Essential Oils



Essential Oils


Katherine A. Hammer

Christine F. Carson



This chapter describes the use of essential oil products for antisepsis and disinfection. Essential oils are plant-derived, volatile, multicomponent liquids that are oily or lipid-like and are produced in many plant tissues. Many have a strong fragrance and have traditionally been extracted by steam or hydrodistillation.1 There is a wealth of scientific data describing the antimicrobial activity of essential oils against many different microorganisms using standard in vitro susceptibility tests, which has been summarized elsewhere.2 This type of data, including minimum inhibitory concentrations (MICs), zones of inhibition, or time kill data, are not discussed in detail in this chapter, where the focus instead is on data pertaining specifically to antisepsis and disinfection applications, including results generated by standard disinfection and antisepsis testing protocols, in-use studies, and in vivo clinical studies. This chapter is also limited to the description of antimicrobial activity relating to plant essential oils and does not include other types of plant extracts, such as hydrosols, preparations from desiccated plant material, or solvent extractions.


CHEMISTRY AND COMPOSITION

Essential oils are typically distilled or pressed from fresh plant material, primarily leaves, and are not water miscible because they are composed largely of hydrocarbons and their oxygenated derivatives. They are generally regarded as secondary metabolites occurring in many, but not all, higher plants. As secondary metabolites, they are not essential for plant growth and often have no known function. Essential oils are complex mixtures, often containing more than 100 volatile, lipophilic, low-molecular-weight compounds that may be roughly divided into two chemical classes: terpenaceous and phenylpropanoid compounds.1 In plants, they are produced via three major biosynthetic pathways. Terpenaceous compounds are produced in plants by the mevalonic or nonmevalonic acid pathways or a combination of both. The nonmevalonic acid pathway is also known as the methylerythritol phosphate pathway.3 Phenylpropanoid compounds are produced by the shikimic acid pathway.4

Terpenaceous compounds arising from the mevalonic and nonmevalonic acid pathways may or may not contain oxygen. Sensu stricto, the term terpene refers to the hydrocarbon compounds from these pathways containing only carbon and hydrogen, whereas terpenoid refers to compounds that also contain oxygen. In practice, the terms are often used interchangeably.1 The former is used in this chapter to refer collectively to terpene and terpenoid compounds. Historically, the word terpene comes to us from turpentine, the liquid originally derived from various Pinus species, although modern turpentine is a petroleum-based product. It was from the original, natural turpentine that the first members of the terpene class of compounds were isolated and characterized.

Terpenes are the largest group of natural compounds with more than 30 000 known structures, ranging from the smaller terpenes found in essential oils to higher terpenes such as beta carotene.5 The basic biosynthetic building block for terpenes is an isoprene unit (C5H8) that is generally joined together in a head-to-tail fashion, although other combinations such as head-to-head do occur. The number of isoprene units from which a terpene is derived, with or without the subsequent loss or addition of carbon atoms, is used to classify the large number of compounds in this class into smaller groups. The terpenaceous compounds in essential oils are almost entirely limited to monoterpenes and sesquiterpenes with a few diterpenes.

Monoterpenes are the most common group of compounds found generally in essential oils.1 In terms of the compounds present in essential oils used commercially in antisepsis and disinfection products, most fall into this chemical class. They are formed when two isoprene (C5H8) units combine yielding a compound with a molecular
formula of C10H16. At first glance, the scope for variation on this theme may seem limited. However, through subsequent substitutions, cyclizations, and/or isomerizations, the derivatives fall into categories of alcohols, esters, phenols, ketones, lactones, aldehydes, and oxides, and about 1500 monoterpenes have been described,6 although not all are found in essential oils.

Many cyclic monoterpenes contain a benzene ring7 that has been shown to make a significant contribution to the antimicrobial activity of both the isolated compound and to essential oils that it occurs in. The antimicrobial activity of the benzene ring may be further enhanced by the attachment of a hydroxyl group to the ring, forming a phenol.8,9,10 Phenolic monoterpenes rank among the most well-known and well-characterized antimicrobial essential oil compounds and include thymol and carvacrol.

Sesquiterpenes are the second most frequent group of compounds found generally in essential oils and arise from the combination of three isoprene units.7 They are not highly prevalent among the oils or compounds used commercially for antisepsis or disinfectant products, although many are known to possess antimicrobial activity. Diterpenes are minor components of essential oils and are not purposefully exploited in commercial antisepsis or disinfectant products, although they may be present in oils that are.

Lastly, phenylpropanoids occur in plant oils much less consistently and generally less abundantly than terpenes but sometimes at high proportions.1 They are derived from the shikimic acid pathway that occurs in microorganisms and plants but never in animals. They are a small group of compounds with only about 50 described. Among these are a few antimicrobial compounds that historically have enjoyed widespread use such as eugenol and anethole.


SPECTRUM AND MODE OF ACTION OF ESSENTIAL OILS

The specific biological activity of an essential oil is a function of its composition. Different oil components inhibit or kill different microorganisms and by different means. Because the term essential oil applies to plant extracts containing a diverse array of antimicrobial components, the mechanism of action for each essential oil is not identical. That said, there are some mechanisms of action that are common to a number of different essential oils.

As a general rule, essential oils show concentration-dependent activity, with minor antimicrobial effects at lower or subinhibitory concentrations and gross effects including death at higher concentrations. Many of the commonly studied essential oils and components, such as oregano oil, thyme oil, and tea tree oil, show bactericidal activity rather than bacteriostatic activity.1,11,12 Understandably, bacteriostatic essential oils would be of limited use for many disinfection applications.

A mechanism of action common to many essential oils and components is the disruption or damage of microbial membranes, which results in the loss of membrane barrier or metabolic function. Damage typically occurs in a concentration-dependent manner, with minor membrane changes occurring at lower concentrations and gross membrane damage occurring at higher concentrations.13 This is often due to the oxygenated monoterpene components, examples of which are thymol, carvacrol, terpinen-4-ol, linalool, and 1,8-cineole. Examples of oils or components shown to elicit membrane damage include oregano oil,14 carvacrol,15,16 thymol,17 1,8-cineole,18 tea tree oil, and terpinen-4-ol.19 Additional mechanisms of action shown for some essential oils and components include inhibition of respiration by tea tree oil20 and inhibition of cell division by cinnamaldehyde.21

Essential oils also vary in their spectrum of antimicrobial activity, which relates to both the chemical composition of the oil and microbial characteristics. As a generalization, essential oils rich in components with higher water solubility, such as the oxygenated monoterpene alcohols, will have higher antibacterial activity, whereas those rich in components with lower water solubility will have lower activity.2 Most essential oils do not show large differences in levels of activity against gram-positive and gram-negative bacteria, although minor variations are common.2 The effect of essential oils on bacterial endospores has not been extensively studied, and available data suggest that essential oils can have sporicidal activity and can also affect the germination and outgrowth of endospores.2 Mycobacteria, which are known to be less susceptible to disinfectants compared to other bacteria, have been tested for susceptibility to essential oils.22,23 Studies indicate that mycobacteria are inhibited by a range of essential oils; however, due to the growth requirements of mycobacteria, special testing conditions must be used, which means that susceptibility results are not directly comparable to those obtained for bacteria. Yeasts are largely susceptible to essential oils at concentrations similar to those affecting bacteria.24 Other fungi, such as dermatophytes and molds, are also inhibited and killed, although the effects vary according to test conditions, concentration, and type of oil.2,25 A number of different viruses have been tested for susceptibility to a range of essential oils, and antiviral activity has thus far been observed for enveloped viruses but not for nonenveloped viruses.24,26,27


COMMERCIAL PRODUCTS CONTAINING VOLATILE OILS OR COMPONENTS

Several essential oil products have been commercially available for decades. These include disinfectants traditionally containing pine oil, such as Pine-O-Cleen® and Pine-Sol®. Pine-O-Cleen® was developed in Australia by
Len Hunter during World War II and now contains benzalkonium chloride as the active ingredient but historically contained pine oil.28 Similarly, the disinfectant Pine-Sol®, developed in the United States in 1929, contained pine oil until recently when it was removed due to lack of availability.29 Interestingly, many Pine-Sol® products contain limonene, among the other components. The product Listerine® was developed in 1879 in the United States.30 Originally developed as a general disinfectant, it was aggressively marketed in the 1920s as a mouthwash, which is how we know it to be used today. Listerine® contains the essential oil components menthol, thymol, methyl salicylate (wintergreen), and eucalyptol. These days, a large number of disinfection and antiseptic products are commercially available that contain essential oils. These range from hand washes and hand gels, to surface disinfectants and sanitizers, to gel products for air conditioning systems. Many of these formulated products also contain other disinfectant ingredients as active components. The essential oil is often not included as an active component but rather as a fragrance or to enhance consumer appeal, particularly for those products marketed as “natural,” “eco-friendly,” or “green.” In these products, the essential oil is likely to be present at concentrations that are too low to have any significant antimicrobial effect.


OILS MOST FREQUENTLY USED IN DISINFECTION

Relatively few of the large number of essential oils produced worldwide in commercial quantities31 are used for disinfection. The oils described in the following discussion include those most commonly included in cleaning, disinfectant, and antiseptic products as well as essential oils that have been investigated specifically for disinfection applications.


Citrus Oils

The two citrus oils most frequently found in disinfectant products are orange and lemon oils. The major citrus oil component limonene (synonymous with dipentene) is also used in some products.

Orange oil, also called sweet orange oil (to differentiate it from bitter orange oil), is obtained from the peel of Citrus sinensis (in the Rutaceae, or citrus tree, family) and contains at least 90% D-limonene.32 Orange oil is the top essential oil produced in the world by volume31 and is obtained largely as a by-product of orange juice production.33 Brazil is the largest global producer of oranges34 and as such also produces the largest quantities of orange oil.31 Lemon oil is obtained from Citrus limon (also in the Rutaceae) and contains D-limonene (60% to 75%) and β-pinene.32 Relatively large volumes of lemon oil are produced compared to many other essential oils, with most produced in Argentina.31 Lemon oil is one of several products obtained from lemon processing, in addition to lemon juice.

Citrus oils are included in a range of “cleaner-disinfectant” type products but are generally not widely used in antisepsis. In addition, citrus oils are included in “degreaser” preparations due to the solvent properties of the essential oil components and the fact that they can be used instead of chlorinated solvents. These products may also be referred to as terpene cleaner.


Tea Tree Oil

Tea tree oil is produced largely from Melaleuca alternifolia (Myrtaceae family). The oil contains monoterpenes and related alcohols, and the most abundant component is terpinen-4-ol, which comprises about 40% of the oil.35 The International Standard ISO 4730 stipulates acceptable ranges for 15 components,36 and oils must meet the compositional criteria in order to be legally sold as “tea tree oil.” Originally obtained from natural stands of trees, the oil is now produced from plantations of M alternifolia in several countries including Australia, Kenya, and China. The essential oil has historically been used as a topical antiseptic in Australia based on its documented broad-spectrum antimicrobial activity.37 Tea tree oil is used primarily in hand hygiene and skin antisepsis products and, to a lesser extent, in products for surface cleaning or disinfection.


Eucalyptus Oil

Eucalyptus oil is produced from a number of Australian Eucalyptus and Corymbia tree species and falls into two broad categories. Oil produced from Corymbia citriodora (formerly Eucalyptus citriodora, also known as lemonscented gum) is used largely in perfumery, whereas the oil used for medicinal or cleaning purposes is produced from a number of species including Eucalyptus globulus. This second oil type contains high levels of 1,8-cineole, also known as eucalyptol, and it is this oil type that is discussed further. The International Standard ISO 3065:2011 specifies that this type of eucalyptus oil must contain 80% to 85% 1,8-cineole.38

Eucalyptus oil in some instances was originally a by-product of timber production but is now a crop in its own right. The largest quantities of cineole-type eucalyptus oil are currently produced in China,31 with other countries including Australia, Spain, Portugal, and Southern Africa producing smaller but still substantial quantities.39 Eucalyptus oil was historically used in a manner similar to tea tree oil, which was as a general medicinal agent in the preantibiotic era. In terms of current commercial uses, it is used mainly in cleaning and disinfectant products and, to a lesser extent, as a medicinal or antiseptic agent.



Pine Oil

Pine oil is obtained by steam distillation from wood, stems, twigs, and leaves of several pine species, including Pinus sylvestris. It is not to be confused with turpentine oil, which is also derived from Pinus species but is distilled from resin harvested from live trees. Pine oils generally contain α-pinene, β-pinene, or α-terpineol as major components.40,41 However, because there are several different types of pine oil, the composition varies. An additional product, pine needle oil, is obtained from the leaves and cones of several Pinus species including P sylvestris.40 Pine oil has a long history as a component of disinfection and cleaning products, as discussed in the following text. Pine oil is not as commonly used now due to declining production and increased cost of the essential oil.29


Other Oils and Components

Essential oils, other than those described earlier, that have been identified in commercially available disinfectants include lavender oil, peppermint oil, lemon myrtle oil (from Backhousia citriodora), and lemongrass oil (from Cymbopogon citratus). Essential oil components have also been identified in disinfectants and antiseptics. These include thymol, a monoterpene and a component of thyme oil (Thymus vulgaris); carvacrol, which is a monoterpene component of both thyme and oregano oils; and eugenol, which is an allylbenzene and is a major component of clove oil. As previously mentioned, the components limonene (from citrus oils) and 1,8-cineole (from eucalyptus oils) are also present in many commercial products.


ANTISEPSIS


Hand Hygiene

Commercial hand hygiene products that contain essential oils or components are increasingly common. Essential oils have been included in both surfactant-based and foaming hand wash products and in leave-on alcohol-based hand rubs. In many instances, the essential oil may be present at relatively low concentrations and as such may not provide any significant antimicrobial activity. At these low concentrations, the essential oil is likely to have been included in the formulation as a fragrance or to increase the general appeal of the product to consumers rather than to have an antiseptic effect.

A limited number of clinical studies have been published that investigate the clinical efficacy of certain hand wash formulations containing essential oils or components.42,43,44,45 Of the four studies identified, two evaluated products containing tea tree oil, one investigated an oregano oil product, and the remaining study investigated a product containing farnesol.

The first of the tea tree oil hand hygiene studies assessed several different hand wash formulations, using volunteers and the hygienic hand wash standard protocol EN 1499.42 The “in-house” products evaluated included a skin wash containing 5% tea tree oil, a skin wash containing both 5% tea tree oil and 10% alcohol, a solution of 5% tea tree oil in water, and a soft soap comparator containing no tea tree oil. Results showed that the skin wash containing both tea tree oil and alcohol, as well as the 5% tea tree oil solution, performed significantly better than the soft soap in reducing numbers of Escherichia coli on hands. Results for the 5% tea tree oil skin wash were not significantly different from soft soap.42 This lack of antibacterial effect compared to soft soap may be due to insufficient contact time or partial inactivation of tea tree oil by other hand wash ingredients, as is mentioned in a subsequent section of this chapter. The second hand wash study evaluating a tea tree oil product, also using protocol based on EN 1499 on hands artificially contaminated with E coli, showed that two commercial hand wash products containing either 0.3% tea tree oil or 0.5% triclosan did not perform significantly better than non-medicated soft soap hand wash, whereas soft soap hand wash followed by the use of 60% propan-2-ol performed significantly better than the nonmedicated tea tree oil and triclosan products.45 A consideration for the outcomes of this last study is that the concentration of tea tree oil included in the commercial product is relatively low, and as such, it is unlikely to exert any profound antibacterial effect.

An experimental hand rub containing a combination of farnesol (a sesquiterpenoid, undisclosed concentration), benzethonium chloride, and polyhexamethylene biguanide was evaluated in human volunteers whose hands were artificially contaminated with Serratia marcescens using a method proposed by the US Food and Drug Administration Tentative Final Monograph protocol. Results showed a 3.2 log reduction in S marcescens count after 1 application of product and a 5.5 log reduction after 10 applications. The comparator commercial hand rub product resulted in log reductions of 3.6 after 1 application and 3.4 after 10 applications. The farnesol product also showed a residual antibacterial effect when tested on ex vivo porcine skin challenged with Staphylococcus aureus or E coli. However, the in vivo and ex vivo studies43,44 did not assess the activity of the experimental hand rub without farnesol, or with farnesol alone, making it difficult to assess the true contribution of farnesol to the final result.

Lastly, a study examining the normal flora of the hands found that significantly fewer bacteria were recovered from hands washed with soap containing 0.5% oregano
oil compared to washing with either water alone or with nonmedicated soap.46 Results obtained after washing with the 0.5% oregano oil soap did not differ significantly from the commercial antibacterial soap, which was Dettol® antibacterial cream soap,46 suggesting that the oregano oil was exerting an antibacterial effect similar to the commercial product.


General Skin Antisepsis

Essential oil products have the potential to be used for the reduction, or removal, of flora from body sites other than hands due to antimicrobial activity. Furthermore, several essential oils, including tea tree oil and eucalyptus oil, were historically used as topical antiseptic agents, indicating a historical precedent for this particular use.37 However, little published data are available on the use of essential oils as topical antiseptics. A number of topical antiseptic or wound care products containing oils such as thyme, lemongrass, basil and rosemary,47 E citriodora oil,48 and tea tree oil49 have been evaluated preclinically. No studies that evaluate products containing essential oils or components for pre- or postoperative infection prevention were identified, and no studies were identified evaluating the clinical effectiveness of topical antiseptic creams, gels, or ointments that contain essential oils. However, two studies evaluating essential oil products for the eradication of methicillin-resistant S aureus (MRSA) were identified, which both describe the use of tea tree oil.

A pilot study showed that 5 of 15 patients using 4% tea tree oil nasal ointment and 5% tea tree oil body wash were cleared of MRSA compared to 2 of 15 (13%) patients receiving standard therapy (2% mupirocin cream and triclosan body wash). However, due to low numbers of patients, the difference was not significant.50 A larger study51 showed that application of a 10% tea tree oil nasal ointment cleared 41% of patients of MRSA, but the standard treatment of 2% mupirocin cream was significantly more effective, with 78% of patients cleared. The use of body wash containing 5% tea tree oil or 4% chlorhexidine gluconate soap to eliminate MRSA carried at sites such as the axillae and groin was also evaluated. Clearance rates in the tea tree oil group were 57% and 80% for axilla and groin, respectively, whereas rates for axilla and groin were 50% and 29%, respectively, in the chlorhexidine group. Overall, the tea tree oil body wash product was considered superior to the chlorhexidine regimen for clearance of skin sites.51 A later study evaluating the use of a 5% tea tree oil body wash to prevent MRSA colonization did not find any difference in rates compared to standard body wash.52 This suggests that tea tree oil products may not actually prevent colonization but may assist in eradication if colonization has already occurred.

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May 9, 2021 | Posted by in MICROBIOLOGY | Comments Off on Essential Oils
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