Ahmad, A.; van Vuuren, S.; Viljoen, A. Unravelling the Complex Antimicrobial Interactions of Essential Oils — The Case of Thymus vulgaris (Thyme).Molecules 2014, 19, 2896-2910.
Abstract
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 represent >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.
Conclusions
The literature is flooded with papers reporting the antimicrobial activity of botanical extracts and isolated natural products. Fewer papers, however, attempt to unravel the potential interaction between molecules, which will inevitably lead to a better understanding of the observed antimicrobial properties. In this study we have used a well-known plant to illustrate some of the fascinating, yet complex interactions which collectively contribute to the activity of a crude essential oil. Many of the essential oil compounds have different modes of actions and therefore, when used in combination, they reduce the concentration needed to achieve an antimicrobial effect. By our own admission, this study simply presents a snapshot of the intricate interactions in Thyme oil, as it would be an impossible task to explore all the possibilities. Further in-depth studies of the modes of action for synergistic combinations are encouraged. We have reported 22 compounds in thyme oil (ignoring the presence of very minor compounds and enantiomers). Theoretically this implies that 4,194,303 combinations exist which could be tested, clearly a Herculean task. This study, however, has shown the complex interactions between several essential oil constituents with many of these combinations being synergistic confirming that synergy is the very premise on which the concept of phytotherapy is based.
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