which have long been recognized for their antimicrobial properties. The results show that the EOs of Zanbthoxylum bungeanum (ZB), Schizonepetae tenuifoliae (ST), Thymus quinquecostatus (TQ), Origanum vulgare (OV), and Eugenia caryophyllata (EC) displayed weak to medium inhibitory activity against DXR, with IC₅₀ values of 78 μg/mL, 65 μg/mL, 59 μg/mL, 48 μg/mL, and 37 μg/mL, respectively. GC-MS analyses of the above oils and further DXR inhibitory activity tests of their major components revealed that eugenol (EC) and carvacrol (TQ and OV) possess medium inhibition against the protein (68.3% and 55.6%, respectively, at a concentration of 20 μg/mL), whereas thymol (ST, TQ, and OV), carveol (ZB), and linalool (ZB, ST, and OV) only exhibited weak inhibition against DXR, at 20 μg/mL (23%−26%). The results add more details to the antimicrobial mechanisms of plant EOs, which could be very helpful in the direction of the reasonable use of EOs in the food industry and in the control of phytopathogenic microbials.

Potential Natural Food Preservatives and Their Sustainable Production in Yeast: Terpenoids and Polyphenols

Terpenoids and polyphenols are high-valued plant secondary metabolites. Their high antimicrobial activities demonstrate their huge potential as natural preservatives in the food industry. With the rapid development of metabolic engineering, it has become possible to realize large-scale production of non-native terpenoids and polyphenols by using the generally recognized as safe (GRAS) strain, Saccharomyces cerevisiae, as a cell factory. This review will summarize the major terpenoid and polyphenol compounds with high antimicrobial properties, describe their native metabolic pathways as well as antimicrobial mechanisms, and highlight current progress on their heterologous biosynthesis in S. cerevisiae. Current challenges and perspectives for the sustainable production of terpenoid and polyphenol as natural food preservatives via S. cerevisiae will also be discussed.

Evaluation of the Antimicrobial Activity and Cytotoxicity of Different Components of Natural Origin Present in Essential Oils

Even though essential oils (EOs) have been used for therapeutic purposes, there is now a renewed interest in the antimicrobial properties of phytochemicals and EOs in particular. Their demonstrated low levels of induction of antimicrobial resistance make them interesting for bactericidal applications, though their complex composition makes it necessary to focus on the study of their main components to identify the most effective ones. Herein, the evaluation of the antimicrobial action of different molecules present in EOs against planktonic and biofilm-forming Gram-positive (Staphylococcus aureus) and Gram-negative (Escherichia coli) bacteria was assessed. The bactericidal mechanisms of the different molecules, as well as their cytocompatibility, were also studied. Carvacrol, cinnamaldehyde, and thymol exhibit the highest in vitro antimicrobial activities against E. coli and S. aureus, with membrane disruption the bactericidal mechanism identified. The addition of those compounds (≥0.5 mg/mL) hampers S. aureus biofilm formation and partially eliminates preformed biofilms. The subcytotoxic values of the tested EO molecules (0.015–0.090 mg/mL) are lower than the minimum inhibitory and bactericidal concentrations obtained for bacteria (0.2–0.5 mg/mL) but are higher than that obtained for chlorhexidine (0.004 mg/mL), indicating the reduced cytotoxicity of EOs. Therefore, carvacrol, cinnamaldehyde, and thymol are molecules contained in EOs that could be used against E. coli– and S. aureus–mediated infections without a potential induction of bactericidal resistance and with lower cell toxicity than the conventional widely used chlorhexidine.

Oxyresveratrol, a Stilbene Compound from Morus alba L. Twig Extract Active Against Trichophyton rubrum

Morus alba L. (mulberry) twig is known to have an inhibitory effect on pathogens in traditional Chinese medicine. In the present study, the dermophytic fungus, Trichophyton rubrum, was used to evaluate the inhibitory effect of total M. alba twig extract and extracts obtained using solvents with different polarities by the method of 96-well MTT colorimetry. The main active substance was isolated and identified by tracking its activity. In addition, the inhibitory effects of active extracts and a single active substance were investigated in combination with miconazole nitrate. Our data indicated that ethyl acetate extracts of mulberry twig (TEE) exhibited a desired inhibitory activity on T. rubrum with the minimum inhibitory concentration (MIC) of 1.000 mg/mL. With activity tracking, the main substance showing antimicrobial activity was oxyresveratrol (OXY), which was isolated from TEE. Its MIC for inhibiting the growth of T. rubrum was 0.500 mg/mL. The combined use of miconazole nitrate and OXY showed a synergistic inhibitory effect, as shown by a significant decrease in the MIC of both components. Based on the OXY content in TEE, the contribution rate of OXY to the inhibitory effect of TEE on T. rubrum was 80.52%, so it was determined to be the main antimicrobial substance in M. alba twig. Copyright © 2017 John Wiley & Sons, Ltd.

Leishmanicidal Activity and Structure-Activity Relationships of Essential Oil Constituents

Several constituents of essential oils have been shown to be active against pathogens such as bacteria, fungi, and protozoa. This study demonstrated the in vitro action of ten compounds present in essential oils against Leishmania amazonensis promastigotes. With the exception of p-cymene, all evaluated compounds presented leishmanicidal activity, exhibiting IC₅₀ between 25.4 and 568.1 μg mL⁻¹. Compounds with the best leishmanicidal activity presented a phenolic moiety (IC₅₀ between 25.4 and 82.9 μg mL⁻¹). Alicyclic alcohols ((−)-menthol and isoborneol) and ketones ((−)-carvone) promoted similar activity against the parasite (IC₅₀ between 190.2 and 198.9 μg mL⁻¹). Most of the compounds showed low cytotoxicity in L929 fibroblasts. Analysis of the structure-activity relationship of these compounds showed the importance of the phenolic structure for the biological action against the promastigote forms of the parasite.