Email updates

Keep up to date with the latest news and content from BMC Complementary and Alternative Medicine and BioMed Central.

Open Access Highly Accessed Research article

In vitro antibacterial activity of some plant essential oils

Seenivasan Prabuseenivasan, Manickkam Jayakumar and Savarimuthu Ignacimuthu*

Author Affiliations

Entomology Research Institute, Loyola College, Chennai – 600 034, India.

For all author emails, please log on.

BMC Complementary and Alternative Medicine 2006, 6:39  doi:10.1186/1472-6882-6-39

The electronic version of this article is the complete one and can be found online at: http://www.biomedcentral.com/1472-6882/6/39


Received:29 July 2006
Accepted:30 November 2006
Published:30 November 2006

© 2006 Prabuseenivasan et al; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background:

To evaluate the antibacterial activity of 21 plant essential oils against six bacterial species.

Methods:

The selected essential oils were screened against four gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Proteus vulgaris) and two gram-positive bacteria Bacillus subtilis and Staphylococcus aureus at four different concentrations (1:1, 1:5, 1:10 and 1:20) using disc diffusion method. The MIC of the active essential oils were tested using two fold agar dilution method at concentrations ranging from 0.2 to 25.6 mg/ml.

Results:

Out of 21 essential oils tested, 19 oils showed antibacterial activity against one or more strains. Cinnamon, clove, geranium, lemon, lime, orange and rosemary oils exhibited significant inhibitory effect. Cinnamon oil showed promising inhibitory activity even at low concentration, whereas aniseed, eucalyptus and camphor oils were least active against the tested bacteria. In general, B. subtilis was the most susceptible. On the other hand, K. pneumoniae exhibited low degree of sensitivity.

Conclusion:

Majority of the oils showed antibacterial activity against the tested strains. However Cinnamon, clove and lime oils were found to be inhibiting both gram-positive and gram-negative bacteria. Cinnamon oil can be a good source of antibacterial agents.

Background

The spread of drug resistant pathogens is one of the most serious threats to successful treatment of microbial diseases. Down the ages essential oils and other extracts of plants have evoked interest as sources of natural products. They have been screened for their potential uses as alternative remedies for the treatment of many infectious diseases [1]. World Health Organization (WHO) noted that majority of the world's population depends on traditional medicine for primary healthcare. Medicinal and aromatic plants which are widely used as medicine and constitute a major source of natural organic compounds.

Essential oils have been shown to possess antibacterial, antifungal, antiviral insecticidal and antioxidant properties [2,3]. Some oils have been used in cancer treatment [4]. Some other oils have been used in food preservation [5], aromatherapy [6] and fragrance industries [7]. Essential oils are a rich source of biologically active compounds. There has been an increased interest in looking at antimicrobial properties of extracts from aromatic plants particularly essential oils [8]. Therefore, it is reasonable to expect a variety of plant compounds in these oils with specific as well as general antimicrobial activity and antibiotic potential [9].

Essential oils (also called volatile oils) are aromatic oily liquids obtained from plant materials (flowers, buds, seeds, leaves, twigs, bark, herbs, wood, fruits and roots). They can be obtained by expression, fermentation or extraction but the method of steam distillation is most commonly used for commercial production. An estimated 3000 essential oils are known, of which 300 are commercially important in fragrance market [7]. Essential oils are complex mixers comprising many single compounds. Chemically they are derived from terpenes and their oxygenated compounds. Each of these constituents contributes to the beneficial or adverse effects.

Essential oils such as aniseed, calamus, camphor, cedarwood, cinnamon, citronella, clove, eucalyptus, geranium, lavender, lemon, lemongrass, lime, mint, nutmeg, orange, palmarosa, rosemary, basil, vetiver and wintergreen have been traditionally used by people for various purposes in different parts of the world (Table 1). Cinnamon, clove and rosemary oils had shown antibacterial and antifungal activity [10]; cinnamon oil also possesses antidiabetic property [11]. Anti-inflammatory activity has been found in basil [12]. Lemon and rosemary oils possess antioxidant property [13,14]. Peppermint and orange oils have shown anticancer activity [15,16]. Citronella oil has shown inhibitory effect on biodegrading and storage-contaminating fungi [17]. Lime oil has shown immunomodulatory effect in humans [16]. Lavender oil has shown antibacterial and antifungal activity; it was also found to be effective to treat burns and insect bites [18].

Table 1. List of selected essential oils and their properties.

In spite of all the information available on the 21 oils selected for this study, we were not able to find antibacterial activity for all those oils. Hence this study was undertaken with the intention of finding out the efficacy of these essential oils as antimicrobial agents for commercial purposes.

Methods

Essential oils

Twenty-one essential oils obtained from Tegraj & Co (P) Ltd, India (commercial producers of plant essential oils and aromatic substances) were used in this study (Table 1). These oils were selected based on literature survey and their use in traditional medicine. Quality of the oils was ascertained to be more than 98% pure.

Test organism

Microorganisms were obtained from the Department of Microbiology, Christian Medical College, Vellore, India and Institute of Basic Medical Sciences (IBMS), Chennai, India. Four strains of gram-negative bacteria [Escherichia coli (ATCC 25922), Klebsiella pneumoniae (ATCC 15380), Pseudomonas aeruginosa (ATCC 27853), Proteus vulgaris (MTCC 1771)] and two strains of gram-positive bacteria [Bacillus subtilis (MTCC 441) and Staphylococcus aureus (ATCC 25923)] were used. The cultures of bacteria were maintained in their appropriate agar slants at 4°C throughout the study and used as stock cultures.

Antibacterial assay

Screening of essential oils for antibacterial activity was done by the disk diffusion method, which is normally used as a preliminary check and to select between efficient essential oils [2]. It was performed using an 18 h culture at 37°C in 10 ml of Mueller Hinton Broth. The cultures were adjusted to approximately 105CFU/ml with sterile saline solution. Five hundred microliters of the suspensions were spread over the plates containing Mueller-Hinton agar using a sterile cotton swab in order to get a uniform microbial growth on both control and test plates. The essential oils were dissolved in 10% aqueous dimethylsulfoxide (DMSO) with Tween 80 (0.5% v/v for easy diffusion) and sterilized by filtration through a 0.45 μm membrane filter. Under aseptic conditions, empty sterilized discs (Whatman no. 5, 6 mm dia) were impregnated with 50 μL of different concentrations (1:1, 1:5, 1:10, 1:20) of the respective essential oils and placed on the agar surface [19]. Paper disc moistened with aqueous DMSO was placed on the seeded petriplate as a vehicle control. A standard disc containing streptomycin (25 μg/disc) was used as reference control. All petridishes were sealed with sterile laboratory parafilm to avoid eventual evaporation of the test samples. The plates were left for 30 min at room temperature to allow the diffusion of oil, and then they were incubated at 37°C for 18 h (18 h was fixed as the optimum since there was no change in the inhibition up to 24 h) After the incubation period, the zone of inhibition was measured with a calliper. Studies were performed in triplicate, and mean value was calculated. The means were analysed by one way analysis of variance (ANOVA) followed by Tukey's post hoc multiple comparison test using SPSS software package version 13.0 for windows. The results were expressed as mean ± SD. P values <0.05 were considered as significant.

MIC assay

Based on the previous screening seven essential oils (Cinnamon, clove, geranium, lemon, lime, orange and rosemary oils) were identified to have potent antibacterial activity and their Minimum Inhibitory Concentrations (MIC) were determined. The agar dilution method recommended by the National Committee for Clinical Laboratory Standards [20] was used with the following modification; a final concentration of 0.5% (v/v) Tween-20 (Sigma) was incorporated into the agar medium to enhance oil solubility. A series of two fold dilution of each oil, ranging from 0.2 to 25.6 mg/ml, was prepared in Muellur Hinton agar at 48°C. Plates were dried at room temperature for 30 min prior to spot inoculation with 3 μl aliquots of culture containing approximately 105 CFU/ml of each organism. Inoculated plates were incubated at 37°C for 18 h and the MIC was determined. Experiments were carried out in triplicate. Inhibition of bacterial growth in the plates containing test oil was judged by comparison with growth in blank control plates. The MICs were determined as the lowest concentration of oil inhibiting visible growth of each organism on the agar plate [21].

Gas chromatography mass spectrometry (GC/MS)

The most potent oil, (cinnamon oil) was analysed using GC/MS (Shimadzu capillary GC-quadrupole MS system QP 5000) with two fused silica capillary column DB-5 (30 μm, 0.25 mm i.d, film thickness 0.25 μm) and a flame ionization detector (FID) which was operated in EI mode at 70 eV. Injector and detector temperatures were set at 220°C and 250°C, respectively. One microliter essential oil solution in hexane was injected and analyzed with the column held initially at 60°C for 2 min and then increased by 3°C/min up to 300°C. Helium was employed as carrier gas (1 ml/min). The relative amount of individual components of the total oil is expressed as percentage peak area relative to total peak area. Qualitative identification of the different constituents was performed by comparison of their relative retention times and mass spectra with those of authentic reference compounds, or by retention indices (RI) and mass spectra.

Results

The anti-bacterial activity of twenty-one selected essential oils against six bacterial species is summarized in Table 2 and 3. The results revealed that the selected essential oils showed antibacterial activity with varying magnitudes. The zone of inhibition above 7 mm in diameter was taken as positive result. Generally most of the tested organisms were sensitive to many of the essential oils. Out of 21 essential oils tested, 19 showed antibacterial activity against one or more bacteria. Cinnamon oil, lime oil, geranium oil, rosemary oil, orange oil, lemon oil and clove oil showed maximum activity against all the bacterial species tested. On the other hand, aniseed oil, eucalyptus oil and camphor oil failed to inhibit any of the tested strains. Both gram-positive and gram-negative bacteria were sensitive to the potent essential oils. P. aeruginosa and P. vulgaris were inhibited by 19 oils, followed by B. subtilis (18 oils), S. aureus (14 oils), E. coli (12 oils) and K. pneumoniae (9 oils). In general cinnamon oil showed significant inhibitory effect against P. aeruginosa (33.3 mm), B. subtilis (29.9 mm), P. vulgaris (29.4 mm), K. pneumoniae (27.5 mm) and S. aureus (20.8 mm). Moderate effects were seen in lime oil, clove oil and lemon oil. No obvious difference in susceptibility was found between gram-negative and gram-positive bacteria. There was no inhibition of growth with the vehicle control (10% DMSO).

Table 2. Antimicrobial activity of 21 essential oils against S. aureus, B. subtilis and K. pneumoniae using disc diffusion method

Table 3. Antimicrobial activity of 21 essential oils against P. vulgaris, P. aeruginosa and E. coli using disc diffusion method

Minimum inhibitory concentration (MIC) for selected seven oils ranged from 0.8 to 12.8 mg/ml (Table 4). This study revealed that cinnamon oil showed maximum activity with MIC values ranging from 0.8 to 3.2 mg/ml followed by clove oil with MIC values ranging from 1.6 to 6.4 mg/ml against all the tested strains where as remaining oils showed moderate MIC values.

Table 4. Minimum Inhibitory Concentration (MIC) of selected essential oils (mg/ml).

GC/MS analysis of cinnamon oil identified thirty-eight phytochemicals as constituents; of these cinnamaldehyde was the major compound (52.4%) followed by Benzaldehyde (12.31%), benzoic acid (8.20%) and benzyl alcohol (2.23%). Remaining chemical compounds were in trace amounts. The major components and their retention times are summarized in Table 5.

Table 5. Major chemical compounds of cinnamon oil (GC-MS analysis)

Discussion

Plant essential oils and extracts have been used for many thousands of years [22], in food preservation, pharmaceuticals, alternative medicine and natural therapies [23,24]. It is necessary to investigate those plants scientifically which have been used in traditional medicine to improve the quality of healthcare. Essential oils are potential sources of novel antimicrobial compounds [25] especially against bacterial pathogens. In vitro studies in this work showed that the essential oils inhibited bacterial growth but their effectiveness varied. The antimicrobial activity of many essential oils has been previously reviewed and classified as strong, medium or weak [26].

In our study, cinnamon, clove, geranium, lemon, lime, orange and rosemary oils exhibited strong activity against the selected bacterial strains. Several studies [27,28] have shown that cinnamon, clove and rosemary oils had strong and consistent inhibitory effects against various pathogens. Even though earlier studies have reported better antimicrobial activity for eucalyptus oil [29,30] our study showed least inhibitory activity of eucalyptus in addition to aniseed and camphor oils. Among all oils analyzed in this work, the essential oil of cinnamon was the most effective as an antibacterial agent. The antibacterial activity has been attributed to the presence of some active constituents in the oils. Our GC-MS study revealed cinnamaldehyde to be the major constituent of cinnamon oil. Cinnamaldehyde was the predominant active compound found in cinnamon oil [31,32]. Earlier studies suggested that the antibacterial activity of cinnamon oil was probably due to their major component, cinnamaldehyde and their properties could be multiple. Cinnamaldehyde is a natural antioxidant and the animal studies suggest that an extract of cinnamon bark taken orally may help prevent stomach ulcer [33]. Cinnamaldehyde was completely inhibiting both sensitive and resistant stain of Helicobacter pylori [34]. Cinnamon oil was not harmful when consumed in food products and it inhibited the growth of molds, yeast and bacteria [27]. Cinnamon extract had a regulatory role in blood glucose level and lipids and it may also exert a blood glucose-suppressing effect [35]. The use of commercial cinnamon preparation produced an improvement of oral candidiasis of HIV-infected patients [36]. Cinnamon oil is locally applied with much benefit in neuralgia and headache. As an antiseptic it is used as an injection in gonorrhea; as germicide it is used internally in typhoid fever. This oil is also used in the treatment of cancer and other microbial diseases [37]. It can be incorporated into creams, lotions, drops, etc. which are applied externally on the body to treat diseases caused by Aspergillus niger [38].

An important characteristic of essential oils and their components is their hydrophobicity, which enable them to partition the lipids of the bacterial cell membrane and mitochondria, disturbing the cell structures and rendering them more permeable [39,40]. Extensive leakage from bacterial cells or the exit of critical molecules and ions will lead to death [41]. Gram-positive bacteria were more resistant to the essential oils than gram-negative bacteria [26]. In the present study, cinnamon, lime, geranium, rosemary, orange, lemon and clove oils were found to be equally effective against both gram-positive and gram-negative organisms.

Conclusion

From this study it can be concluded that many essential oils possess antibacterial activity. Cinnamon oil has the most potential bactericidal properties. We believe that the present investigation together with previous studies provide support to the antibacterial properties of cinnamon oil. It can be used as antibacterial supplement in the developing countries towards the development of new therapeutic agents. Additional in vivo studies and clinical trials would be needed to justify and further evaluate the potential of this oil as an antibacterial agent in topical or oral applications.

Competing interests

The author(s) declare that they have no competing interests.

Authors' contributions

SP and MJ have carried out the experimental part such as selection of essential oils, inoculum preparation and antimicrobial evaluation. SI supervised the work, evaluated the results and corrected the manuscript for publication. All authors read and approved the final manuscript.

Acknowledgements

We thank ICMR, New Delhi for providing financial support. We also thank the Department of Microbiology, Institute of Basic Medical Sciences (IBMS), University of Madras, Chennai and Christian Medical College, Vellore, India for providing the bacterial cultures. We are thankful to Mr. Sankar AGM, R & D Section, Nicholas Piramal India Limited, Chennai for helping in GC/MS analysis.

References

  1. Tepe B, Daferera D, Sokmen M, Polissiou M, Sokmen A: In vitro antimicrobial and antioxidant activities of the essential oils and various extracts of Thymus eigii M. Zohary et P.H. Davis.

    J Agric Food Chem 2004, 52:1132-1137. PubMed Abstract | Publisher Full Text OpenURL

  2. Burt SA: Essential oils: their antibacterial properties and potential applications in foods: a review.

    Inter J Food Microbiol 2004, 94:223-253. Publisher Full Text OpenURL

  3. Kordali S, Kotan R, Mavi A, Cakir A, Ala A, Yildirim A: Determination of the chemical composition and antioxidant activity of the essential oil of Artemisia dracunculus and of the antifungal and antibacterial activities of Turkish Artemisia absinthium, A. dracunculus, Artemisia santonicum, and Artemisia spicigera essential oils.

    J Agric Food Chem 2005, 53:9452-9458. PubMed Abstract | Publisher Full Text OpenURL

  4. Sylvestre M, Pichette A, Longtin A, Nagau F, Legault J: Essential oil analysis and anticancer activity of leaf essential oil of Croton flavens L. from Guadeloupe.

    J Ethnopharmacol 2006, 103:99-102. PubMed Abstract | Publisher Full Text OpenURL

  5. Faid M, Bakhy K, Anchad M, Tantaoui-Elaraki A, Alomondpaste : Physicochemical and microbiological characterizations and preservation with sorbic acid and cinnamon.

    J Food Prod 1995, 58:547-550. OpenURL

  6. Buttner MP, Willeke K, Grinshpun SA: Sampling and analysis of airborne microorganisms. In Manual of Environmental Microbiology. Edited by Hurst CJ, Knudsen GR, McInerney MJ, Stetzenbach LD, Walter MV. ASM Press: Washington, DC; 1996:629-640. OpenURL

  7. Van de Braak SAAJ, Leijten GCJJ: Essential Oils and Oleoresins: A Survey in the Netherlands and other Major Markets in the European Union. CBI, Centre for the Promotion of Imports from Developing Countries, Rotterdam.

    1999, 116.

  8. Milhau G, Valentin A, Benoit F, Mallie M, Bastide J, Pelissier Y, Bessiere J: In vitro antimicrobial activity of eight essential oils.

    J Essent Oil Res 1997, 9:329-333. OpenURL

  9. Darokar MP, Mathur A, Dwivedi S, Bhalla R, Khanuja SPS, Kumar S: Detection of antibacterial activity in the floral petals of some higher plants.

    Curr Sci 1998, 75:187. OpenURL

  10. Ouattara B, Simard RE, Holley RA, Pitte GJP, Begin A: Antibacterial activity of selected fatty acids and essential oils against six meat spoilage organisms.

    Inter J Food Microbiol 1997, 37:155-162. Publisher Full Text OpenURL

  11. Subash Babu P, Prabuseenivasan S, Ignacimuthu S: Cinnamaldehyde-A potential antidiabetic agent.

    Phytomed, in press. OpenURL

  12. Singh S, Majumdar DK: Effect of Ocimum sanctum fixed oil on vascular permeability and leucocytes migration.

    Indian J Exp Biol 1999, 37:1136-1138. PubMed Abstract OpenURL

  13. Calabrese V, Randazzo SD, Catalano C, Rizza V: Biochemical studies on a novel antioxidant from lemon oil and its biotechnological application in cosmetic dermatology.

    Drugs Exp Clin Res 1999, 25:219-225. PubMed Abstract OpenURL

  14. Aruoma OI, Spencer JP, Rossi R, Aeschbach R, Khan A, Mahmood N, Munoz A, Murcia A, Butler J, Halliwell B: An evaluation of the antioxidant and antiviral action of extracts of rosemary and Provencal herbs.

    Food Chem Toxicol 1996, 34:449-456. PubMed Abstract | Publisher Full Text OpenURL

  15. Kumar A, Samarth RM, Yasmeen S, Sharma A, Sugahara T, Terado T, Kimura H: Anticancer and radioprotective potentials of Mentha piperita.

    Biofactors 2004, 22:87-91. PubMed Abstract | Publisher Full Text OpenURL

  16. Arias BA, Ramon-Laca L: Pharmacological properties of citrus and their ancient and medieval uses in the Mediterranean region.

    J Ethnopharmacol 2005, 97:89-95. PubMed Abstract | Publisher Full Text OpenURL

  17. De Billerbeck VG, Roques CG, Bessiere JM, Fonvieille JL, Dargent R: Effects of Cymbopogon nardus (L.) W. Watson essential oil on the growth and morphogenesis of Aspergillus niger.

    Can J Microbiol 2001, 47:9-17. PubMed Abstract | Publisher Full Text OpenURL

  18. Cavanagh HM, Wilkinson JM: Biological activities of lavender essential oil.

    Phytother Res 2002, 16:301-308. PubMed Abstract | Publisher Full Text OpenURL

  19. NCCLS (National Committee for Clinical Laboratory Standards): Methods for dilution antimicrobial susceptibility tests of bacteria that grow aerobically. In Approved Standard M100-S12. Wayne. PA, NCCLS; 2002. OpenURL

  20. Prudent D, Perineau F, Bessiere JM, Michel GM, Baccou JC: Analysis of the essential oil of wild oregano from Martinique (Coleus aromaticus Benth.) – evaluation of its bacterioatatic and fungistatic properties.

    J Essen Oil Res 1995, 7:165-173. OpenURL

  21. Delaquis PJ, Stanich K, Girard B, Mazza G: Antimicrobial activity of individual and mixed fractions of dill, cilantro, coriander and eucalyptus essential oils.

    Inter J Food Microbiol 2002, 74:10-109. OpenURL

  22. Jones FA: Herbs – useful plants. Their role in history and today.

    Euro J Gastroenterol Hepatol 1996, 8:1227-1231. OpenURL

  23. Reynolds JEF: Martindale – the Extra Pharmacopoeia. 31st edition. London. Royal Pharmaceutical Society of Great Britain; 1996.

  24. Lis-Balchin M, Deans SG: Bioactivity of selected plant essential oils against Listeria monocytogenes.

    J Appl Bacteriol 1997, 82:759-762. OpenURL

  25. Mitscher LA, Drake S, Gollapudi SR, Okwute SK: A modern look at folkloric use of anti-infective agents.

    J Nat Prod 1987, 50:1025-1040. PubMed Abstract | Publisher Full Text OpenURL

  26. Zaika LL: Spices and herbs: their antibacterial activity and its determination.

    J Food Saf 1988, 23:97-118. OpenURL

  27. Matan N, Rimkeeree H, Mawson AJ, Chompreeda P, Haruthaithanasan V, Parker M: Antimicrobial activity of cinnamon and clove oils under modified atmosphere conditions.

    Int J Food Microbiol 2006, 107:180-185. PubMed Abstract | Publisher Full Text OpenURL

  28. Aureli P, Costantini A, Zolea S: Antibacterial activity of some plant essential oils against Listeria monocytogences.

    J Food Prot 1992, 55:344-348. OpenURL

  29. Cimanga KK, Kambu , Tona L, Apers S, De Bruyne T, Hermans N, Totte J, Pieters L, Vlietinck AJ: Correlation between chemical composition and antibacterial activity of essential oils of some aromatic medicinal plants growing in the Democratic Republic of Congo.

    J Ethnopharmacol 2002, 79:213-20. PubMed Abstract | Publisher Full Text OpenURL

  30. Takarada K, Kimizuka R, Takahashi N, Honma K, Okuda K, Kato T: A comparison of the antibacterial efficacies of essential oils against oral pathogens.

    Oral Microbiol Immunol 2002, 19:61-4. Publisher Full Text OpenURL

  31. Simic A, Sokovic MD, Ristic M, Grujic-Jovanovic S, Vukojevic J, Marin PD: The chemical composition of some Lauraceae essential oils and their antifungal activities.

    Phytother Res 2004, 18:713-717. PubMed Abstract | Publisher Full Text OpenURL

  32. Baratta MT, Dorman HJ, Deans SG, Figueiredo AC, Barroso JG, Ruberto G: Antimicrobial and antioxidant properties of some commercial essential oils.

    Flav Fragr J 1998, 13:235-244. Publisher Full Text OpenURL

  33. Blumenthal M: The Complete Commission E Monographs, Therapeutic Guide Herbal Medicines. Boston, Mass: Integrative Medicine Communications; 1998:110.

  34. Ali1 SM, Khan AA, Ahmed I, Musaddiq M, Ahmed KS, Polasa H, Venkateswar Rao L, Habibullah CM, Sechi LA, Ahmed N: Antimicrobial activities of Eugenol and Cinnamaldehyde against the human gastric pathogen Helicobacter pylori.

    Annals of Clinical Microbiology and Antimicrobials 2005, 4:20. BioMed Central Full Text OpenURL

  35. Kim SH, Hyun SH, Choung SY: Anti-diabetic effect of cinnamon extract on blood glucose in db/db mice.

    J Ethnopharmacol 2006, 104:119-123. PubMed Abstract | Publisher Full Text OpenURL

  36. Quale JM, Landman D, Zaman MM, Burney S, Sathe SS: In vitro activity of Cinnamomum zeylanicum against azole resistant and sensitive Candida species and a pilot study of cinnamon for oral candidiasis.

    Am J Chin Med 1996, 24:103-109. PubMed Abstract | Publisher Full Text OpenURL

  37. Nadkarni KM: Indian Meteria Medica. Popular Prakashan, Bombay, India; 1976:228-231.

  38. Pawar VC, Thaker VS: In vitro efficacy of 75 essential oils against Aspergillus niger.

    Mycoses 2006, 49:316-323. PubMed Abstract | Publisher Full Text OpenURL

  39. Knobloch K, Weigand H, Weis N, Schwarm H-M, Vigenschow H: Action of terpenoids on energy metabolism. In Progress in Essential Oil Research: 16th International Symposium on Essential Oils. Edited by Brunke EJ. De Gruyter, Berlin; 1986:429-445. OpenURL

  40. Sikkema J, De Bont JAM, Poolman B: Interactions of cyclic hydrocarbons with biological membranes.

    J Biol Chem 1994, 269:8022-8028. PubMed Abstract | Publisher Full Text OpenURL

  41. Denyer SP, Hugo WB: Biocide-induced damage to the bacterial cytoplasmic membrane. In Mechanisms of Action of Chemical Biocides. Edited by Denyer SP, Hugo WB. The Society for Applied Bacteriology, Technical Series No 27. Oxford Blackwell Scientific Publication, Oxford; 1991:171-188. OpenURL

  42. Rastogi RP, Mehrotra BN: Compendium of Indian medicinal plants. Volume I–IV. Central Drug Research Institute, Lucknow, and National Institute of science communication, New Delhi, India; 1994.

  43. Rastogi RP, Mehrotra BN: Glossary of Indian Medicinal Plants. National Institute of science communication, New Delhi, India; 2002.

Pre-publication history

The pre-publication history for this paper can be accessed here:

http://www.biomedcentral.com/1472-6882/6/39/prepub