Major bioactivities and mechanism of action of essential oils and their components Nizar Y. Saad, a,b * Christian D. Muller b and Annelise Lobstein b ABSTRACT: Essential oils are gaining increasing interest for their antimicrobial and antiviral properties, as well as for their preventive and therapeutic actions against many human pathologies. Herein, we present an overview on new discoveries in essential oil research, discussing antimicrobial activity, as well as immunomodulatory, anti-apoptotic, anti-angiogenic and anti-tumoural properties. In addition, we emphasize recent advances in the identification of bioactive components and understanding of their mechanism of action. We discuss their molecular diversity and wide spectrum of activity as well as their structure–activity relationships and capability of targeting paradoxical responses triggered by different genes and pathways. Finally, we emphasize the effort required to isolate and identify the bioactive components of essential oils and to determine their cytotoxicity as their specificity. Thus, new approaches to specifically address bioactive components to selected targets could enhance the latter property in order to accommodate any cytotoxicity towards dysfunctioning loci. Copyright © 2013 John Wiley & Sons, Ltd. Keywords: essential oils; antimicrobial activities; immuno-modulatory; anti-tumour; anti-apoptotic and anti-angiogenic properties Introduction Currently, several questions are raised concerning safety and the ratio of benefits to side effects of compounds used in medicine or in the food industry. Moreover, the extensive use of antibiotics in human medication as well as in animal farms, is leading to the emergence of resistant bacterial strains. Additionally, a growing number of allergic cases (allergies following absorption through the oral route, and others observed via dermal applications) in the modern world has become a real health problem. [1] Therefore, it is necessary to find alternative treatments that can fight bacteria resistant to antibiotics and heal or at least alleviate allergic reactions. Thus, two strategies can be undertaken to achieve these two objectives: (1) development of a new generation of antibiotics, and (2) search for natural products whose antibacterial, antifungal and antiviral proper- ties have been known for centuries in the field of alternative and popular medicines. Scientific evidence for these and other properties, discussed below, are already available, which has encouraged the increasing integration of natural products into modern medicine. [2] Among the natural products found in aromatic plants are known secondary metabolites and essential oils. Essential oils are volatile liquids, or semi-liquids, extracted from plants, usually by steam vaporization and cold-press techniques. However, contrary to what was mentioned in a 2000 review on the biological activity of essential oils and their constituents, [3] solvent extraction, simultaneous distillation–extraction (SDE) and supercritical fluid extraction (SFE) are not methods for producing essential oils, by definition. Indeed, using such methods would lead to the extraction and identification of many bioactive compounds that are not usual constituents of essential oils. Essential oils are complex mixtures of monoterpene and ses- quiterpene hydrocarbons (10 and 15 carbon atoms, respectively) and their oxygenated derivatives (alcohols, aldehydes, ketones) as well as phenylpropanoids. In some cases, essential oils may encompass other chemical families like fatty acids, oxides and sulfur derivatives. Essential oils have gained interest as potential sources for bioactive natural molecules and are the subject of studies for their possible use as alternative medications for the treatment of infectious diseases. The long known antimicrobial actions of essential oils are now being extensively scientifically reviewed and applied in health and industry fields. [4] Various in vitro studies have confirmed the inhibitory action of essential oils against bacteria, fungi, yeasts, viruses and protozoa. [5–7] Many regions, such as the Mediterranean region, are rich in aromatic plants, and production of essential oils from these plants can be a profitable source for the economic and ecological development for these countries. In this review, we describe methods of analysis of the antibacterial activity of essential oils. Following, we will review the published data on antimicrobial activity of essential oils, and discuss the structure–activity relationships and mecha- nisms of action of their active components. Finally, an overview about other preventive and therapeutic properties of essential oils, such as anti-inflammatory, immunomodula- tory, anti-angiogenic, anti-tumour and pro-apoptotic properties will be given and the mechanisms of action and putative target pathways discussed. Other biological activities of essential oils and their components, including anti-allergic, enzyme inhibitory, psychological, antimutagenic, antiviral, insect repellent and molluscicidal, as well as various uses and applications of essential oils are already argued [3] and are not discussed in this review. * Correspondence to: Nizar Youssef Saad, Faculté de Pharmacie, Université de Strasbourg, 74 route du Rhin, CS 60024, 67401 Illkirch Cedex, France. E-mail: nizar.saad@etu.unistra.fr a Faculty of Science II, Lebanese University, Fanar-Matn, Lebanon b Laboratoire d’Innovation Thérapeutique, UMR 7200 CNRS, Faculté de Pharmacie, Université de Strasbourg, France Flavour Fragr. J. 2013, 28, 269–279 Copyright © 2013 John Wiley & Sons, Ltd. Review Received: 16 January 2013, Revised: 13 March 2013, Accepted: 13 March 2013 Published online in Wiley Online Library: 9 May 2013 (wileyonlinelibrary.com) DOI 10.1002/ffj.3165 269