The chemical biology of antimicrobial bioactivity derived from kānuka (Kunzea robusta)

Abstract

Natural products are sourced from the environment where immense potential for bioactive compounds exist. Traditional medicine has explored these compounds, with many discoveries based on medicinal plants still in use today. The native flora in Aotearoa (New Zealand) contains many endemic species that are yet to be extensively investigated, for example Kunzea robusta or kānuka. Kānuka is considered a taonga species to Māori, the indigenous people of Aotearoa, due it to extensive use in Māori traditional medicine. Unlike its close relative mānuka, the bioactive potential of kānuka has yet to be extensively explored. Therefore, the aims of this thesis were to determine its medicinal potential by investigating the antifungal, antibacterial, and anti-P. acnes bioactivities of kānuka oil. Chemical profiles were also investigated, and statistical analyses correlated the chemical composition of kānuka oil extracts to the aforementioned bioactivities, thus identifying potential lead drug compounds. Methods: Using 99 samples of kānuka oil extracted from leaves collected from various locations in Te Tairawhiti via steam distillation, chemical profiles were determined using gas chromatography mass spectrometry (GC-MS) analysis. The growth of Candida albicans, methicillin-resistant Staphylococcus aureus (MRSA) and Propionibacterium acnes was evaluated in the presence of all kānuka extracts to represent each bioactivity. Multivariate statistical analysis was then used to identify the compounds within the kānuka extracts that positively correlate with the three aforementioned bioactivities. Results: Bioassays were conducted using minimum inhibitory concentration (MIC) of 5 x 10- 5 % (v/v) for antifungal and anti-MRSA bioactivity and a MIC of 500 x 10 -5 % (v/v) for anti- P. acnes bioactivity. Seasonal and geographical variation for all three bioactivities was detected. Antifungal bioactivity assays revealed median percent growth of treated C. albicans cells to range between 38-54% with more variation in growth observed in season one (spring). The most potent extract in this experiment was seen in season one from land block H; however, in season two (autumn) land block H was the least potent (96%) and land block F was the most potent (32%). Antibacterial bioactivity assays revealed median percent growth of treated MRSA to range between 35-79% with variation consistent in both seasons. The most potent effects were seen in season one from land block H (28%) with the less effective coming from land block E where in some cases there was no effect on treated cell conditions. Finally, anti- P. acnes bioactivity assays revealed median percent growth of treated P. acnes cells to range between 62-73% with more variation observed in season two. The most potent effects were seen in season one from land block B (~43%) with the less effective coming from season two land block H (89%). GC-MS analysis distinguished α-pinene as the most abundant compound (~34%) and multivariate analysis identified lead compounds such as α-Murolene, isoamyl isovalerate for antifungal activity and compounds such as limonene and nerolidol for anti-MRSA and anti-P. acnes bioactivity. The unidentified compound, unknown was particularly interesting as it was the only compound to positively correlate to all three bioactivities. Conclusion: Kānuka oil extracted from Te Tairawhiti exhibits antifungal, anti-MRSA and anti-P. acnes properties that show seasonal and geographical variation. The results from this study provide scientific validation for the medicinal benefits of kānuka oil that can be further explored for pharmaceutical potential.