**Abstract**

Essential oils (EO) are volatile, non-lipid-based oils produced as a plant defense mechanism. Studies from our group have validated the potential usefulness of EOs to synergistically and additively work with antibiotics. In this book chapter, we aim to outline some background on the EOs and their uses and applications, to discuss the different mechanisms of action in partnering with antibiotics, and, finally, to explore their potential use against multidrug-resistant bacteria. Applications of EO in therapy will enable the revival of previously sidelined antibiotics and enhance the development of new drug regimens to better mitigate what may be the biggest health challenge by year 2050.

**Keywords:** lavender oil, cinnamon bark oil, peppermint oil, multidrug-resistant bacteria, synergistic interaction, antimicrobial

## **1. Introduction**

Essential oil (EO) is a concentrated mixture of organic compounds. EOs are produced by plants as a form of defense in addition to being an attractant to insects for dispersion of pollens and seeds [1, 2]. These oils are formed by the glandular trichomes and specialized secretory structure like secretory hairs, ducts, cavities, and glands; they then diffuse to the surface organs of plant such as leaves and flowers [3, 4]. The process of EOs formation involves three pathways which are the methyl-D-erythritol-4-phosphate (MEP), mevalonate, and malonic acid pathways [5]. The MEP and mevalonate pathways contribute in the biosynthesis of isoprenoids, whereas the malonic acid pathway will form the phenolic compounds [6, 7].

EOs have been used for many years for different purposes, such as to preserve raw and processed food because it can inhibit the growth of microorganisms like bacteria, viruses, and fungi [1, 8, 9]. Besides food, EO was also utilized in the area of perfumery for many years especially for ancient civilizations of India, Greece, Egypt, and Rome [10, 11].

In addition, EOs also serve as an alternative medicine that is important for local populations to treat severe burns to accelerate healing [11] and also for diseases such as leishmaniasis, schistosomiasis, and malaria [12, 13]. To date, approximately 10% of all EOs have been analyzed and commercially used as an insect repellent, attributed by its low toxicity to mammalian cells and the environment [10, 14]. However, certain EOs may cause toxicity or allergies which results in health and safety problems. Hence, national and international organizations have set standards to control the use of EOs [15].


**Table 1.**

*EOs extracted from plant parts.*

EOs can be found in various plants species, in particular those that belong to the Coniferae, Myrtaceae, Rutaceae, Labiatae, Umbelliferae, Alliaceae, and Zingiberaceae families [16, 17]. EOs are derived from different plant parts, such as flowers, leaves, wood, roots, seeds, rhizomes, and fruits [18]. See **Table 1** for examples of EOs found in each of the plant parts.
