Long before the discovery of microorganisms, it was acknowledged that plants had certain therapeutic and healing properties especially the fact that they possessed antibacterial activity. This dates back to ancient times and they have even gained mention in religious scriptures and by prophets such as the prophet Muhammad (PBUH) who has mentioned that nigella sativa oil commonly known as black seed oil (an essential oil extracted from nigella seeds) has the cure for every illness except death (ref x) and thus certain oils have been used in the middles eastern subcontinent and generally worldwide for thousands of years.
Essential oils posses a distinct odour and are isolated from many different parts of the plant including the leaf, root, bark, flower, bud etc and are normally extracted by steam or hydro-distillation . These oils usually evaporate in the presence of air at moderate temperatures, thus explaining the use of the terms volatile or ertheral oils to describe such “essential” oils.
The term Essential was selected to describe these oils because they represent the “essence” i.e. the characteristic odour of the plant. (tanker et al 1990)
They have been around for thousands of years and have found a wide range of uses in society ranging from their use as aphrodisiacs to their use as an antibacterial. Traditionally plant extracts have been used in food preservation and are also widely used in the perfume industry. Interestingly plant extracts have also been used for the production of many drugs and
It has been documented by xxx that 98% of all drugs on the market are derived from plants (ref xx find) important examples would include digioxin from digitalis (use ref on desktop)
Recently, many novel drugs such as the anti-malarial drug artemisin have been extracted from the artermisia plant (ref x)
And plant extracts have also been used to create novel anti-cancer drugs such as vincristine and vinblastine from atropa belladonna. The potent opiod analgesics morphine has been extracted from the plant papaver somenfarum aswell. Recent research has shown potential for develeoping new HIV drugs from plants. (ref2)
Essential oils have been used for many years for their antiseptic i.e. antibacterial/antifungal properties and therapeutic properties aswell as their aromatic properties. Fast tracking to the present day their uses remain the same, however with technological advancements in analytical chemistry much more is now understood in relation to their mechanism of action, particularly at the antimicrobial level. (Ref 2)
The discovery of antibiotics such as penicillin from fungal sources and other more effective synthetic chemical drugs lead to a dramatic decline in the use of plant products for its antibiotic properties, but they are nevertheless still and sometimes exclusively used by some populations, furthermore there are many traditional herbalists who deal only in natural, herbal products. (ref xx)
We are now however witnessing a resurgence into the research of plant products as antimicrobials, as it is now widely understood that any antibiotic has a limited lifespan and due to the overreliance on such drugs and consequently over and unnecessary prescribing of antibiotics; antibiotic resistance has become a huge problem worldwide. There is a therefore a constant requirement to discover new antimicrobials to stop the spread of multi-drug resistant pathogens.
There has been a huge increase in to the research of traditional folk medicine in order to identify leads to develop novel therapeutic drugs more specifically antibiotics. Recent studies have suggested that numerous plant species have antimicrobial properties, and such plant based antibiotics have a huge therapeutic potential as they can help with the problem of antibiotic resistance and also exert their activity with fewer adverse effects that are often associated with synthetic antibiotics (ref 7)
Furthermore the interest in aromatherapy a branch of alternative medicine has made it a financially lucrative market for pharmaceutical companies to enter.
This rise in interest from pharmaceutical companies has had the effect of raising the profile of plant products as drugs as these companies pay particular attention to quality control, testing and are also subject to stringent regulations thus their safety is on par with conventional drugs.
The therapeutic properties of essential oils have been well documented; results have been varied, with toxicity and side effects being very frequently implicated, and thus a possible explanation for their decline in use (REF 1) however due to reasons mentioned above we are now experiencing a rise in their use.
The oils are extracted from various aromatic plants; in the main these plants are found in countries where the temperature is warm i.e. countries such as those in the Middle East where they form a major part of the traditional pharmacopeia. There are numerous methods of extraction, which may include the use of liquid carob dioxide or microwaves, or more commonly distillation with use of steam or boiling water. Different methods are employed depending on the nature of the oil being extracted and the species from which it is being extracted from. For example in the extraction of Citrus oil steam distillation is preferred. However in other cases such as for use in the perfume industry the oil is extracted with lipophillic solvents.
Depending on the method used for extraction, the chemical profile of the essential oil obtained will vary, and the method of extraction used will differ according to the purpose for which the essential oil will be used e.g. in food preservation or perfume etc. (Ref 2)
The quality, quantity and composition of the essential oil extracted can vary according to the climate, composition of the soil, age of the plant and also the part of the plant from which it is extracted. (ref 2 Masotti et al 2003) so therefore in order to obtain an oil with a consistent composition it is imperative that the oil is extracted under the same conditions from the same part of the plant, that has been grown in the same soil under the same temperature. The majority of the essential oils used in industry are analysed using a combination of gas chromatography and mass spectrometry allowing analytical monographs to be obtained.
Essential oils are naturally occurring complex mixtures which contain on average 100 different components at varying concentrations. (ref2) Plants synthesize a vast range of aromatic substances consisting of chemical compounds which have hydrogen, carbon and oxygen as a basic framework. These can then be divided in to two groups, the hydrocarbons which consist mainly of the terpenes and other oxygenated compounds which are effectively alcohols, aldehydes: esters, ketones, phenols and their oxygenated derivatives (ref 1 #76).
The advancement of technology has allowed identification of many different components of essential oils, but many more remain to be discovered, currently oils are analysed using Gas chromatography and mass spectrometry, this technique however is limited in its uses as it relies on interpretation from the operator and also the fact that some components are so minute that they do not even register on the gas chromatography detectors. These minority compounds can however, be identified via smelling by experienced perfumers.
Studies have found that the majority of essential oils contain 2 or 3 major components that make up the bulk of the oil; with the other components being present in trace amounts. ( ref 2) For example geraniol (70%) and geranyl acetate (15%) have been found to be the main components of palmarosa essential oil and citronelleol (xx%) and geraniol are the major components of geranium essential oil.(guenther etal 1982). It is usually these major components that are responsible for the biological properties of these oils.
Compounds associated with antimicrobial properties
The components that are associated with the antibacterial properties of these oils can be divided in to numerous categories and will be described below.
Phenolics and Polyphenols
Phenolics also known as phenols, consists of a hydroxyl group that is directly bonded to an aromatic hydrocarbon group. Phenol being the simplest compound of this class. (insert ref) They have documented antimicrobial properties and are used as disinfectants.(Refere) . Some compounds contain 2 hydroxyl groups e.g. Catechol and some compounds even posses 3 OH groups such as pyrogallol. It is understood that the position and number of OH groups on the phenol group is associated with the compounds toxicity to microorganisms and therefore increasing the number of OH groups will give rise to an increase in toxicity. (ref 1 look no 76)
The antimicrobial activity of phenols against microorganisms includes mainly; the inhibition of enzymes by the oxidized compounds either by a nonspecific reaction with proteins or through the interaction of the oxidized species with sulfhydryl groups. (ref 1 no 137)
Eugenol is an example of a phenolic compound found in some essential oils that posses antibacterial properties, it is compound that is commonly found in clove oil. It has been discovered that eugenol inhibits the growth of both fungi and bacteria (ref 1 58+224)
Quinones are a class of organic compounds; and consist of an aromatic ring with a ketone group substituted at 2 different positions. Quinones are regularly used in the dye industry and are also found in henna which gives henna its dyeing properties. Their toxic properties arise from the fact that quinones provide a large supply of free radicals and can thus react with amino acids present in proteins that have a lone pair of electrons. (ref 1 no 210) this reaction would normally cause the inactivation of the protein therefore resulting in cell death.
There are many targets for quinines in bacterial cells such as the cell wall, cell membrane and can also inhibit enzymes. St Johns wart, typically used for its anti-depressant and mood lifting properties contains Hypericin which is an example of a quinone; more specifically an anthraquinone, moreover it has also been reported that Hypericin has antibacterial activity. (ref 1 no 58)
Flavanoids are phenolic compounds, produced by plants as secondary metabolites to protect from attacks by insects or microorganisms. Flavanols are also potent anti-oxidants and are found in many dietary foods, used by humans to protect from certain conditions such as cancer, heart disease and stroke. They posses antimicrobial activity and this could be due to their ability to interact with proteins and other bacterial cell components such as the cell wall etc. Dietary sources that provide a rich source of flavanoids include: citrus fruits, tea, wine and dark chocolate. Catechin is found in large proportions in green teas and also in oolong tea and it has been shown to have antibacterial activity (ref 1 no 227)
The term “tannin” is used to describe any large polymeric phenolic compound, with differing molecular weights typically ranging from 500 to 3000 (ref 1 no 87) These “tannins” are found in many different parts of the plant and they protect these plants from invading microorganism and insects. (ref 1 no 192). They have astringent properties and are also used in the process of tanning leather; these astringent properties have been used for centuries and evidence suggest consuming large amount of beverages such as tea especially green teas and wines can help treat and even prevent certain ailments. Their mechanism of action is similar to that of quinones described above, as they also interact with proteins in a nonspecific manner causing the inactivation of these proteins and hence cell death. (ref 1 no 87 +210)
Coumarins are again phenolic substances made up of benzopyrene i.e. composed of “fused benzene and alpha-pyrone rings” (ref 1 no 161). They are produced by many different species of plants notably being found in high concentrations in the tonka bean. Medically they are extensively used for their anti-coagulant properties and drugs such as warfarin are given to cardiac patients regularly. (ref 1 no 223) they have also been shown to posses anti inflammatory and vasodilatory properties (ref 1 no 177,152).
Comuarains are also used as rodenticides where they antagonise the Vitmain K cycle. Vitamin K is involved in the production of key clotting factors, thus inhibiting the vitamin K cycle causes death of the rodent by severe haemorrhage. Coumarins are therefore used with caution in humans and patients receiving anti-coagluant therapy i.e. patients on warfarin require close monitoring.
More importantly there are antibacterial effects associated with coumarins, and it has been shown in studies that Hydroxycinnamic acids which are similar to coumarins have inhibitory effects against gram-positive bacteria (ref 1 no 66). Furthermore Comuarins have been shown to stimulate macrophages and thus can have the effect of combating infections or prevent the manifestation of infection and disease (ref 1 no 37). To summarise, the antibacterial properties of coumarins do show some promise, but nevertheless further research in to coumarins and their antimicrobial properties is necessary before they can be seen as a real alternative to conventional antibiotics.
Terpenes & Terpeniods
Terpenes are produced by plants as secondary metabolites and are widely used in the perfume industry, as they have a pleasant fragrance. They are hydrocarbons derived from isoprene which is a 5-carbon-base unit with a molecular formula of C5H8. They form compounds of differing structures and function (ref 2). The simplest of terpenes is hemiterpene and this consists of a single isoprene unit. Isoprene units can however, be combined to form monoterpenes (C10H16), sesquiterpenes (C15H24), diterpenes (C20H32), triterpenes(C30H48) and tetraterpenes (C40H64). (ref 1,2).
The monoterpenes i.e those terpenes formed when two isoprene units combine (C10H16) is the most frequently occurring terpene, constituting approximately 90% of all the essential oils. (ref2)
Terpenoids are very similar to terpenes , and occur when oxygen is either added or a methyl group is removed from a terpene.(ref 1+2) They are also known as isoprenoids or terpenenes, and the nomenclature of terpenoids is also similar to terpenes e.g. hemiterpenoid, monoterpenoid, sesquiterpenoids etc. Common terpenoids include menthol and camphor (ref 1)
Several studies indicate that the terpenoids have antimicrobial (ref 1 no 144.196.221) , antifungal (ref 1 no 122,179,180) and also anti-protozoal activity (ref 1 no 78,237). Interestingly studies have reported that the majority of essential oils were active against fungi, with a relatively much smaller proportion of oils tested had activity against bacteria. (ref 1 no 39). There mechanism of action is again unclear and it is suggested that they interacts with proteins in a non-specific manner.
Alkaloids are heterocyclic compounds containing nitrogen. Morphine isolated from the opium poppy, papver somniferum and its derivative codeine is a common example of an alkaloid being used for its therapeutic properties (ref 1 no 690)
There a certain groups of alkaloids that have been found to posses antimicrobial activity. These are the diterpenoid alkaloids; Japodagrin and japodagrone are examples and are extracted from the root of Jatropha podagrica Hook. (ref 9) and they have shown to have activity against gram positive bacteria. Diterpenoid alkaloids that have been extracted from the buttercup plant also have antibacterial properties (ref 1 no 163). It has also been documented that certain other groups of alkaloids posses activity against the HIV virus (ref 1 200), and also against trypanosomes (ref 1 no 73) and plasmodia (ref1 163). The activity is assumed to be due to the ability of the alkaloid to intercalate with DNA. (ref1 no176)
Lectins and Polypeptides
Polypeptides have documented antibacterial activity, studies have found that peptides that are active against bacteria are often positively charged and also contain a disulfide bond (ref 1 no 19,253). This can also explain the mechanism of action of peptides against microorganism as it is understood that they disrupt the membrane by forming ion channels in the membrane. (ref 1 no 222,253).
It can thus be understood from the above that plant products have many different components, and have a wide range of activities. The above mentioned components are found to be present in essential oils at varying concentrations, and provide activities against bacteria, viruses, fungi etc
The primary reason for choosing to study geranium oil is the fact that they has not been much research carried out on this particular oil, also the fact that this oil contains reasonable concentrations of terpeniods which generally posses antimicrobial properties. Furthermore recent studies have shown that geranium oil has showed promising antibacterial acivity against P-aerugios.