History

Essential oil-containing aromatic plants have been used for anti-infectious purposes for millennia. Sewage, rotting garbage, sick people, environmental pollution, and other sources of unpleasant smells reveal the presence of proliferating microbial toxins. Without knowing the details of what pathogenic agents were present, people understood that where there were bad vapors, diseases lurked. Aromatic plants have been the primary antidote for these ‘evil spirits.’ Traditional medical systems, such as Ayurveda, had a general concept of microscopic pathogens and knew that substances such as essential oils counteracted those toxins.

Historically, people have known that essential oils had many uses, including medicines and preservatives of foods. The medicinal powers of essential oils have been utilized in various forms for millennia, such as unguents, lotions, perfumes, perfumed waters, fragrant baths and massage, incense, and innumerable other preparations. Essential oil preparations were highly esteemed by the ancient physicians. Essential oil-containing spices were one of the primary methods of food preservation; the search for those spices led to the discovery of the New World.

The anti-microbial actions of essential oils are one of the most extensively studied aspects of botanical medicine. Research into the antiseptic properties of essential oils has been going on since the 1880’s, starting with oils such as oregano, cinnamon, and clove. By the 1930’s a considerable amount of conclusive studies had been amassed, including proof that essential oils used in perfumes had antibiotic powers, but these were eclipsed by the discovery of penicillin and the emergence of antibiotic drugs. The origins of the current aromatherapy movement can be credited largely to the research and work of people such as Dr. Jean Valnet in the 1960’s.

Essential Oil Research

Thousands of studies on the anti-microbial effects of essential oils are available online in hundreds of databases and journals. Research has been conducted on a vast array of essential oils and essential oil constituents from a multitude of species, varieties and chemotypes of aromatic plants, including details of their geographic origins, harvesting and extraction, analytical methods, and comparative results.

Many of these studies have been performed on aromatic plants with a known history of use in ethnobotanical medicine. In these cases, testing the plant’s oil components against microbial strains frequently confirms what local practitioners have known. In some cases new information emerges which might improve the use of the plant. For example, the entire aerial portion of a species may be used traditionally for anti-infectious purposes, but testing will reveal that the strongest concentrations of effective compounds are in the leaves rather than the flowers, or that the flowers contain properties that are best used in other ways.

Frequently, the oils used in research are distilled in small quantities for the purpose of the study only, and are therefore not found on the market. Bringing these oils into commerce would expand the range of natural medicines available to practitioners, create new products, and give farmers new sources of income.

The body of research includes not only examination of the antibiotic, antiviral, and anti-fungal properties of the oils, but other important functions including anti-inflammatory, immune-stimulating, hormone-regulating, anti-parasitic, and anti-cancer powers. Furthermore, the research is not limited to human health and medical applications, but extends to other important fields including veterinary medicine, food preservation, natural flavoring and fragrancing, industrial applications, and agricultural uses such as pesticides and fertilizers.

Even a brief perusal of the available literature reveals that essential oils represent a vast and underutilized botanical resource, a non-toxic and ecologically sustainable industry capable of replacing a high percentage of the toxic fossil fuel-derived chemical compounds now routinely used in almost every consumer product. Essential oils are steadily moving to the next level of practical application in a new generation of non-toxic products; this will reduce the toxic biological burden at the root of countless diseases and hasten the transition from unsustainable fossil fuel-based economies and industries to sustainable plant-based economies and industries. This is the true potential of botanical medicines: agents of healing on all levels, including ecological, agricultural, industrial, economic, and medical.

In the field of natural medicine, essential oils are important anti-infectious and anti-microbial agents, whose importance grows as microbial resistance to antibiotic and antiviral drugs increases. Research papers frequently conclude with the observation that these oils are promising alternatives for standard anti-microbial drugs. The challenge in applying this information is making the transition from in-vitro studies on pure microbial strains grown in petri dishes to the realities of human physiology.

Because of their potency and documented pharmaceutical efficacy, essential oils represent an important interface between allopathic and herbal systems of medicine; as their antibiotic powers are recognized they will first replace routine prescriptions for easily treatable conditions, and will later be applied to more serious conditions.

Testing Oils for Antimicrobial Properties

Essential oils are being tested in-vitro, in-vivo in animals, and in human clinical trials.

In-vitro testing of essential oils against microbial strains generally proceeds in three stages. The first step is distillation of the oil from a specific species of plant. The second is analysis of the chemical constituents of the oil, done through gas chromatography. The third is in-vitro testing of the oil or its primary compounds against various strains of microbes.

The most commonly used in-vitro method for determining which pathogens are susceptible to which oils or oil constituent is the agar diffusion technique; this is the same method used to determine the bactericidal activity of antibiotic drugs. The procedure begins by inoculating a standardized microbial strain on agar medium. A series of sterile paper disks are saturated with different oils at different concentrations, and placed over the culture; alternately, the oil is dropped directly into holes in the medium. After a latency period the inhibition halo around the disks is measured, and the anti-microbial activity of the oil is rated according to its size and essential oil concentration.

Although the agar diffusion technique is the most commonly used, it is not necessarily the most accurate, as the essential oils have volatile as well as aqueous properties. Newer methods are now being employed to test the anti-microbial effects of essential oil vapors, rather than direct aqueous contact on disks. Results show that the concentrations of oils needed in the vapor state are far less than when applied in agar medium, thus confirming that not only are the oils highly anti-microbial, but that they are also more potent than originally thought.

The Role of Essential Oil Production in Plant Physiology

Traditional medical systems such as Ayurveda and Chinese medicine are fundamentally systems of “eco-physiology,” which describe the functioning of the human body using terms and concepts derived from observing the elements and energetic patterns of planetary biospheric physiology. If students contemplate these principles deeply, they begin to develop a kind of “macro-thinking” that reveals not just the basic elemental correspondences taught in acupuncture colleges, but vast patterns of interrelationships between living beings and the underlying commonalities of biological functions. When this type of synthetic and integrative thinking is combined with an understanding, even rudimentary, of botany, physiology, and chemistry, a truly holistic vision of life emerges. A holistic vision of life awakens a sense of reverence for the intelligence operating within every aspect of nature, and this awakening in turn is the foundation of spiritual wisdom.

In the field of essential oil chemistry, numerous parallels and examples of biological unity can be discovered using this macro-thinking at the intuitive level, which reveal why and how essential oils work. The lungs, for example, have a similar structure to trees: the trachea is the trunk, the bronchi are the large branches, bronchioles are smaller branches, and alveoli are the leaves. Likewise, the majority of essential oils used for treating upper respiratory conditions and the mucous membrane level of the lungs are derived from the leaves of trees, such as eucalyptus and ravensare, or from needles of conifers such as pine, spruce, and fir. In Chinese medical terms, these oils are specifically for Wind Cold and Wind Heat, i.e. airborne pathogens; likewise, the oils produced within these leaves and needles are released directly into the air.

The immunological functions of essential oils within plants are also directly related to their effects on human immunity. Essential oils are secondary metabolic byproducts which serve several physiological purposes, including anti-bacterial, anti-vital, and wound healing. The molecules within an essential oil can be thought of as the expression of the plant’s immunological intelligence; when we utilize essential oils we are using botanical immuno-chemical intelligence to repel and destroy pathogens common to both plants and humans, and to activate healing processes that are likewise similar in both. It is interesting to note that most aromatic plants are not vulnerable to common pathogens and pests that affect non-aromatic plants; likewise, those who have worked with essential oils during times of epidemics, such as distillers and professional perfumers, have a historical reputation of being less vulnerable to contagious illnesses.

Modes of Antimicrobial Actions

Different molecular compounds work differently against different microbes. One of the major models of anti-microbial action that has been confirmed is cellular membrane toxicity caused by monoterpenoid components.

Although essential oils are complex mixtures, research suggests that the monoterpenes, being lipophilic, diffuse into cell membranes and cause them to expand, thereby increasing their fluidity, disordering membrane structures, and inhibiting membrane-embedded enzymes. Studies on the effects of essential oils such as oregano, ravensare, and tea tree show that they cause rapid cellular damage to bacteria. By inhibiting the enzymes and cofactors involved in the respiratory electron transport chain spanning the cytoplasmic membranes of bacteria and mitochondrial membranes of yeast, the monoterpenes cause inhibition of respiratory oxygen of microbes. Even slight changes to the structural integrity of cell membranes can detrimentally affect cellular metabolism. In the case of monoterpene toxicity, potassium ions are lost, which disrupts ionic homeostasis and disturbs chemiosmotic control of energy-dependent processes such as metabolism and motility.

Antibiotic drugs interrupt specific metabolic pathways, such as the formation of a particular protein used to build a cell membrane; bacteria can learn how to resist this specific disrupting influence within ten days of being exposed to the drug. The biochemical action of essential oils prevents this from happening; by blocking the entire cellular respiratory function, bacteria are simply suffocated. Some of the strongest anti-microbial compounds, such as the phenols thymol and carvacrol, can completely block oxygen intake in cell membranes.

Essential oils not only neutralize pathogenic germs, but also help to restore and correct the underlying humoral terrain, such as expectoration from congested mucous membranes, as well as enhancing and stimulating the immune system. Used properly, essential oils do not harm or disrupt beneficial intestinal flora. They are effective against bacteria, viruses, parasites, fungus, and yeast. Because of their wide-spectrum action against pathogens and immune enhancing functions, essential oils are an increasingly important alternative to antibiotic therapy.

Antimicrobial Compounds in Essential Oils

For safe and effective therapeutic use of essential oils it is important to have a basic knowledge of their constituents. Essential oils usually contain several major compounds and many more minor compounds, sometimes numbering hundreds. Researchers have identified many specific compounds responsible for the anti-microbial powers of oils; however, like other herbal preparations and phyto-medicines, the effect of synergies is probably more biocompatible and therapeutically balanced than using an isolated active ingredient.

The chemical compounds in essential oils fall into two primary groups: hydrocarbons, which are mainly terpenes (monoterpenes, diterpenes, and sesquiterpenes), and oxygenated compounds which are phenols, terpene alcohols, esters, ethers, aldehydes, ketones, and oxides. While some compounds are distinctly more anti-microbial than others, there are many adjunctive uses for the less potent anti-microbial oils.

Terpenes

Terpenes, which include monoterpenes, sequiterpenes, and diterpenes, comprise a large group of molecules found in some form in almost all essential oils, with a wide range of therapeutic functions.

Different monoterpenes have anti-inflammatory, antiseptic, antiviral and antibacterial properties; some are stimulating with a tonic effect; others are expectorant and mucous membrane stimulants and decongestants; others are atmospheric antiseptic agents. Limonene, found in most citrus oils, has expectorant and antiviral actions. Pinene, found in pine, cypress, and tea tree oils and cymene, found in thyme oil, have powerful antiseptic actions. Other monoterpenes include camphene (firs, lemongrass, sweet fennel, nutmeg, cypress, valerian); sabinene (juniper berry, cedarwood, rose geranium); and cadinine (hyssop, myrrh, tea tree).

Sesquiterpenes have outstanding anti-inflammatory properties which can be used in conjunction with stronger anti-microbials or for chronic inflammation following infections. The sequiterpene group contains chamazulene (chamomiles, blue yarrow), farnesol (chamomiles, rose), valeranon (valerian), and santalol (sandalwood). Jatamansi (Himalayan spikenard) is nearly one hundred percent sesquiterpenes.

Diterpenes are rarely found in essential oils, as they are less volatile and distill only in minute amounts.

Terpene Alcohols

Terpene alcohols as a group are among the most beneficial and versatile of the essential oil compounds. They have broad spectrum anti-infectious, antibacterial, antifungal, and antiviral properties; they also possess uplifting and energizing stimulant and tonic properties. In general they are nontoxic, non-irritating, and relatively safe.

Two of the more common terpenic alcohols found in anti-microbial oils are terpinen-4-ol in Melaleuca alternifolia (tea tree) and Origanum majorana (sweet marjoram), and alpha terpineol, found in Ravensara aromatica (ravensare) and Eucalyptus radiata. Other important terpenic alcohols with anti-microbial actions are linalool, found in Coriandrum sativum (coriander), thyme (linalool chemo-type), and different species of lavender; geraniol, found in Cymbopogon martinii (palmarosa) and thyme (geraniol chemo-type); thujanol, found in thyme (thujanol chemo-type) and Origanum majorana (sweet marjoram); and menthol, found in Mentha piperita (peppermint) and Mentha arvensis (field mint).

Phenols

Phenols are among the most potent of the anti-microbial compounds. Phenols have powerful broad spectrum anti-infectious and antibacterial functions, are antiseptic and disinfectant, and have strong anti-parasitic properties. They have moderately strong tonic, stimulant, anti-viral, anti-fungal, and immune enhancing properties.

Although they have excellent antiseptic properties, phenols are skin and mucous membrane irritants which can be caustic, especially when used neat. Oils high in phenols should be used in low concentrations and for short periods of time, after which they should be replaced by others that are less potentially toxic.

Three of the most important phenols from essential oils are thymol, carvacrol, and eugenol. Thymol is found in high concentration in oils such as Thymus vulgaris (thyme) and Trachyspermum ammi (ajowan). Carvacrol is found in oils such as Origanum compactum (oregano), Origanum heracleoticum (Greek oregano), Corydothymus capitatus (Spanish oregano), Satureja montana (savory), and Thymus serpyllum (wild thyme). Eugenol is found in oils such as Eugenia caryophyllus (clove), Cinnamomum verum (Ceylon cinnamon leaf), and Ocimum gratissimum (basil eugenol chemo-type).

Aldehydes

Monoterpene aldehydes are found primarily in the lemon-scented oils. This group contains oils that have anti-inflammatory, anti-infectious, anti-fungal, anti-bacterial, and disinfectant powers. These oils must be used with caution as they can cause skin irritation.

Monoterpene aldehydes include citral (bergamot, lemon, lime, lemongrass, melissa), geranial (petitgrain, orange, lemon, mellisa), neral (verbena, lemongrass, lemon), and citronellal (citronella, grapefruit, rose, melissa).

Aromatic aldehydes are among the most powerful broad-spectrum anti-infectious and antibacterial compounds found in essential oils. They have moderately powerful antiviral, anti-fungal, and anti-parasitic functions, and moderately strong functions as immune system stimulants and general tonics. They are dermo-caustic and must be diluted appropriately.

Cinnamic aldehyde is an aromatic aldehyde with potent anti-microbial power. It is found in oils from Cinnamomum verum or zeylandicum (Ceylon cinnamon bark), Cinnamomum cassia (Chinese cinnamon bark), and Cinnamomum loureirii (Vietnamese cinnamon bark).

Other aromatic aldehydes are cuminal, found in Cuminum cyminum (cumin), and phellandral, found in Eucalyptus polybractea (blue mallee eucalyptus).

Ketones

Ketones are some of the most toxic of the compounds found in essential oils. However, some ketone-containing oils have excellent therapeutic value, although they are not generally considered strongly anti-microbial. Some oils containing ketones aid in wound healing and dissolving mucus, some are immune system stimulants, and some are anti-fungal. They can be used effectively in conjunction with stronger anti-microbial oils. Oils such as hyssop, eucalyptus and rosemary have moderate amounts of ketones; peppermint, spearmint, and rose geranium, which contain menthone, can be very beneficial when used properly.

Esters

Esters are not major anti-microbials, but they can be used in conjunction with stronger anti-microbial oils. Esters found in essential oils are normally very fragrant with a fruity aroma. Their therapeutic effects are balancing to the nervous system, calming, anti-inflammatory, and antispasmodic. An example of a well-known ester is linalyl acetate, which is found in lavender, clary sage, and petitgrain. Some esters also have anti-fungal and anti-microbial properties: geranium oil, which contains geranyl acetate, and helichrysum, which contains neryl acetate, possess anti-fungal properties; lemongrass oil, which contains geranyl acetate and linalyl acetate, has been found to be bactericidal against Helicobacter pylori (see below). These components are normally gentle in their actions and can be used with a wide safety margin.

Oxides

Oils containing oxides are generally camphoraceous in nature. As a group they are considered to have only mild anti-infectious effects, but they have excellent expectorant properties that can be used in conjunction with other oils. One of the most well-known of the non-toxic oxides is cineol (eucalyptol), which is found in eucalyptus and rosemary oils; these oils combine well with phenol-rich oils such as thyme and oregano for treatment of respiratory viral and bacterial infections.

In-vitro, In-vivo, and Clinical Testing

An extensive amount of documentation exists for in-vitro, in-vivo animal testing, and human clinical trials using essential oils. Below is a brief sample of some of these trials, to give a general overview of the range of possibilities that essential oils have for future clinical practice, including antibacterial, antiviral, and-fungal, and acaricidal.

Herpes Simplex

Essential oil of Melissa officinalis (lemon balm) was found to inhibit Herpes simplex virus type 2 (HSV-2), indicating that the oil contains an anti-HSV-2 substance.

(Antiviral activity of the volatile oils of Melissa officinalis L. against Herpes simplex virus type-2.; Allahverdiyev A, Duran N, Ozguven M, Koltas S.; Tropical Diseases Center, Faculty of Medicine, Cukurova University, Adana, Turkey.)

UTI

The antibacterial activity of essential oils extracted from Ocimum gratissimum (basil eugenol chemo-type), Cymbopogum citratus (lemon grass), and Salvia officinalis (sage) was assessed on bacterial strains derived from 100 urine samples. Salvia officinalis showed enhanced inhibitory activity, with 100 percent efficiency against Klebsiella and Enterobacter species, 96 percent against Escherichia coli, 83 percent against Proteus mirabilis, and 75 percent against Morganella morganii.

(Antibacterial activity of essential oils on microorganisms isolated from urinary tract infection; Rogério Santos Pereira; Tânia Cristina Sumita; Marcos Roberto Furlan; Antonio Olavo Cardoso Jorge; Mariko Ueno; Universidade de Taubaté. Taubaté, SP, Brasi)

Helicobacter pylori

Japanese researchers found that Cymbopogon citratus (lemongrass) and Lippia citriodora (lemon verbena) oils were bactericidal against Helicobacter pylori at 0.01 percent. In in-vivo studies, the density of H. pylori in the stomach of mice treated with lemongrass was significantly reduced compared with untreated mice. Resistance to lemongrass did not develop, whereas resistance to clarithromycin developed under the same conditions. The researchers concluded that the essential oils are bactericidal against H. pylori without the development of acquired resistance, and since resistance to antibiotics is emerging, that these essential oils may have potential as new and safe agents for inclusion in anti-H. pylori regimens.

(Antimicrobial activity of essential oils against Helicobacter pylori.; Ohno T, Kita M, Yamaoka Y, Imamura S, Yamamoto T, Mitsufuji S, Kodama T, Kashima K, Imanishi J.; Third Department of Internal Medicine, Kyoto Prefectural University of Medicine, Kyoto, 602-8566, Japan.)

Hepatitis

Fifty patients with chronic hepatitis C and ten with chronic hepatitis B were treated with essential oils such as ravensare, thyme, laurel, niaouli, and helichrysum, either in monotherapy or combined with standard allopathic drugs (interferon for hepatitis C). In patients with HCV treated with bitherapy and essential oils, tolerance and response to treatment was improved (80 percent good tolerance and 100 percent complete response especially for genotype 1). For patients with HCV treated with monotherapy (essential oils only), an improvement in hepatitis was noted in 64 percent of cases. For HBV, two cures were obtained with essential oils in monotherapy.

(The role of aromatherapy in the treatment of viral hepatitis; A.M. Giraud-Robert International Journal of Aromatherapy; Volume 15, Issue 4, 2005, Pages 183-192)

Oregano Oil Effective Again Shigella Dysentery

The results of this test showed that origanum volatile oil has obvious protective effect on mice infected with two strains of Shigella, and that it had germistatic and germicidal effects on dysentery bacteria. The researchers concluded that Origanum volatile oil is an effective medicine against the infection of dysentery bacteria.

(Experimental study on the antibacterial effect of origanum volatile oil on dysentery bacilli in vivo and in vitro; Liao F, Huang Q, Yang Z, Xu H, Gao Q.; Department of Microbiology, Tongji Medical College, Huazhong University of Science and Technology, Wuhan 430030, China.)

Anti-Fungal (Tinea / Ringworm)

Japanese researchers conducing both in-vitro and in vivo experiments found that the essential oils of cinnamon bark, lemongrass, thyme, lavender, tea tree, and citronella oils (in increasing effectiveness) had potent anti-Trichophyton actions by vapor contact.

(In-vitro and in-vivo anti-Trichophyton activity of essential oils by vapour contact.; Inouye S, Uchida K, Yamaguchi H.; Teikyo University Institute of Medical Mycology, 359 Otsuka, Hachioji, Tokyo 192-03, Japan.)

Malodor of Tumor Necrosis

Oils of eucalyptus and tea tree have been found to be highly effective in removing the malodor of necrotic tumors in cancer patients, thereby improving the quality of life. Necrotic neoplastic ulcers are usually superinfected with anaerobic bacteria such as Bacteroides, Enterobacter, or Escherichia coli species. Additionally, these oils promote ulcer healing and re-epithelization. Adverse effects are uncommon and are usually limited to minor irritation at the time of application; the beneficial effects, however, have been quite pronounced.

(Antibacterial Essential Oils Reduce Tumor Smell and Inflammation in Cancer Patients Journal of Clinical Oncology, Vol 23, No 7 (March 1), 2005: pp. 1588-a-1589)

Scabies

The acaricidal activity of Melaleuca alternifolia (tea tree) oil and some of its individual active components were tested on the itch mite Sarcoptes scabiei var hominis. A five per cent concentration of the oil was highly effective in reducing mite survival times, both in vitro and in vivo. The researchers suggest that because of increased resistance against anti-ectoparasitic compounds, tea tree oil has a potential role as a new topical acaricide, and confirm terpinen-4-ol as the primary active component.

(Acaricidal activity of Melaleuca alternifolia (tea tree) oil: in vitro sensitivity of sarcoptes scabiei var hominis to terpinen-4-ol.; Walton SF, McKinnon M, Pizzutto S, Dougall A, Williams E, Currie BJ.; Menzies School of Health Research, and Northern Territory Clinical School, Flinders University, Darwin, Australia.)

References

Determining the Antimicrobial Actions of Tea Tree Oil; Sean D. Cox, Cindy M. Mann, Julie L. Markham, John E. Gustafson, John R. Warmington and S. Grant Wyllie

Antiviral and Antimicrobial Properties of Essential Oils; Dominique Baudoux

General Antimicrobial Effect

Bioactivity of essential oils of selected temperate aromatic plants: antibacterial, antioxidant, antiinflammatory and other related pharmacological activities; Katya P Svoboda and Janice B Hampson; Plant Biology Department, SAC Auchincruive, Scotland, UK.

Terpenoids and Their Effects on Conifer Insects; Linda A. Mahaffey, Colorado State University; BI570 Spring 2004

Antimicrobial activity of the essential oil of Cestrum; Diurnum; Bhattacharjee I., Ghosh A. and Chandra G.; Mosquito Research Unit, Department of Zoology, The Burdwan University, India

Essential Oils Gain Credibility in the War on Pathogens; Marilyn Vail;

Essential Oils - Nature's Powerful Anti-Viral Weapons; Melodie Kantner