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The Antibacterial Action of Eugenol, Thyme Oil, and Related Essential Oils Used in Dentistry
College of Dentistry New York University New York, New York The worldwide overuse of antibiotics has caused microorganisms to develop resistance to current antibiotics and to become virulent. Microorganisms, however, do not appear to develop tolerance or resistance to the antibacterial effects of essential oils such as clove oil (eugenol) and thyme oil (thymol). Such oils provide an effective, powerful, and cost-effective means of infection control in the dental practice. This article discusses the antibacterial action of eugenol, thyme oil, and related essential oils used in clinical dentistry. Introduction Essential oils have been used since the 19th century for root canal therapy, temporary fillings, and periodontal therapy. Thyme oil is the most germicidal essential oil Thymol, a component of thyme oil, has been used in several medicaments. During the first half of the 20th century, thymol was used in alcohol solutions for cavity sterilization, but alcohol decreased the effectiveness of Essential! Oils. Instead of eliminating the alcohol from the solution, practitioners mistakenly discontinued using thymol in dental procedures. Dental cements. bases, liners, and impression pastes that are based on the reaction between zinc oxide and eugenol [which forms zinc oxide-eugenol or ZOE) are antibacterial because eugenol is slowly released from the set mixture.' ZOE is biocompatible with the pulp and provides valuable anodyne and abundant effects as a temporary and permanent luting agent. It is an excellent thermal insulating base and sealer against micro leakage. ZOE is the material of choice for the indirect pulp capping of deep cavities in restorative dentistry because it is palliative and germicidal. Eugenol is the main component [800/0) of clove oil, which is obtained by pressing or distilling the buds, leaves, and stems of the evergreen tree Eugenia caryophyllata. Clove oil is sometimes preferred over eugenol in impression pastes because it is less irritating to the oral tissues. The History of Essential Oils Cloves (from the French word dou, meaning "small nail") are a tropical fruit t grown on the Spice Islands of Madagascar, Zanzi bar. Cloves Ware unknowvn in ancient Egypt, Rome, and Greece, but were used by the emperor of China in 200 BC as a mouth perfume. The first medicinal text reference to cloves was by AI-Gazzar in the 10th century, when they were used to control mouth odors and pain. Eugenol was the first essential oil proved to be a significant germicide and sedative for dental use. This was discovered in 1837 b' Bonastre, who used a combination of magnesium oxide and eugenol as a dental filling material. In 1876, Chisholm improved the formula by substituting zinc oxide for oxide. He was the first to develop (recommend ZOE as a temporary. Cast restorations were perfected within as (by Taggart in 1907, and this development led to the use of ZOE cements that possessed a thin film And that were biocompatible and suitable as temporary, intermediate, and permanent restorative cements. Most of the dental filling available in the late 19th and early 20th centuries were based on the eschaotic principle that local remedies had to destroy soft tissue on contact to be antiseptics. Such products included phenol (carbolic Reid), cresols, formaldehyde, and silver lensotrate. These substances were blended with zinc oxide and used as cements. Phenol was used in dentistry in the late 19th and early 20th centuries as an antiseptic for cavity sterilization and as a root canal paste when mixed with zinc oxide, A germicidal standard of reference is the phenol coefficient, which compares the effectiveness of germicides with that of phenol. a coal tar distillation product. The germicidal action of the phenol compounds is probably due to the destruction of the cell membrane. Phenol activity is increased by the substitution in the ring, as shown in the topical antiseptic phenolic compounds used as hand cleaners such as chloroxylenol, hexachlorophene, and triclosan. In 1905, Long' reported that phenols were used more frequently than any other drugs in dentistry as an antiseptic and analgesic. This essential oil group was antiseptic, analgesic, abundant, well penetrating, and comparatively nonirritating. It also acted as a disinfectant in root canals and carious cavities and did not destroy tissues. Essential oils are valuable constituents of many dental mecli-caments. The essential oils are fragrant, volatile essences that evaporate completely and leave only temporary stains. Thyme has been used in medicine since ancient times, and was Isolated by Neuman in 1719. Thyme oil, which is derived from the aromatic mint shrub Thymus vulgarjs, yields thymol (powder) and carvacrol (pH 5.3) as its major active ingredients. Thymol and carvacrol also can be obtained from caraway, marjoram, oregano (origanum), rosemary, and savory plants. Both thymol and carvacrol were used by dentists in the 19th century. In 1885, Gorgas used thymol and carvacrol to treat supurating pulps, alveolar abscesses, stomatitis, and odontalgia. In 1892, Clifford described the use of thymol in root canal therapy when combined with iodine in a product called Arjstol. In 1902, Coleman recommended thymol as an antiseptic for endodontics and reported that it had the highest antiseptic power of all essential oils). Rodney Price, chairman of the research section of the American Dental Association in 1923, recommended thymol for sterilization in root canal therapy. In 1938, the Accepted Dental Remedies of the ADA stated that the antiseptic efficiency of thymol was higher than that of phenol as a canal filling material and for direct pulp capping. In 1944, thymol solution in direct and indirect pulp capping procedures. At seven years, the caries in each patient had been arrested; the cavities were completely sterile; and the pulp had remained. Although thymol was 24 times more active than phenol, its insolubility in water and its relatively moderate germicidal strength in a 50% ethyl alcohol solution caused it to be discontinued as a dental therapeutic. Thymol was also used in a solution of one part thymol, one part ethyl alcohol. and two parts ether called Hartman's solution." The solvents decreased the thymol activity. However, thymol is recognized as a powerful antiseptic, germicide, fungicide, and counterirritant and is currently used in several root canal materials. Currently, one of the most common uses of thymol is as the main ingredient of Listerine, which contains ethyl alcohol as a solvent. Carvacrol, the isomer of thymol. is not used in dentistry today, although it is known to be a powerful antiseptic in dental preparations. Eucalyptus oil (pH 5.0) is distilled from the leaves of the blue gum tree. Eucalyptus is native to Australia. The oil has been used in root canal pastes and mouthrinses and as a solvent for gutta-percha. Eucalyptol is the active antiseptic ingredient in eucalyptus oil. Peppermint oil (pH 5.6) is distilled from the leaves and the flowering tops of the mint plant Mentha arvenjs var. pjperascens. This process yields menthol as the active ingredient used in toothpastes and as a genera] antiseptic. Lemon oil (pH 3.5) is derived from Citrus limonum and yields the active ingredient limonene, which also is found in orange oil and grapefruit oil. Lemon oil is used as a cleanser and disinfectant with antiseptic properties. The cellular building block of the essential oils is the isoprene unit, which joins in a regular head-to-tail way called the isoprene rule to form a multitude of organic compounds. The isoprene unit has two available, reactive, double-bond sites allowing polymers, ring formation, and cross-linking to occur in substances from vitamin A to natural rubber. When an essential oil (eg, eugenol, a weak acid) reacts with zinc oxide (a base) in the presence of water, a salt neutralization reaction occurs and forms a five-membered, chelated ring structure where the zinc ion attaches to two eugenol molecules." This is called the zinc eugenolate complex . Clinical Study Materials and Methods The following essential oils were. tested for their germicidal activity: eugenol, thyme oil, eucalyptus oil, Because eugenol and thyme oil were the most active, an additional test was conducted to determine if the addition of thymol or carvacrol to eugenol would increase the germicidal action of ZOE. The liquid portion consisted of two parts eugenol to one part thymol or two parts eugenol to one part carvacrol." Isoeugenol and vanillin, the phenolic components of clove oil that are derived from eugenol, were also tested to determine their germicidal action. In addition, two parts eugenol were combined with one part thyme oil to determine if thyme oil would increase the germicidal action of ZOE. The following bacteria were obtained from the culture collection of the Department of Microbiology, New York University College of Dentistry: 1) Streptococcus mutans OMZ 176, a gram-positive coccus; 2) Staphylococcus aureus, a gram-positive coccus; 3) Klebsiella pneumoniae A TCC 33452, a gram-negative rod; and 4) Actinomyces viscosus gram-positive rod. These virulent bacteria were chosen because they represented some of the most important pathogenic microorganisms found in the mouth. A viscosus is one of the major inducers of supragingival and subgingival plaque, which leads to gingivitis, the early stage of periodontal disease. For the inhibition assay, the bacteria (grown in a liquid culture) were homogenized and then inoculated onto a blood-agar medium, which had been prepared by aseptically adding 50 mL of citrated bactericidal at 24 hours. Thyme a sheep blood to 1 L of sterilized showed the greatest germicidal blood agar base. at 24 hours. One drop of an essential oil was almost three times greater mixed with one scoop of zinc oxide than that of eugenol. Because eugenol and thyme oil were the most active, an additional test was conducted to determine if the addition of thymol or carvacrol to eugenol would increase the germicidal action of ZOE. The liquid portion consisted of two parts eugenol to one part thymol or two parts eugenol to one part carvacro1. Isoeugenol and vanillin, the phenolic components of clove oil that are derived from eugenol. were also tested to determine their germicidal action. In addition, two parts eugenol were combined with one part thyme oil to determine if thyme oil would increase the germicidal action of ZOE. The following bacteria were obtained from the culture collection of the Department of Microbiology, New York University College of Dentistry: 1) Streptococcus mutans OMZ These virulent bacteria were chosen because they represented some of the most important pathogenic microorganisms found in the mouth. S mutuns is one of the major etiologic agents of dental caries. S oureus is the most common aerobic microorganism associated with oral infections such as periodontal abscesses, osteomyelitis, and the invasion of the mouth in cases of xerostomia. This bacterium develops a strong resistance to antibiotics. It also contributes to clinical cases of halitosis and has been isolated from human bite wounds. A viscous is one of the major inducers of supragingival and subgingival plaque, which leads to gingivitis, the early stage of periodontal disease. For the inhibition assay, the bacteria (grown in a liquid culture) were homogenized and then inoculated onto a blood-agar medium, which had been prepared by aseptically adding 50 mL of citrated sheep blood to 1 L of sterilized blood agar base.b One drop of an essential oil was mixed with one scoop of zinc oxide (0.1 gm). and the mixture was then placed in the center of an inoculated blood agar plate. The plates were incubated at 36°C (97°F) and examined at 24 hours, 48 hours, 72 hours, 96 hours, one week, two weeks, one month, two months, three months, and four months. The zone of inhibition of bacterial growth, termed the inhibition halo effect, was measured in millimeters. Results Table I lists the zones of inhibition board with thyme oil, eugenol, eucalyptus oil, peppermint oil, and lemon oil when placed in direct conduct with bacteria. Table II lists the zones of inhibition obtained with thymol or carvacrol added to ZUE. The essential oils are effective against both gram-positive and gram-negative rods and cocci bacteria. All the essential oils were Conclusions The three components of clove oil (eugenol, isoeugenol, and vanillin) are germicidal and share a similar phenolic structure, A dental cement made of ZOE is germicidal, and its germicidal strength can be doubled by adding thymol and quadrupled by adding carvacrol, the liquid isomer of thymol. Carvacrol is the most effective phenol, followed by thymol. Thyme oil is the most germicidal essential oil. The addition of either thymol or carvacrol to ZOE can benefit the patient in indirect pulp capping procedures because thymol and carvacrol are abundant and palliative to the pulp, Vanillin (synthetic vanilla) is germicidal. A cement based on vanillin is ant bactericidal and more pleasant to the taste than eugenol or other essential oils. Phenol is self-limiting; that is, its degradation products restrict its action. The essential oils, on the other hand, are not self-limiting; they penetrate well; and they are long-lasting. The author has successfully treated periodontal abscesses by using eugenol and thyme oil as irrigating agents to anesthetize the gingival before lancing an abscess and has dipped the ends of curettes and scalers in eugenol and thyme oil for abundant, anodyne, and antiseptic effects. Practitioners can use dental tape impregnated with eugenol or thymol as an adjunct to periodontal treatment or for the agents' antiseptic and sedative actions. A few crystals of thymol added to the hydrocolloid will impart its germicidal action on the microorganisms in the water. Also, several crystals of thymol in a slurry of pumice and water used with a polishing lathe-often one of the most contaminated niches in the dental office-will prevent the growth of microorganisms. Alcohol is not an effective vehicle for the essential oils because it reduces their effectiveness; The essential oils are best used in their liquid form or dissolved in eugenol. The active components of the essential oils (i.e., thymol] are best used dissolved in the liquid monohydric phenols such as eugenol and carvacrol. The active phenolic components such as carvacrol, eucalyptol, eugenol, isoeugenol, Iimonene, menthol, thymol, and vanillin possess marked germicidal properties against bacteria and fungi. The study presented here concurs with other reports that describe thymol and carvacrol as more antiseptic than eugenoI.9.11.13.18.>4.JS The most germicidal dental cement is made with carvacrol and zinc oxide. Practitioners can use the essential oils and their active components for analgesia, antisepsis, cavity sterilization in cements, periodontal procedures such as scaling and curettage, periodontal abscesses, alveolar osteitis (dry socket), oral infections such as osteomyelitis, and halitosis." The essential oils are effective against the most common pathogenic microorganisms found in the mouth." The addition of thyme oil to eugenol doubles the germicidal action of ZOE dental cement. Dental medicaments made from essential oils are anti-inflammatory because phenolic compounds act as nonspecific counter-irritants in addition to being germicidal. Part-owners are becoming increasingly concerned about disease and infection control. The overuse of antibiotics has caused microorganisms to develop resistance to current antibiotics and to become more virulent. Microorganisms, however, do not appear to develop a tolerance or resistance to the antibacterial effect of essential oils such as clove oil (eugenol) and thyme oil (thymol and carvacrol). Such oils provide an effective, powerful, cost-effective means for infection control in dentistry. |
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