Anti-Listeria monocytogenes activity of essential oils components of conifers

Mechanism of bacterial inactivation by (+)-limonene and its potential use in food preservation combined processes

This work explores the bactericidal effect of (+)-limonene, the major constituent of citrus fruits' essential oils, against E. coli. The degree of E. coli BJ4 inactivation achieved by (+)-limonene was influenced by the pH of the treatment medium, being more bactericidal at pH 4.0 than at pH 7.0. Deletion of rpoS and exposure to a sub-lethal heat or an acid shock did not modify E. coli BJ4 resistance to (+)-limonene. However, exposure to a sub-lethal cold shock decreased its resistance to (+)-limonene. Although no sub-lethal injury was detected in the cell envelopes after exposure to (+)-limonene by the selective-plating technique, the uptake of propidium iodide by inactivated E. coli BJ4 cells pointed out these structures as important targets in the mechanism of action. Attenuated Total Reflectance Infrared Microspectroscopy (ATR-IRMS) allowed identification of altered E. coli BJ4 structures after (+)-limonene treatments as a function of the treatment pH: β-sheet proteins at pH 4.0 and phosphodiester bonds at pH 7.0. The increased sensitivity to (+)-limonene observed at pH 4.0 in an E. coli MC4100 lptD4213 mutant with an increased outer membrane permeability along with the identification of altered β-sheet proteins by ATR-IRMS indicated the importance of this structure in the mechanism of action of (+)-limonene. The study of mechanism of inactivation by (+)-limonene led to the design of a synergistic combined process with heat for the inactivation of the pathogen E. coli O157:H7 in fruit juices. These results show the potential of (+)-limonene in food preservation, either acting alone or in combination with lethal heat treatments.

Application of anti-listerial bacteriocins: monitoring enterocin expression by multiplex relative reverse transcription–PCR

Listeriosis is a deadly food-borne disease, and its incidence may be limited through the biotechnological exploitation of a number of anti-listerial biocontrol agents. The most widely used of these agents are bacteriocins and the Class II enterocins are characterized by their activity against Listeria. Enterocins are primarily produced by enterococci, particularly Enterococcus faecium and many strains have been described, often encoding multiple bacteriocins. The use of these strains in food will require that they are free of virulence functions and that they exhibit a high level expression of anti-listerial enterocins in fermentation conditions. Multiplex relative RT (reverse transcription)–PCR is a technique that is useful in the discovery of advantageous expression characteristics among enterocin-producing strains. It allows the levels of individual enterocin gene expression to be monitored and determination of how expression is altered under different growth conditions.

Essential oils in combination and their antimicrobial properties

Essential oils (EOs) have been long recognized for their antibacterial, antifungal, antiviral, insecticidal and antioxidant properties. They are widely used in medicine and the food industry for these purposes. The increased interest in alternative natural substances is driving the research community to find new uses and applications of these substances. EOs and their components show promising activities against many food-borne pathogens and spoilage microorganisms when tested in vitro. In food systems, higher concentrations of EOs are needed to exert similar antibacterial effects as those obtained in in vitro assays. The use of combinations of EOs and their isolated components are thus new approaches to increase the efficacy of EOs in foods, taking advantage of their synergistic and additive effects. The purpose of this review is to provide an overview on the antimicrobial efficacy of these combinations. A survey of the methods used for the determination of the interactions and mechanisms involved in the antimicrobial activities of these combinations are also reported.

In vitro lethal effect of ajowan (Trachyspermum ammi L.) essential oil on hydatid cyst protoscoleces

Various chemical scolicidal agents have been used for inactivation of hydatid cyst protoscolices, but most of them are associated with adverse side effects. Since ajowan (Trachyspermum ammi) has been shown to have a number of medicinal properties, in this study the scolicidal effect of the essential oil (EO) from the fruits of this herbal plant was investigated. Ajowan EO was obtained by hydrodistillation method. Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) were employed to determine the chemical composition of the EO. Protoscoleces were exposed to various concentrations of EO (3, 5 and 10mg/mL) for 10, 20, 30, and 60 min. Viability of protoscolices was confirmed by 0.1% eosin staining. A total of 18 compounds representing 99.54% of the total oil, were identified. Thymol (50.07%), γ-terpinene (23.92%), and p-cymene (22.9%) were found to be the major EO constituents. While the mortality rate of protoscolices in the control group was 6.67%, scolicidal power of ajowan EO at concentration of 3mg/mL was 31.34, 35.98, 45.17, and 51.58% after 10, 20, 30, and 60 min, respectively. The EO at concentration of 5mg/mL killed 51.89, 72.20, 88.64, and 100% of protoscolices after 10, 20, 30, and 60 min, respectively. One hundred percent scolicidal activity was observed with ajowan EO at concentration of 10mg/mL after 10 min of exposure. The results of this study revealed that the EO of ajowan is rich in thymol, γ-terpinene and p-cymene, has high scolicidal power and it may be used as a natural scolicidal agent.

Chemical composition and antimicrobial effects of essential oils of Eucalyptus globulus, Myrtus communis and Satureja hortensis against Escherichia coli O157:H7 and Staphylococcus aureus in minced beef

Essential oils (EOs) extracted by hydrodistillation from leaf parts of Algerian Eucalyptus globulus, Myrtus communis and Satureja hortensis were analyzed by gas chromatography/mass spectrometry (GC/MS). The main components of EOs obtained were γ-terpinene (94.48%), 1,8-cineole (46.98%) and carvacrol (46.10%), respectively, for E. globulus, M. communis and S. hortensis. The in vitro antimicrobial activity of the EOs was evaluated against Staphylococcus aureus CECT 4459 and Escherichia coli O157:H7 CECT 4267 using the agar diffusion technique. Results revealed that E. globulus and S. hortensis EOs had more antibacterial effects than that from M. communis. Minimal inhibitory concentrations (MIC) showed a range of 0.05-0.22% (volume by volume [v/v]). Sensitivity of gram-positive S. aureus was much higher than that of gram-negative E. coli. Plant EOs were added to minced beef (two-fold MIC value) at 0.10-0.44%, experimentally inoculated with the same pathogens at a level of 5 × 10(5) colony forming units (cfu)/g and stored at 5 ± 2 °C. Results showed that the EOs of E. globulus and S. hortensis had remarkable antibacterial properties, higher than that of M. communis, against S. aureus and E. coli. Indeed, a reduction of 5.8 log cfu/g (70.74% of reduction) was recorded after 7 days of storage for S. hortensis against E. coli. However, regarding S. aureus, both S. hortensis and E. globulus caused a highly significant (p < 0.05) decrease of microbial counts, most evident after 5 days of storage; S. aureus numbers were 3.50 and 2.50 cfu/g, respectively, corresponding to a reduction of 2.20 and 3.20 log cfu/g (38.60 and 56.14% of reduction) after 1 week of storage. Sensory evaluation revealed that the aroma of minced beef meat treated with EOs was acceptable by panelists at the levels used.

Phylogenetic relationships of the genus Chamaecyparis inferred from leaf essential oil

The species differentiation between Chamaecyparis formosensis, C. obtusa var. formosana, and C. obtusa, based on the composition of the leaf essential oils, was studied. The characterization of the oils by GC-FID and GC/MS analyses showed remarkable differences between these three essential oils. Cluster analysis (CA) and principal-component analysis (PCA) distinguished three groups of essential oils. The C. formosensis oil was dominated by α-pinene while those isolated from C. obtusa var. formosana and C. obtusa were characterized by high levels of (-)-thujopsene and α-terpinyl acetate, respectively. Moreover, the phylogenetic relationships of the genus Chamaecyparis were in agreement with previous findings based on morphological and molecular evidence. In addition, the essential oils from C. obtusa var. formosana could be classified into three chemical types, according to their different characteristic main compounds (β-elemol, (-)-thujopsene, and cis-thujopsenal). The biochemical correlations between the major constituents of the Chamaecyparis species were examined and their relationship is discussed.

Antioxidant and antibacterial activity of essential oil and extracts of bay laurel Laurus nobilis Linnaeus (Lauraceae) from Portugal

Laurus nobilis L. is an aromatic plant frequently used as a spice in Mediterranean cookery and as a traditional medicine for the treatment of several infectious diseases. The aim of this study was to characterise the antibacterial and antioxidant activities of bay laurel essential oil (EO), ethanolic extract (EE) and hot/cold aqueous extract (AE). The major components detected in bay laurel EO were eucalyptol (27.2%), α-terpinenyl acetate (10.2%), linalool (8.4%), methyleugenol (5.4%), sabinene (4.0%) and carvacrol (3.2%). The EO exhibited strong antibacterial activity against all tested foodborne spoilage and pathogenic bacteria, whereas this activity was less pronounced or even nonexistent in the EE and AE. In contrast, EO exhibited low antioxidant activity compared to extracts (EX), and among the EX, the hot AE revealed the highest antioxidant ability. The results show that bay laurel EO and its EX have potential as natural alternatives to synthetic food preservatives, in order to enhance food safety and increase food shelf life.

Comparative anti-infectious bronchitis virus (IBV) activity of (-)-pinene: effect on nucleocapsid (N) protein

In the present study, anti-IBV (infectious bronchitis virus) activities of (-)-pinenes were studied by MTT assay, as well as docking and molecular dynamic (MD) simulations. The CC₅₀ values of (-)-α-pinene and (-)-β-pinene were above 10 mM. And the maximum noncytotoxic concentrations (TD₀) of (-)-α-pinene and (-)-β-pinene were determined as 7.88 ± 0.06 and 6.09 ± 0.31 mM, respectively. The two compounds were found to inhibit IBV with an IC₅₀ of 0.98 ± 0.25 and 1.32 ± 0.11 mM. The MTT assay showed that the inhibitions of (-)-pinenes against IBV appear to occur moderately before entering the cell but are much stronger occur after penetration of the virus into the cell. Molecular simulations indicated that (-)-α-pinene and (-)-β-pinene specifically interact with the active site which is located at the N terminus of phosphorylated nucleocapsid (N) protein, with the former being more potent than the latter. The binding energies of them are -36.83 and -35.59 kcal mol-1, respectively. Results presented here may suggest that (-)-α-pinene and (-)-β-pinene possess anti-IBV properties, and therefore are a potential source of anti-IBV ingredients for the pharmaceutical industry.

Essential oils composition in two Rosmarinus officinalis L. varieties and incidence for antimicrobial and antioxidant activities

The essential oil composition of Rosmarinus officinalis var. typicus and var. troglodytorum endemic to Tunisia, and growing wild in different bioclimates, was determined by GC and GC-MS. Oils were assessed for their antimicrobial and antioxidant activity. A variation of the chemical composition attributed to varieties rather than to bioclimates was revealed. 1.8-Cineole (47.2-27.5%) and camphor (12.9-27.9%) were identified as the main constituents of var. typicus and var. troglodytorum, respectively. The principal component analysis performed on oil constituents for all the populations allowed the distinction of two distinct population groups in accordance to the varietal subdivision. Based on the determination of the diameter of inhibition and the determination of the minimum inhibitory concentration, a low to moderate antimicrobial activity according to oils was revealed against eight bacteria tested. However, oils from var. troglodytorum showed higher bactericidal effect than those from var. typicus. The oils' antioxidant activity, determined by 1,1-diphenyl-1-picrylhydrazyl (DPPH) assay, ferric reducing (FRAP) assay and β-carotene bleaching test, was relatively high. The highest activity was found in oils from var. troglodytorum and in one population of var. typicus from the upper semi-arid bioclimate.

Chemical Composition of Fatty Acid and Unsaponifiable Fractions of Leaves, Stems and Roots of Arbutus unedo and in vitro Antimicrobial Activity of Unsaponifiable Extracts

The chemical composition of the fatty acid and unsaponifiable fractions of the leaves, stems and roots of Arbutus unedo L. were determined using gas chromatography and gas chromatography-mass spectrometry. The fatty acid fractions of the leaves, stems and roots contained 38.5%, 31.3% and 14.1% palmitic acid, respectively, along with other long-chain fatty acids (up to C22). The chemical composition of the unsaponifiable fractions differed: the leaf and stem fractions contained high levels of aliphatic (32.1% and 62.6%, respectively) and terpenic compounds (49.6% and 25.7%, respectively), and the root fraction mainly contained esters, of which the most abundant was benzyl cinnamate (36.6%). The antimicrobial activities of the unsaponifiable fractions against nine species of microorganisms were assessed. The unsaponifiable leaf and stem extracts inhibited the growth of Klebsiella pneumoniae, Enterococcus faecalis and Candida albicans.

Activities of ten essential oils towards Propionibacterium acnes and PC-3, A-549 and MCF-7 cancer cells

Ten essential oils, namely, mint (Mentha spicata L., Lamiaceae), ginger (Zingiber officinale Rosc., Zingiberaceae), lemon (Citrus limon Burm.f., Rutaceae), grapefruit (Citrus paradisi Macf., Rutaceae), jasmine (Jasminum grandiflora L., Oleaceae), lavender (Mill., Lamiaceae), chamomile (Matricaria chamomilla L., Compositae), thyme (Thymus vulgaris L., Lamiaceae), rose (Rosa damascena Mill., Rosaceae) and cinnamon (Cinnamomum zeylanicum N. Lauraceae) were tested for their antibacterial activities towards Propionibacterium acnes and in vitro toxicology against three human cancer cell lines. Thyme, cinnamon and rose essential oils exhibited the best antibacterial activities towards P. acnes, with inhibition diameters of 40 ± 1.2 mm, 33.5 ± 1.5 mm and 16.5 ± 0.7 mm, and minimal inhibitory concentrations of 0.016% (v/v), 0.016% (v/v) and 0.031% (v/v), respectively. Time-kill dynamic procedures showed that thyme, cinnamon, rose, and lavender essential oils exhibited the strongest bactericidal activities at a concentration of 0.25% (v/v), and P. acnes was completely killed after 5 min. The thyme essential oil exhibited the strongest cytotoxicity towards three human cancer cells. Its inhibition concentration 50% (IC₅₀) values on PC-3, A549 and MCF-7 tumor cell lines were 0.010% (v/v), 0.011% (v/v) and 0.030% (v/v), respectively. The cytotoxicity of 10 essential oils on human prostate carcinoma cell (PC-3) was significantly stronger than on human lung carcinoma (A549) and human breast cancer (MCF-7) cell lines.

Evaluation of diffusion and dilution methods to determine the antibacterial activity of plant extracts

The aim of this study was to evaluate diffusion and dilution methods for determining the antibacterial activity of plant extracts and their mixtures. Several methods for measurement of the minimal inhibitory concentration (MIC) of a plant extract are available, but there is no standard procedure as there is for antibiotics. We tested different plant extracts, their mixtures and phenolic acids on selected gram-positive (Staphylococcus aureus, Bacillus cereus, and Listeria monocytogenes) and gram-negative bacteria (Escherichia coli O157:H7, Salmonella Infantis, Campylobacter jejuni, Campylobacter coli) with the disk diffusion, agar dilution, broth microdilution and macrodilution methods. The disk diffusion method was appropriate only as a preliminary screening test prior to quantitative MIC determination with dilution methods. A comparison of the results for MIC obtained by agar dilution and broth microdilution was possible only for gram-positive bacteria, and indicated the latter as the most accurate way of assessing the antimicrobial effect. The microdilution method with TTC (2,3,5-triphenyl tetrazolium chloride) or INT (2-p-iodophenyl-3-p-nitrophenyl-5-phenyl tetrazolium chloride) to indicate the viability of aerobic bacteria was found to be the best alternative approach, while only ATP determination was appropriate for microaerophilic Campylobacter spp. Using survival curves the kinetics of bacterial inactivation on plant extract exposure was followed for 24h and in this way the MIC values determined by the microdilution method were confirmed as the concentrations of extracts that inhibited bacterial growth. We suggest evaluation of the antibacterial activity of plant extracts using the broth microdilution method as a fast screening method for MIC determination and the macrodilution method at selected MIC values to confirm bacterial inactivation. Campylobacter spp. showed a similar sensitivity to plant extracts as the tested gram-positive bacteria, but S. Infantis and E. coli O157:H7 were more resistant.

Chemical composition and antibacterial activity of the essential oils of Callistemon citrinus and Callistemon viminalis from South Africa

The chemical composition and the antibacterial activity of the essential oils obtained by hydrodistillation from the leaves of Callistemon citrinus and Callistemon viminalis were analyzed by GC and GC/MS. Twenty-four and twelve components were identified for C. citrinus and C. viminalis, representing 92.0% and 98.3% of the total oils. The major components of C. citrinus and C. viminalis were 1,8-cineole (61.2% and 83.2%) and α-pinene (13.4% and 6.4%), respectively. The in vitro antibacterial activity of the essential oils was studied against 12 bacteria strains using disc diffusion and broth microdilution methods. The oils exhibited strong zone of inhibitions against some bacteria such as S. faecalis (20.3-24.0 mm), both strains of S. aureus (23.0-26.3 mm), B. cereus (17.3-19.0 mm) and S. macrcesens (11.3-23.7 mm) when compared to standard antibiotics gentamycin and tetracycline used as controls. Expect for P. aeruginosa and S. macrcescens, the MIC values of both essential oils ranged from 0.31-2.50 mg/mL.

Antimicrobial activity of plant essential oils using food model media: efficacy, synergistic potential and interactions with food components

The aim of this study was to optimise the antimicrobial efficacy of plant essential oils (EOs) for control of Listeria spp. and spoilage bacteria using food model media based on lettuce, meat and milk. The EOs evaluated were lemon balm, marjoram, oregano and thyme and their minimum inhibitory concentrations (MIC) were determined against Enterobacter spp., Listeria spp., Lactobacillus spp., and Pseudomonas spp. using the agar dilution method and/or the absorbance based microplate assay. MICs were significantly lower in lettuce and beef media than in TSB. Listeria strains were more sensitive than spoilage bacteria, and oregano and thyme were the most active EOs. EO combinations were investigated using the checkerboard method and Oregano combined with thyme had additive effects against spoilage organisms. Combining lemon balm with thyme yielded additive activity against Listeria strains. The effect of simple sugars and pH on antimicrobial efficacy of oregano and thyme was assessed in a beef extract and tomato serum model media. EOs retained greater efficacy at pH 5 and 2.32% sugar, but sugar concentrations above 5% did not negatively impact EO efficacy. In addition to proven antimicrobial efficacy, careful selection and investigation of EOs appropriate to the sensory profile of foods and composition of the food system is required. This work shows that EOs might be more effective against food-borne pathogens and spoilage bacteria when applied to foods containing a high protein level at acidic pH, as well as moderate levels of simple sugars.

Phytotherapeutic prevention of dental biofilm formation

The antimicrobial and biofilm formation preventive properties of Mentha piperita and Rosmarinus officinalis essential oils and chlorhexidine were assessed against Streptococcus mutans and Streptococcus pyogenes. 26 and 20 compounds were identified by GC and GC-MS analysis in hydrodistilled oils from M. piperita and R. officinalis, respectively. The minimal bactericidal concentrations (MBC) of the M. piperita and R. officinalis oils and chlorhexidine were (6000, 2000, 8000 ppm) and (1000, 4000, 1000 ppm) for S. mutans and S. pyogenes, respectively. The decimal reduction time (D) of S. mutans exposed to the oils at their MBC levels was 2.8 min while chlorhexidine showed a longer time. The D values of S. pyogenes on exposure to the MBC levels of M. piperita and R. officinalis oils and of chlorhexidine were 2.14, 4.28 and 2.8 min, indicating a higher efficacy of M. piperita oil. Biofilm formation was performed by growing S. mutans culture with and without essential oils in LB medium in polystyrene tubes. In vitro biofilm inhibitory properties were in the order M. piperita > R. officinalis > chlorhexidine. In vivo experiments on the antibiofilm properties revealed that all concentrations of the oils were significantly (p < 0.001) more effective than chlorhexidine. In conclusion, essential oils may be considered as safe agents in the development of novel antibiofilm agents.

Phytotherapeutic inhibition of supragingival dental plaque

Antimicrobial activities and biofilm-formation preventive properties of Mentha piperita and Cuminum cyminum essential oils and chlorhexidine were assessed against Streptococcus mutans and Streptococcus pyogenes. Gas chromatography (GC) and gas chromatography-mass spectrometry (GC-MS) analysis led to the identification of 26 and 32 compounds in the essential oils of M. piperita and C. cyminum, respectively. Minimal bactericidal concentrations (MBC) of the oils and chlorhexidine and microbial decimal reduction time (D value) were determined. Antibacterial and in vivo biofilm preventive efficacies of all the concentrations of M. piperita oil were significantly (p<0.001) higher. The biofilm inhibitory properties in planktonic cultures were in M. piperita > chlorhexidine > C. cyminum order. In vivo experiments conducted on male and female volunteers who brushed with essential oil blended toothpastes indicated that lower concentrations of the oils, in particular the M. piperita oil, were significantly higher (p<0.001) and effective during the course of the study as compared to chlorhexidine. In conclusion, there may be a potential role for essential oils in the development of novel anticaries treatments.

Chemical composition and antimicrobial activity of the essential oil from Satureja horvatii Silic (Lamiaceae)

The present paper describes the chemical composition and antimicrobial activity of the essential oil of the endemic species Satureja horvatii Silic, collected in Montenegro. The essential oil was obtained from the aerial parts of the plant by hydrodistillation and analyzed by GC-MS. From the 34 compounds representing 100 % of the oil, the major compound was the phenolic monoterpene thymol (63.37 %). The oil contained smaller amounts of g-terpinene (7.49 %), carvacrol methyl ether (4.92 %), carvacrol (4.67 %), p-cymene (4.52%), a-terpinene (1.81 %), borneol (1.58 %), a-thujene (1.56 %), b-caryophyllene (1.55 %) and b-myrcene (1.44 %). The antimicrobial activity of the essential oil of S. horvatii was evaluated using the agar diffusion and broth microdilution methods. The essential oil exhibited antimicrobial activity to varying degrees against all the tested strains. The maximum activity of S. horvatii oil was observed against Gram-positive bacteria (Micrococcus luteus, Staphylococcus epidermidis, Staphylococcus aureus and Enterococcus faecalis) and against the yeast (Candida albicans). The oil exhibited moderate activity against the Gram-negative bacteria Escherichia coli and Klebsiella pneumoniae and weak activity against Pseudomonas aeruginosa. This study confirms that the essential oil of S. horvatii possesses antimicrobial activities in vitro against medically important pathogens.

Growth response of Salmonella typhimurium in the presence of natural and synthetic antimicrobials: estimation of MICs from three different models

The aim of this study was to determine the MICs of 14 antimicrobials for Salmonella Typhimurium with three methods and to check the influence of experiment duration on the estimation of MICs. The growth of Salmonella Typhimurium in a brain heart infusion medium containing various concentrations of natural aromatic compounds, organic acids, or salts was monitored by absorbance measurements for 24 or 72 h. Three different ways of analyzing optical density (OD) curves were tested for the determination of MICs. Both quantitative methods gave similar MICs for most of the compounds. The semiquantitative method does not allow estimating the MIC for all compounds. Noticeable differences were found between MICs obtained for 24- or 72-h experiments, whatever the method used. The proposed methods and models can be used for the estimation of MICs from OD data. MICs could be used for a quantitative approach to Salmonella Typhimurium growth.

Chemical composition and antimicrobial activity of volatile components from capitula and aerial parts of Rhaponticum acaule DC growing wild in Tunisia

The chemical composition of the volatile fractions obtained by steam distillation from the capitula (C) and the aerial parts of Rhaponticum acaule DC were analysed by GC-MS. From the 57 identified constituents, representing 95.5% and 96.3% of the two oils, respectively, methyl eugenol, epi-13 manool, beta-ionone, beta-bisabolol, 1-octadecanol, phytol and farnesyl acetate were found to be the main components. Furthermore, the oils were tested against six Gram-positive and Gram-negative bacteria and four phytopathogenic fungi. It was found that oils from both parts of R. acaule, and especially that of C, exhibited interesting antibacterial activity, but no antifungal activity was observed.