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Essential Oils as Potent Antimicrobial Agents

Rakesh K. Sindhu1*, Prabhjot Kaur1, Parneet Kaur1, Sumitra Singh2 and Simona Cavalu3

1Chitkara College of Pharmacy, Chitkara University, Punjab, India

2Department of Pharmaceutical Sciences, Guru Jambheshwar University of Science and Technology, Hisar, Haryana, India

3University of Oradea, Faculty of Medicine and Pharmacy, Oradea, Romania

Abstract

Over the last few decades, it has been investigated for the essential oils in variety of aromatic and herbal plants. Essential oils are defined as the concentrated hydrophobic liquids containing volatile chemical compounds incorporated normally in various plant parts during their secondary metabolism. They show extraordinary power in the area of biomedicine as they disrupt many bacteria, parasites, and viruses. The occurrence of various "aldehydes, phenolics, terpenes, and other antimicrobial compounds means that essential oils are used in a wide variety of bacteria". The utilization of essential oils as antimicrobials and the food preservatives is of great concern because the synthetic oils and drugs show various side effects. Therefore, the essential oils can be used as their alternative. Essential oils seem to be a possible option in synthetic compounds, particularly on account of the obstruction that has been progressively evolved by pathogenic microorganisms. The antitumor and antiviral, antioxidants, antifungal, and antibacterial action of essential oils and their constituents have been generally contemplated. Hereby, the antimicrobial efficiency can be evaluated by using various methods like dilution method and time kill methods. This chapter was centred around the antimicrobial action of different fundamental oils. It highlights the general explanation of essential oils including their methods of extraction and applications, the chemical composition, their mechanism of action on different microbes (bacteria, fungi, and viruses), evaluation of the antimicrobial efficacy, and their future perspectives.

Keywords: Essential oils, aromatic oils, antimicrobial agents, aldehyde, terpenes

1.1 Introduction

Natural herbs serve as a wide source of medicinal agents and variety of modern drugs. There are number of phytoconstituents present in the plants and herbs that have been investigated for their use in the treatment of various infectious diseases [1]. The use of medicinal plants by the mankind for the treatment of various ailments is being done since ancient times. The presence of numerous bioactive compounds in the medicinal plants plays a vital role in discovery of drugs [2]. Since few years, interest has been seen in the development of the therapeutic properties which are concentrated embodiments of different plant parts like buds, fruits, leaves, and flowers. Essential oils of plants are basically auxiliary metabolites of plants, which is accounted for having different biological activities like antimicrobial, anticancer, analgesic, and anti-inflammatory [3, 4].

Essential oils, also referred to as volatile oils, are a complex mixture that comprises of various single components. Essential oils possess gigantic business in the worldwide market attributable to extraordinary flavor, scent properties and biological activities as well [5-7]. They are utilized in aromatherapy as well as for treating various diseases including disorders, malignant growth, diabetes, and Alzheimer's [8]. Several researchers in the past have recognized the antimicrobial efficiency of essential oils and their chemical compounds [9-13]. They serve various activities like antibacterial, antiviral, and antifungal [14]. There are many essential oils tested for their potent antibacterial efficacy against "phytopathogenic bacteria" under different conditions in vivo and in vitro [15-17]. Essential oils and their natural active ingredients contain antimicrobial substances found in food and the way they are used in foods by providing alternatives to the conventional fungicides and bactericides [18]. The antifungal activity is mainly due to the lipophilic properties of the essential oils as it provides cell wall penetration and affects the enzymes which are involved in the synthesis of cell wall, therefore altering the morphological characteristics of fungi [19]. The essential oils are characterized into different types based on their origin and the chemical components present in them. Each of these components exhibit different modes of action, which may vary with the type of essential oil or the microbes used. "Gram-positive bacteria" are found to be more liable to essential oils than "gram-negative bacteria". Gram-positive bacteria lead to the ease in the lipophilic compound infiltration of the essential oils. Thus, a series of biochemical reaction is accountable for the antimicrobial activity of the essential oils. They depend on the nature of the chemical components [20, 21]. Various factors that affect the antimicrobial efficacy are taken into consideration depending upon the active constituents present in them. This antimicrobial activity can be evaluated by using various methods like dilution method, time kill methods, agar disk diffusion method, and checkerboard testing. Essential oils can also be used in combination with the antibiotics for improving their effectiveness. Different antibiotics to which various microbes have become resistant lead to the increase in their efficacy when given along with the essential oils. Therefore, the essential oils serve as a powerful tool possessing antimicrobial efficacy, which can be evaluated using different techniques including dilution method, agar disk diffusion method, time kill method, and checkerboard testing.

The objective of this chapter is to highlight the mechanism of action like how they inhibit the microbes based on a series of biochemical reactions and focusing role of essential oils as antimicrobial agents.

1.2 Essential Oils

Essential oils are defined as blended hydrophobic beverages with chemical reactions from plants. The essential oils can be obtained themselves in the chloroplasts of the leaf by hydrolysis of specific glycosides. They can be found in various plant parts like leaves, seeds, roots, wood, rhizome, bark, and resin. Certain plants that are rich in the essential oils include almond, oregano, rose, bergamot, jasmine, eucalyptus, and juniper. The essential oils present in different parts of the plants may differ from each other in properties, constituents, etc. [22]. The medicinal and aromatic plants produce a number of secondary metabolites among which the terpenes, terpenoids, and aromatic phenols play an important role in the composition of essential oils [23]. There are two common pathways namely shikimic acid pathway and mevalonate pathway for the synthesis of terpenoid derivatives [24]. These oils generally get evaporated at ordinary temperature when comes in exposure with air. There different categories including terpenoids (hemiterpenes, monoterpenes, sesquiterpenes, and diterpenes) and functional groups (hydrocarbon, aldehyde, alcohol, phenol, and ether) present in them [25, 26]. The essential oils have been regarded by the science as far as its usefulness. These are considered as the "chemical weapons" of the plant kingdom as the compounds present in them can deter the insects and ensure the plant's protection against the microbial attacks. They may additionally act as the "plant pheromones" with an end goal to attack and cause the seduction of pollinators. The oxygenated atoms of the essential oils, which serve as chemical messengers, bring life to the plants, therefore helping in development and invigorating healings [22]. There are around 3,000 essential oils that have been depicted from which 300 of them are used financially in the flavoring and aromas advertise [27].

1.2.1 Classification of Essential Oils

This classification can be widely done on the basis of chemical composition, methods of extraction, and aroma, which is described in Figure 1.1 [28].

1.2.2 Methods of Extraction

The essential oils need to be extracted in many forms, which may enhance their bioactivity in one or the other way. These essential oils have various applications in pharmaceutical, cosmetics, and in food safety. The extracting techniques may vary depending on the properties and compounds as required in the herbal extract. The extraction methods used are the main factors to ensure the quality of essential oil. Inappropriate extraction techniques may result in the destruction and alter the action of the phytoconstituents present in the essential oils. This may result in numerous effects including the stain effect, physical change of essential oils, off flavor/odor, and loss of the pharmacological components [29, 30]. There is a rapid increase in the world production and consumption of essential oils and perfumes. There are several extraction techniques that can be used, e.g., soxhlet extraction, cold pressing method, steam distillation, solvent extraction, and supercritical fluid extraction [31].

Figure 1.1 Classification of essential oils [28].

1.2.3 Applications

The use of essential oils is generally different depending upon the source, extraction procedure, quality, etc. The current applications of the essential oils have been observed in the fabrication of aromas, beauty products, cleansing gels, and shampoos...