The long-term use of antibiotics has led to widespread bacterial resistance and so there is an urgent need to identify new antibiotics. Traditional medicine based on natural products has often led to the discovery of new drugs, and ecological cues can also reveal bioactive natural compounds. Kino from eucalypt trees (Corymbia and Eucalyptus spp.) is a trunk exudate that contains high levels of potentially useful polyphenols. Aboriginal communities in Australia have used these kinos to treat ailments such as diarrhoea, scabies, haemorrhage and chronic bowel inflammation. Crude kino extracts from Corymbia citriodora, C. torelliana and their hybrid were tested for antimicrobial activity against three Gram-negative bacteria, two Gram-positive bacteria and a unicellular fungus. All crude extracts showed antimicrobial activity against all the microorganisms, with the highest zone of inhibition observed against Pseudomonas aeruginosa. The MIC of all extracts was 200 μg/mL. The extracts were not cytotoxic within 48 hours, although minor morphological changes of Caco-2 cells were observed. None of the extracts reduced the ability of microorganisms to form biofilm within 24 h. One of these eucalypt species, C. torelliana, also has an unusual ecological relationship with stingless bees, which are strongly attracted to the resin of its fruits and disperse its seeds. Seven flavanones from C. torelliana kino were identified by spectroscopic and spectrometric methods including UV, 1D and 2D NMR and UPLC-HR-MS. These were 3,4',5,7-tetrahydroxyflavanone (1), 3',4',5,7-tetrahydroxyflavanone (2), 4',5,7-trihydroxyflavanone (3), 3,4',5-trihydroxy-7-methoxyflavanone (4), (+)-(2S)-4',5,7-trihydroxy-6-methylflavanone (5), 4',5,7-trihydroxy-6,8-dimethylflavanone (6) and 4',5-dihydroxy-7-methoxyflavanone (7). All seven flavanones were active against P. aeruginosa at 50 μg/well but only one compound, (5), was active against Staphylococcus aureus. Another compound, (6), greatly increased biofilm formation by both P. aeruginosa and S. aureus. The presence and location of methyl groups in the flavanone structure were characters that determined their antibacterial and biofilm-stimulating activities. Two of the most abundant flavanones in C. torelliana kino were then tested against these two bacteria at a wider range of concentrations. Compound (1) inhibited planktonic growth of P. aeruginosa but also increased its biofilm formation. In contrast, compound (3) inhibited planktonic growth of S. aureus and, only at sub-inhibitory concentrations, reduced its biofilm formation. Both flavanones reduced adhesion of P. aeruginosa and S. aureus to Caco-2 cells and both elicited no signs of cytotoxicity in Vero-2 cells. This thesis, therefore, demonstrated that combining traditional medicinal knowledge with ecological cues can reveal natural compounds with antibiotic potential.
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Submitted in the fulfilment of the requirements of the degree of Doctor of Philosophy, University of the Sunshine Coast, 2015.
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