Publications

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Ahmad, A., Van Vuuren, S.F., Viljoen, A.M. 2014. Unravelling the complex antimicrobial interactions of essential oils – the case of Thymus vulgaris (Thyme). Molecules 19: 2896-2910.
Thymus vulgaris has gained tremendous popularity as an ornamental, culinary herb and its use in phytotherapy is well established and supported in the literature. The objective of this study was to explore possible interactions between selected molecules within Thymus vulgaris essential oil (TvEO) to gain a better understanding of how this complex essential oil exerts its antimicrobial activity. Evaluation of the antimicrobial efficacy and interactions were assessed on the essential oil and volatile constituents against various pathogens. Interactions between molecules at various ratios were graphically observed through the construction of isobolograms. Gas chromatography-mass spectrometry (GC-MS) analysis revealed 22 compounds which collectively represents >95% of the oil composition. Based on their minimum inhibitory concentration (MIC) values they were categorised into weak (≥4 mg mL-1), moderate (2-4 mg mL-1) and noteworthy active (≤2 mg mL-1) compounds. For the combination study, 21% synergistic, 42% additive, 36% indifferent and 1% antagonistic interactions were observed. Most of the interactions were observed between the weak and highly active molecules, and interestingly, no synergistic interaction was observed between the highly active compounds. Synergistic and additive interactions between the strong and weaker antimicrobial constituents present in TvEO enhance the antimicrobial efficacy of this commercially important essential oil. The essential oil composition of three Myrtaceous species (Leptospermum petersonii, L. scoparium and Kunzea ericoides) belonging to the tea tree group were analysed using gas chromatography coupled to mass spectrometry (GC-MS). The major compounds determined from the mean ± SD of the monthly samples collected for one calendar year in L. petersonii are citronellal (11.4±4.3%), citronellol (17.5±7.1%), neral (19.7±1.6%) and geranial (34.7±3.3%). The major compounds in L. scoparium are eudesma-4(14)-11-diene(11.6±2.4%), α-selinene(10.4±2.3%) and (E)-methyl cinnamate (12.6±3.8%). The major compounds in K. ericoides are α-pinene (37.6±6.3%) and p-cymene (13.5±4.1%). The essential oils show some promising antimicrobial activity against selected micro-organisms when investigated using the minimum inhibitory concentration assay. Highest sensitivities were noted for the Brevibacteria (lowest MIC value of 0.06 mg/ml), a genus associated with foot odour. When the different essential oils were combined in various ratios and tested againstStaphylococcus aureus, Pseudomonas aeruginosa and Candida albicans, a predominantly additive effect was noted.




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