http://naturalingredient.org/wp/wp-content/uploads/Burt_Essential_Oils_A_Review_2004.pdf

Essential oils: their antibacterial properties and potential applications in foods--a review.

Plant essential oils for pest and disease management

Aims: The minimum inhibitory concentration (MIC) of oregano essential oil (OEO) and two of its principle components, i.e. thymol and carvacrol, against Pseudomonas aeruginosa and Staphylococcus aureus was assessed by using an innovative technique. The mechanism of action of the above substances was also investigated. Methods and Results: The applied technique uses 100-well microtitre plate and collects turbidimetric growth data. To produce the inhibition profiles, a wide range of concentrations were tested for each of the three compounds, as well as for carvacrol‐thymol mixtures. Following a specific mathematical analysis of the observed inhibition profiles from all compounds, it was suggested that mixtures of carvacrol and thymol gave an additive effect and that the overall inhibition by OEO can be attributed mainly to the additive antimicrobial action of these two compounds. Addition of low amounts of each additive: (a) increased permeability of cells to the nuclear stain EB, (b) dissipated pH gradients as indicated by the CFDA-SE fluorescent probe irrespective of glucose availability and (c) caused leakage of inorganic ions. Conclusions: Mixing carvacrol and thymol at proper amounts may exert the total inhibition that is evident by oregano essential oil. Such inhibition is due to damage in membrane integrity, which further affects pH homeostasis and equilibrium of inorganic ions. Significance and Impact of the Study: The knowledge of extent and mode of inhibition of specific compounds, which are present in plant extracts, may contribute to the successful application of such natural preservatives in foods, since certain combinations of carvacrol‐ thymol provide as high inhibition as oregano essential oil with a smaller flavour impact.

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A study of the minimum inhibitory concentration and mode of action of oregano essential oil, thymol and carvacrol.

In recent years, the use of essential oils (EOs) derived from aromatic plants as low-risk insecticides has increased considerably owing to their popularity with organic growers and environmentally conscious consumers. EOs are easily produced by steam distillation of plant material and contain many volatile, low-molecular-weight terpenes and phenolics. The major plant families from which EOs are extracted include Myrtaceae, Lauraceae, Lamiaceae, and Asteraceae. EOs have repellent, insecticidal, and growth-reducing effects on a variety of insects. They have been used effectively to control preharvest and postharvest phytophagous insects and as insect repellents for biting flies and for home and garden insects. The compounds exert their activities on insects through neurotoxic effects involving several mechanisms, notably through GABA, octopamine synapses, and the inhibition of acetylcholinesterase. With a few exceptions, their mammalian toxicity is low and environmental persistence is short. Registration has been the main bottleneck in putting new products on the market, but more EOs have been approved for use in the United States than elsewhere owing to reduced-risk processes for these materials.

Essential oils in insect control: low-risk products in a high-stakes world.

Aims: To quantify the antibacterial properties of five essential oils (EO) on a non-toxigenic strain of Escherichia coli O157:H7 in the presence and absence of a stabilizer and an emulsifier and at three different temperatures.

Methods and Results: Five EOs known to exhibit antibacterial properties were screened by disc diffusion assay and the most active were selected for further study in microdilution colorimetric assays. Oregano (Origanum vulgare) and thyme (Thymus vulgaris; light and red varieties) EO had the strongest bacteriostatic and bactericidal properties, followed by bay (Pimenta racemosa) and clove bud (Eugenia caryophyllata synonym: Syzygium aromaticum) EO. Oregano oil was colicidal at 625 μl l−1 at 10, 20 and 37°C. The addition of 0·05% (w/v) agar as stabilizer reinforced the antibacterial properties, particularly at 10°C, whereas 0·25% (w/v) lecithin reduced antibacterial activity. Scanning electron micrographs showed extensive morphological changes to treated cells.

Conclusions: Oregano and thyme EO possess significant in vitro colicidal and colistatic properties, which are exhibited in a broad temperature range and substantially improved by the addition of agar as stabilizer. Bay and clove bud EO are less active. Lecithin diminished antibacterial properties. The bactericidal concentration of oregano EO irreversibly damaged E. coli O157:H7 cells within 1 min.

Significance and Impact of the Study: Oregano and light thyme EO, particularly when enhanced by agar stabilizer, may be effective in reducing the number or preventing the growth of E. coli O157:H7 in foods.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1472-765X.2003.01285.x

Antibacterial activity of selected plant essential oils against Escherichia coli O157:H7.

The fumigant toxicity of 28 essential oils extracted from various spice and herb plants and some of their major constituents were assessed for adult coleopteransRhyzopertha dominica, Oryzaephilus surinamensis, Tribolium castaneum, andSitophilus oryzae. Three groups of active materials were distinguished: (1) The compounds terpinen 4-ol, 1,8-cineole, and the essential oils of three-lobed sage, sage, bay laurel, rosemary, and lavender were most active againstR. dominica; (2) The compounds linalool,α-terpineol, and carvacrol and the essential oils of oregano, basil, Syrian marjoram, and thyme were most active againstO. surinamensis; and (3) the compound 1,8-cineole and the essential oils anise and peppermint were active againstT. castaneum.

Fumigant toxicity of essential oils against four major stored-product insects.

Antifungal activity of oregano and thyme essential oils applied as fumigants against fungi attacking stored grain

The major component of the trail pheromone of the South American leaf-cutting ant,Atta sexdens rubropilosa Forel, is 3-ethyl-2,5-dimethylpyrazine (II). Methyl and ethyl phenylacetate and methyl 4-methylpyrrole-2-carboxylate (I), which is the major component of the trail pheromone ofA. texana (Buckley) andA. cephalotes (L.), were also identified and may be minor components. The pheromone is stored in the poison gland.Atta sexdens sexdens (L.) also responds strongly to the pyrazine, which in large amounts evokes a weak response fromA. texana, A. cephalotes, andAcromyrmex octospinosus (Reich). Foraging workers ofAtta sexdens rubropilosa did not preferentially pick up baits impregnated with the pyrazine. The pyrazine was puffed into the nests ofA. cephalotes, and a particular response called “milling” was noted.

Uzi Ravid

Papers

Fumigant toxicity of essential oils against four major stored-product insects.

Antifungal activity of oregano and thyme essential oils applied as fumigants against fungi attacking stored grain

The major component of the trail pheromone of the leaf-cutting ant,Atta sexdens rubropilosa forel