Traditional medicine and ecological cues can both help to reveal bioactive natural\ncompounds. Indigenous Australians have long used kino from trunks of the eucalypt tree,\nCorymbia citriodora, in traditional medicine. A closely related eucalypt, C. torelliana, produces\na fruit resin with antimicrobial properties that is highly attractive to stingless bees. We tested\nthe antimicrobial activity of extracts from kino of C. citriodora, C. torelliana C. citriodora, and\nC. torelliana against three Gram-negative and two Gram-positive bacteria and the unicellular fungus,\nCandida albicans. All extracts were active against all microbes, with the highest activity observed\nagainst P. aeruginosa. We tested the activity of seven flavonoids from the kino of C. torelliana against\nP. aeruginosa and S. aureus. All flavonoids were active against P. aeruginosa, and one compound,\n(+)-(2S)-40,5,7-trihydroxy-6-methylflavanone, was active against S. aureus. Another compound,\n40,5,7-trihydroxy-6,8-dimethylflavanone, greatly increased biofilm formation by both P. aeruginosa\nand S. aureus. The presence or absence of methyl groups at positions 6 and 8 in the flavonoid A ring\ndetermined their anti-Staphylococcus and biofilm-stimulating activity. One of the most abundant and\nactive compounds, 3,40,5,7-tetrahydroxyflavanone, was tested further against P. aeruginosa and was\nfound to be bacteriostatic at its minimum inhibitory concentration of 200 g/mL. This flavanonol\nreduced adhesion of P. aeruginosa cells while inducing no cytotoxic effects in Vero cells. This study\ndemonstrated the antimicrobial properties of flavonoids in eucalypt kino and highlighted that\ntraditional medicinal knowledge and ecological cues can reveal valuable natural compounds.
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