2.1 The History of essential oils
The Emergence and spread of drug-resistant pathogens are considered as the major serious problems to the public health because it reduces the effectiveness of antibiotic treatment, which could lead to an increase in limitations of the available antibiotics, morbidity and mortality (Jones, 2001). All of the previous reasons have prompted scientists to find alternative antimicrobial agents for the treatment of resistant pathogenic microorganisms including the use of essential oils.
The essential oils and their derivatives from natural origin such as plant oils have been known and used for a long time in food reservation. The ancient Egyptian people and other nation for example China and India have been used essential oils (EOs) for many centuries. the production of the essential oils returns back to 2000 years ago, in the middle east, with the beginnings of more modern technologies occurring in Arabia in the 9th century. Plant extracts, in addition to participating in taste and flavor, can act against Gram-positive and negative pathogens. Nowadays, several of the essential oils are still utilized as medication in the public health in India.
2.1 Essential oils
Essential oils are the volatile fluids chemically complex mixtures, they are produced from the secondary metabolism of aromatic plants. These oils are termed ‘essential’ because they represent the most important part of the plant. they are obtained from plant material (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits, and roots). [12], [13], [14], [15], [16], [17] .Essential oils are widely distributed in all plant kingdom. The essential oils are complex mixers Consisting of a lot of components, derived from terpenes and terpenoids (isoprenoids) and aromatic and aliphatic aldehydes and phenols, all characterized by low molecular weight [20]. They considerably are responsible for the distinctive odor of plants and contributes to the advantageous or disadvantageous effects [21]. There are many processes of essential oils extraction. They can be obtained by mechanical expression, hydrodistillation, fermentation or extraction and steam distillation that is the most common used for commercially produced it. The application of ingredients essential oils in foods, drinks and cosmetics is very important, both for the growing interest of consumers in use of natural ingredients and also increasing concern about possibility unsafe of synthetic additives [24].Essential oils are commercially important especially for the pharmaceutical, agronomic, food, sanitary, cosmetic and perfume industries [23, 25]. Essential oils exhibit various biological activities like antibacterial [26, 27, 28], antifungal [29, 30], antiviral [31, 32], insecticidal [33] antioxidant [34, 35, 36], anticancer activity [37, 38, 39], anti-inflammatory [40, 41, 42], Antistaphylococcal activity [43], Antimycotic activities [44] and antidiabetic [45, 46, 47], etc. Some oils are also used in food preservation, aromatherapy [48] and fragrance industries [49].
2.1.1 Properties of essential oils
The Biological activities of antimicrobial EOs are directly linked to the presence of their bioactive volatile ingredients (Mahmoud & Croteau, 2002). EOs chemically composed of terpene compounds (mono-, sesqui- and diterpenes), alcohols, acids, esters, epoxides, aldehydes, ketones, amines and sulfides (Bakkali, Averbeck, Averbeck, & Idaomar, 2008). The EOs have antiseptic characteristics such as antibacterial, antifungal, insecticidal activities (Bassol” and Juliani, 2012). The plants are producing EOs for their characteristics to defend against predators and herbivores. The Most of the oils have toxic effects to cells without to be able to make mutations and carcinogens (Bakkali et al., 2008). The properties of antimutagenic can be due to several effects likes suppressing the entrance of the mutagen, deactivation of the mutagen, and activation of the cell to produce antioxidants (Bakkali et al., 2008). In fact, there are diverse studies that are conducted to display the antimicrobial activity of plant products. EOs considered as the best antimicrobial candidates to use as preservatives in the food system (Tajkarimi et al., 2010).
2.1.2 Antimicrobial activity of essential oils
The antimicrobial effects of EOs have been covered a wide spread of microorganisms for many years, but their mechanism(s) of action are still not fully understood. many mechanisms have been suggested to demonstrate the actions of the chemical compounds included in the EOs (Burt, 2004; Cox et al., 2000). The antimicrobial activity of essential oils cannot be approved according to just one component due to they have different ingredients (Bajpai, Baek, & Kang, 2012). Feature lipophilic components of EOs contribute to the antimicrobial effect because they are able to be accumulated in the lipid bilayer of the cell membrane, which will follow the loss of ions and reduce the ATP that lead to cell death (Bakkali et al., 2008; Oussalah et al., 2006; Quir”s-Sauceda et al., 2014). The EOs have the capacity due to their hydrophobicity to go through the cell membrane and mitochondria then ,lead them to high permeability which allows the cells to be more sensitive to other antimicrobial agents, that causes the death of the cell (Solorzano-Santos and Miranda-Novales, 2012). Furthermore, EOs have hydroxyl group which is might block the genetic material composition (Hern”ndez-Ochoa et al., 2011). There are ingredients in the EOs might bind to proteins that cause cell death due to inhibiting the activity of metabolic enzymes (Lacroix, 2007). The EOs can kill the bacteria by other ways such as making the membrane more rigid, reducing respiratory activity, and coagulation of cytoplasmic material (Hyldgaard et al., 2012).
2.2 Essential oil safety
Generally, the essential oils are safe with minimum negative effects. Many of EOs have been approved as food additives by the U.S. Food and Drug Administration [76]. commonly, the most side effects could be appear in the mucous membranes, skin irritation, and allergies, particularly for oils having aldehydes and phenols. Allergy caused from inhaled essential oils can happen; however, data about exposure levels are limited and several of the reports regarded perfumes rather than aromatherapy essential oils [77]. An unusual case of airborne contact dermatitis was reported only once in context to aromatherapy without massage [78]. The aromatherapy employs Mixture Unknown of these essential oils without discovering their plant sources. It has been reported allergies in some cases, mostly with local management. with the period of time, the oils could be oxidized, and they are reported for the convert in their chemical composition on storage for a long time. a few previous studies have observed that using these oils are not safe, but the most of them have not proved these oils if used in aromatherapy are harmful [80].
2.3 Some plants used in treatment
Many plants have been reported to use in the aromatherapy due to the presence of essential and volatile oils in different plants' materials like flowers, barks, stem, leaves, roots, fruits etc.
2.3.1 Eucalyptus
Eucalyptus (Eucalyptus globulus) belongs to the family of Myrtaceae, is an evergreen plant. The oils of E. globule were used as a controller and stimulator for the different systems. Throat infections, catarrh, coughs, bronchitis, asthma and sinusitis associated with the respiratory system, could be treating by oils of this plant. Furthermore, the antioxidant, anti-inflammatory and antibacterial activities of the Eucalyptus oil were evidenced. several previous studies have been approved the efficacy of Eucalyptus oil as a good cure for many infectious diseases. Promising results make it possible to use this oil a wide range to treat multifactorial diseases of various origins in humans [44,46].
2.3.2 Lavender
Lavender (Lavandula officinalis Chaix.) belongs the family of Lamiaceae, is a good herb of the garden [38]. when the antibiotics failing to work, the oil of Lavender has antibacterial feature against a lot of bacterial species, but mechanisms are still not completely understood. Using Lavender oil as therapeutic drugs was documented for medication of abrasions, burns, stress, headaches, in a promotion of new cell growth, skin problems, painful muscles and promoting an immune system [47,48,54].
2.3.3 Peppermint
Peppermint [Mentha piperita Linn. (M. Piperita)] belongs to the family of Lamiaceae. The biological action resulted to menthol that is the major component of peppermint oil. Peppermint oil has been studied and documented for its anti-inflammatory, analgesic, antimicrobial, antiseptic, antispasmodic. It was observed that it can calm down much bacterial, fungal, and viral infections when applied in the form of a vapor balm.
2.3.4 Rosemary
Rosemary (Rosmarinus officinalis Linn.) belonging to the family of Lamiaceae bears. there are three types of the rosemary (silver, gold and green stripe); the green that is used for its pharmaceutical properties.