Phytochemistry and bioactive properties of plant volatile oils : antibacterial, antifungal and antioxidant activities
Aromatic and medicinal plants have been recognized since antiquity as possessing biological activities; chief amongst these are their antibacterial, antifungal and antioxidant properties. In this study, the chemical composition, the antimicrobial and in vitro antioxidant bioactivities the volatile oils extracted by hydrodistillation from aromatic and medicinal members of the plant families Geraniaceae: geranium (Pelargonium graveolens L'Herit); Lamiaceae: melissa (Melissa officinalis L.), monarda (Monarda citriodora var. citriodora Cerv. ex Lag.), oregano (Origanum vulgare ssp. hirtum (Link) Letsw.) and thyme (Thymus vulgaris L.); Myristicaceae: nutmeg (Myristica fragrans Houtt.); Myrtaceae: clove (Syzygium caryophyllus Gaertn.); Piperaceae: black pepper (Piper nigrum L.) and Umbelliferae: lovage (Levisticum officinalis L.) were investigated. The chemical percentage composition of the volatile oils extracted from black pepper, clove, geranium, lovage (from both leaf and stem material), melissa, monarda, nutmeg, oregano and thyme were analysed using gas chromatography and mass spectroscopy. The findings of these analyses confirmed that volatile oils from aromatic and medicinal members of different plant families are principally mixtures of mono- and sesqui- terpenoids compounds. Furthermore, oil samples extracted from species of the same plant family, samples sourced from different parts of the same plant and a commercial and authenticated volatile oil described as from the same species may exhibit different compositions, i.e. the percentage composition and variation in individual phytochemicals. A series of experiments were carried out in an attempt at assessing the antibacterial properties of black pepper, clove, geranium, melissa, nutmeg, oregano and thyme volatile oils and their main components. A collection of 25 test microorganisms [9 Gram-positive and 16 Gram-negative strains] including human, plant and veterinary pathogens and food spoilage organisms. All the volatile oils demonstrated some degree of antiseptic activity with the oils of oregano and thyme being particularly active. The phenyipropanoid eugenol and the phenolic constituents carvacrol and thymol were found to be strongly antiseptic. The antifungal activity of black pepper, clove, melissa, oregano and thyme volatile oils against the agriculturally important Aspergillus species Aspergillus flavus (Link) Fries and Aspergillus niger van Tieghen, and the Fusarium species Fusarium culmorum W.G. Smith was investigated. All the volatile oil samples demonstrated antifungal properties with variable degrees of efficacy across the concentration levels used in this study, with the volatile oils of oregano and thyme being particularly inhibitory. The in vitro antioxidant properties of the volatile oils of black pepper, clove, nutmeg, oregano and thyme and their major components were evaluated by using a method routinely used in this laboratory, a simplified plate diffusion technique for determining lipid antioxidant activity using linoleic acid/β-carotene. To investigate further the potential antioxidant activity of these plant extracts and their main components and characterise their underpinning mechanism of action, an attempt to develop and optimize current antioxidant screening techniques was carried out. These methods included a thiobarbituric reactive species assay, a conjugated diene assay and a free-radical trapping assay. Finally, a series of feeding trials were carried out to assess whether volatile oils feed at various concentrations prior to and during pregnancy can beneficially affect the compositional levels of major fatty acids in a maternal and foetal/neonatal model extracted from cholesteryl ester, triacylglyceride, free fatty acid and phosphoglyceride lipid fractions in an organ dependent manner. Particular focus centered upon the saturated fatty acids lauric, palmitic and stearic acids; the essential fatty acids[LA](18:2n-6) and α-linoleic [LA] (18:2n-6) and α-linolenic [ALA (18:3n-3) acids and the long chain polyunsaturated fatty acid metabolic derivatives, e.g. arachidonic acid [ARA] (20:4n-6), docosahexaenoic [DHA] (22:6n-3) and eicosapentnoic acid [EPA] (20:5n-3). The volatile oil of oregano was administered orally to female rats at 167mg Kgˉ¹; 334 mg Kgˉ¹ and 843 mg Kgˉ¹ concentration levels immediately prior to and during pregnancy to determine whether they affected the fatty acids composition in a variety of major maternal and neonatal organs. In addition to the aforementioned experiment, further feeding trails were carried out using the volatile oils of clove and nutmeg at a 5Oµg gˉ¹ concentration level.