• Antibacterial activities and interactions of some essential oils against important human pathogenic bacteria: an in vitro study
  • Mohsen Sheikhrezaee,1 Mitra Mehrabani,2 Alireza Golnaraghi,3,* Farzad Kobarfard,4 Omid Hosseini,5 Zahra Sheykhrezaee,6
    1. Department of Traditional Medicine, Faculty of Persian Medicine, Kerman University of Medical Sciences
    2. Department of Traditional Pharmacy, Faculty of Persian Medicine, Kerman University of Medical Sciences
    3. Department of Biodiversity, BoomZista Institute
    4. Department of Medicinal Chemistry, School of Pharmacy, Shahid Beheshti University of Medical Sciences
    5. Department of Medicinal Chemistry, School of Pharmacy, Shahid Beheshti University of Medical Sciences
    6. Department of Nature & Health, BoomZista Institute


  • Introduction: Emergence of antibiotic-resistant microbial strains and other issues related to antibiotic production/applications have revealed the fact that there are still challenges to overcome microbial epidemics and the treatment of infectious diseases. Persian medicine and its capacity could be considered as one of the choices to solve such issues. Although the words microbe or microbial diseases have not been mentioned in Persian medicine original books, the term Havaye Vabaee shares many similarities with contagious diseases. Particularly, a range of air-borne infectious diseases transmitted through inhalation fall into this category. Medicinal plants contain a variety of natural compounds with different benefits and properties. Applications of medicinal plants have been frequently described in Persian medicine. Several herbal elements, utilized in prevention and treatment of Havaye Vabaee diseases, are now classified into anti-oxidant, anti-microbial and bolstering immune system categories.
  • Methods: Antibacterial activities of essential oils (EOs) of some widely used herbal elements, including blacksamson echinacea, dill, chamomile, cinnamon, cumin, eucalyptus, lavender, myrtle, pennyroyal, peppermint, rosemary and thyme, whose antimicrobial effects are indirectly described in the old literature of traditional medicine as antiseptic agents and treatments of diseases caused by Havaye Vabaee, were investigated. Antibacterial activities of the EOs were evaluated against some important human pathogenic Gram-negative (Escherichia coli, Klebsiella pneumonia, Listeria monocytogenes and Pseudomonas aeruginosa) and Gram-positive (Staphylococcus aureus, Staph. epidermidis, and Streptococcus pyogenes) bacteria. To this end, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and double-diffusion assay (DDA) tests were utilized using a microdilution method. On the basis of MIC and DDA values, three EOs of cinnamon, eucalyptus and thyme with the highest antibacterial activities were then selected for checkerboard tests where antibacterial effects of double combinations of the EOs against E. coli and Staph. aureus were evaluated. The result of a checkerboard test was applied as input to calculate a fractional inhibitory concentration (FIC) index.
  • Results: Based on the results, the lowest MICs of the EOs were obtained for cinnamon followed by thyme and eucalyptus. These EOs also had the lowest values in MBC tests as compared with other EOs studied. The MIC and MBC values obtained for cinnamon were comparable with those obtained for the positive control used in this study (Gentamycin). While the reactions of the three Gram-positive bacteria to EOs were almost similar to each other, MIC and MBC values evaluated for the four Gram-negative bacteria were almost considerably different. DDA tests, in agreement with MIC and MBC assays, revealed the largest inhibition zones of the three EOs. Thyme followed by cinnamon showed the highest inhibition zones against different pathogenic bacteria. The values obtained were comparable with those measured for Gentamycin in most of EO-bacterium combinations. The FIC index values ranged from 1.0625 to 2 for Staph. aureus, and from 2 to 4 for E. coli. The lowest and the highest values were recorded for cinnamon-eucalyptus/Staph. aureus, and for cinnamon-thyme/E. coli and eucalyptus-thyme/E. coli combinations, respectively. The FIC index values were greater than 0.5 and less than 4, indicating “no interaction” between components of the combinations. The values calculated for cinnamon-thyme/E. coli and eucalyptus-thyme/E. coli combinations were 4, close to the cut-off value for antagonistic interactions.
  • Conclusion: The results indicated significant antibacterial properties of the three EOs. This characteristic can be attributed to their major chemical components and different antibacterial mechanisms involved. The differences observed in bacterial resistance and susceptibility to EOs are mainly due to the variation in bacterial cell membranes. Application of herbal elements is the main strategy in Persian medicine against different diseases, including those caused by Havaye Vabaee. Such combinations possibly affect a variety of biochemical and physiological processes in the human body and produce a plethora of interactions on the target microbes. Our study suggests that thyme oil must be used with caution as an antibacterial in combination with cinnamon or eucalyptus oils because antagonism may predominate; little is known about the mechanisms of antagonistic interaction. The results clearly indicate the necessity of investigation on the interactions of medicinal plants before formulating them as medicinal conoctions. We should warn patients to avoid the simultaneous use of herbal elements with chemical drugs without consulting with their physician, and try to change the general belief saying that use of herbal medicines is completely safe and healthy.
  • Keywords: Persian medicine, essential oils, combinations, antibacterial, antagonism