The presence, yield and composition of secondary metabolites in plants, viz. the volatile components and those occurring in essential oils, can be affected in a number of ways, from their formation in the plant to their ¼nal isolation. Several of the factors of in½uence have been studied, in particular for commercially important crops, to optimize the cultivation conditions and time of harvest and to obtain higher yields of high-quality essential oils that ¼t market requirements. In addition to the commercial importance of the variability in yield and composition, the possible changes are also important when the essential oils and volatiles are used as chemotaxonomic tools. Knowledge of the factors that determine the chemical variability and yield for each species are thus very important. These include: (a) physiological variations; (b) environmental conditions; (c) geographic variations; (d) genetic factors and evolution; (e) political/social conditions; and also (f) amount of plant material/space and manual labour needs.

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The essential oil composition of Cananga odorata, Boswellia thurifera, Cymbopogon citratus, Marjorana hortensis, Ocimum basilicum, Rosmarinus ocinalis, Cinnamomum zeylanicum and Citrus limon was analysed by GC and GC±MS, and their antimicrobial and antioxidant activity tested. Twenty-®ve dierent genera of bacteria and one fungal species were used in this study as test organisms. These included animal and plant pathogens, food poisoning and spoilage bacteria and the spoilage fungus Aspergillus niger. The volatile oils exhibited considerable inhibitory eect against all the tested organisms. The oils also demonstrated antioxidant capacities, comparable with a-tocopherol and butylated hydroxytoluene (BHT). The method adopted in this study was the modi®ed thiobarbituric acid reactive species (TBARS) assay. The antioxidant activity was carried out under dierent conditions by using egg yolk and rat liver in the absence and presence of the radical inducer 2,2 H -azobis (2-amidinopropane) dihydrochloride (ABAP). #

Aims: Thymus species are wild species mostly found in the arid lands of Portugal. Possible antimicrobial properties of Thymus essential oils have been investigated. The chemical composition of the essential oils and the antimicrobial activity of Thymus mastichina (L) L. subsp. mastichina, T. camphoratus and T. lotocephalus from different regions of Portugal were analysed. Methods and Results: Hydrodistillation was used to isolate the essential oils and the chemical analyses were performed by gas chromatography (GC) and GC coupled to mass spectrometry. The antimicrobial activity was tested by the disc agar diffusion technique against Candida albicans, Escherichia coli, Listeria monocytogenes, Proteus mirabilis, Salmonella spp. and Staphylococcus aureus. Pure linalool, 1,8-cineole and a mixture (1 : 1) of these compounds were included. Linalool, 1,8-cineole or linalool ⁄ 1,8-cineole and linalool ⁄ 1,8-cineole ⁄ linalyl acetate were the major components of the essential oils, depending on the species or sampling place. The essential oils isolated from the Thymus species studied demonstrated antimicrobial activity but the micro-organisms tested had significantly different sensitivities. Conclusions: The antimicrobial activity of essential oils may be related to more than one component. Significance and Impact of the Study: Portuguese endemic species of Thymus can be used for essential oil production for food spoilage control, cosmetics and pharmaceutical use. Further studies will be required to elucidate the cell targets of the essential oil components.

Thymbra capitata and Thymus species are commonly known in Portugal as thyme and they are currently used as culinary herbs, as well as for ornamental, aromatizing and traditional medicinal purposes. The present work reports on the state of the art on the information available on the taxonomy, ethnobotany, cell and molecular biology of the Portuguese representatives of these genera and on the chemotaxonomy and antibacterial, antifungal and antioxidant activities of their essential oils and other volatile-containing extracts.

Antilisterial activities of Thymbra capitata and Origanum vulgare essential oils were tested against 41 strains of Listeria monocytogenes. The oil of T. capitata was mainly constituted by one component, carvacrol (79%), whereas for O. vulgare three components constituted 70% of the oil, namely, thymol (33%), gamma-terpinene (26%), and p-cymene (11%). T. capitata essential oil had a significantly higher antilisterial activity in comparison to O. vulgare oil and chloramphenicol. No significant differences in L. monocytogenes susceptibilities to the essential oils tested were registered. The minimum inhibitory concentration values of T. capitata essential oil and of carvacrol were quite similar, ranging between 0.05 and 0.2 microL/mL. Antioxidant activity was also tested, the essential oil of T. capitata showing significantly higher antioxidant activity than that of O. vulgare. Use of T. capitata and O. vulgare essential oils can constitute a powerful tool in the control of L. monocytogenes in food and other industries.

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