The volatile oils of black pepper [Piper nigrum L. (Piperaceae)], clove [Syzygium aromaticum (L.) Merr. & Perry (Myrtaceae)], geranium [Pelargonium graveolens L'Herit (Geraniaceae)], nutmeg [Myristica fragrans Houtt. (Myristicaceae), oregano [Origanum vulgare ssp. hirtum (Link) Letsw. (Lamiaceae)] and thyme [Thymus vulgaris L. (Lamiaceae)] were assessed for antibacterial activity against 25 different genera of bacteria. These included animal and plant pathogens, food poisoning and spoilage bacteria. The volatile oils exhibited considerable inhibitory effects against all the organisms under test while their major components demonstrated various degrees of growth inhibition.

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-2672.2000.00969.x

Antimicrobial agents from plants: antibacterial activity of plant volatile oils.

The antimicrobial activity of plant oils and extracts has been recognized for many years. However, few investigations have compared large numbers of oils and extracts using methods that are directly comparable. In the present study, 52 plant oils and extracts were investigated for activity against Acinetobacter baumanii, Aeromonas veronii biogroup sobria, Candida albicans, Enterococcus faecalis, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Salmonella enterica subsp. enterica serotype typhimurium, Serratia marcescens and Staphylococcus aureus, using an agar dilution method. Lemongrass, oregano and bay inhibited all organisms at concentrations of ≤ 2.0% (v/v). Six oils did not inhibit any organisms at the highest concentration, which was 2.0% (v/v) oil for apricot kernel, evening primrose, macadamia, pumpkin, sage and sweet almond. Variable activity was recorded for the remaining oils. Twenty of the plant oils and extracts were investigated, using a broth microdilution method, for activity against C. albicans, Staph. aureus and E. coli. The lowest minimum inhibitory concentrations were 0.03% (v/v) thyme oil against C. albicans and E. coli and 0.008% (v/v) vetiver oil against Staph. aureus. These results support the notion that plant essential oils and extracts may have a role as pharmaceuticals and preservatives.

Antimicrobial activity of essential oils and other plant extracts

Organic acids, such as malate, citrate and oxalate, have been proposed to be involved in many processes operating in the rhizosphere, including nutrient acquisition and metal detoxification, alleviation of anaerobic stress in roots, mineral weathering and pathogen attraction. A full assessment of their role in these processes, however, cannot be determined unless the exact mechanisms of plant organic acid release and the fate of these compounds in the soil are more fully understood. This review therefore includes information on organic acid levels in plants (concentrations, compartmentalisation, spatial aspects, synthesis), plant efflux (passive versus active transport, theoretical versus experimental considerations), soil reactions (soil solution concentrations, sorption) and microbial considerations (mineralization). In summary, the release of organic acids from roots can operate by multiple mechanisms in response to a number of well-defined environmental stresses (e.g., Al, P and Fe stress, anoxia): These responses, however, are highly stress- and plant-species specific. In addition, this review indicates that the sorption of organic acids to the mineral phase and mineralisation by the soil's microbial biomass are critical to determining the effectiveness of organic acids in most rhizosphere processes.

Organic acids in the rhizosphere: a critical review

Laccases of fungi attract considerable attention due to their possible involvement in the transformation of a wide variety of phenolic compounds including the polymeric lignin and humic substances. So far, more than a 100 enzymes have been purified from fungal cultures and characterized in terms of their biochemical and catalytic properties. Most ligninolytic fungal species produce constitutively at least one laccase isoenzyme and laccases are also dominant among ligninolytic enzymes in the soil environment. The fact that they only require molecular oxygen for catalysis makes them suitable for biotechnological applications for the transformation or immobilization of xenobiotic compounds.

Fungal laccases - occurrence and properties.

/pdf/the-structure-and-function-of-fungal-laccases-25lk9xuy0a.pdf

The structure and function of fungal laccases

Oxalate secretion by fungi provides many advantages for their growth and colonization of substrates. The role of oxalic acid in pathogenesis is through acidification of host tissues and sequestrati...

Oxalate production by fungi : its role in pathogenicity and ecology in the soil environment

Fifty-one essential oils extracted from plants of known origin were tested for their antimicrobial activity against three bacteria, Pseudomonas aeruginosa, Staphylococcus aureus, Escherichia coli and four yeasts, Torulopsis utilis, Schizosaccharomyces pombe, Candida albicans and Saccharomyces cerevisiae using the drop diffusion method. All showed antimicrobial activity against at least one of the micro-organisms. Following this preliminary screening, 13 essential oils showing antimicrobial activity against at least five of the micro-organisms were tested in the range 50 micrograms ml-1 to 500 micrograms ml-1 using broth micro dilution techniques with dimethylsulphoxide (DMSO) as a dispersing solvent. The concentration of most of the oils required for total inhibition of growth was > 500 micrograms ml-1. Further studies on the antimicrobial action of cinnamon oil in the range 10-150 micrograms ml-1 showed that 50-fold higher activity was found when no dispersing solvent was used.

/pdf/antimicrobial-action-of-essential-oils-the-effect-of-40mnxo1tcy.pdf

Antimicrobial action of essential oils : the effect of dimethylsulphoxide on the activity of cinnamon oil

. The essential oil extracted from clove (Syzygium aromaticum) is used as a topical application to relieve pain and promote healing in herbal medicine and also finds use in the fragrance and flavouring industries. Clove oil has two major components, eugenol and β-caryophyllene, which constitute 78% and 13% of the oil, respectively. Clove oil and these components are generally recognized as ‘safe’, but the in-vitro study here demonstrates cytotoxic properties of both the oil and eugenol, towards human fibroblasts and endothelial cells. Clove oil was found to be highly cytotoxic at concentrations as low as 0.03% (v/v) with up to 73% of this effect attributable to eugenol. β-caryophyllene did not exhibit any cytotoxic activity, indicating that other cytotoxic components may also exist within the parent oil.

/pdf/cytotoxicity-of-clove-syzygium-aromaticum-oil-and-its-major-43681vguye.pdf

Cytotoxicity of clove (Syzygium aromaticum) oil and its major components to human skin cells

Lavender (Lavandula angustifolia) oil, chiefly composed of linalyl acetate (51%) and linalool (35%), is considered to be one of the mildest of known plant essential oils and has a history in wound healing. Concerns are building about the potential for irritant or allergenic skin reactions with the use of lavender oil. This study has demonstrated that lavender oil is cytotoxic to human skin cells in vitro (endothelial cells and fibroblasts) at a concentration of 0.25% (v/v) in all cell types tested (HMEC-1, HNDF and 153BR). The major components of the oil, linalyl acetate and linalool, were also assayed under similar conditions for their cytotoxicity. The activity of linalool reflected that of the whole oil, indicating that linalool may be the active component of lavender oil. Linalyl acetate cytotoxicity was higher than that of the oil itself, suggesting suppression of its activity by an unknown factor in the oil. Membrane damage is proposed as the possible mechanism of action.

/pdf/cytotoxicity-of-lavender-oil-and-its-major-components-to-1x54agergw.pdf

Christine S. Evans

Papers

Oxalate production by fungi : its role in pathogenicity and ecology in the soil environment

Antimicrobial action of essential oils : the effect of dimethylsulphoxide on the activity of cinnamon oil

Cytotoxicity of clove (Syzygium aromaticum) oil and its major components to human skin cells

Cytotoxicity of lavender oil and its major components to human skin cells.

Antimicrobial action of palmarosa oil (Cymbopogon martinii) on Saccharomyces cerevisiae