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.

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

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.

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.