Antibacterial Properties of Flavonoids from Kino of the Eucalypt Tree, Corymbia torelliana.

Traditional medicine and ecological cues can both help to reveal bioactive natural compounds. Indigenous Australians have long used kino from trunks of the eucalypt tree, , in traditional medicine. A closely related eucalypt, , produces a fruit resin with antimicrobial properties that is highly attractive to stingless bees. We tested the antimicrobial activity of extracts from kino of , × , and against three Gram-negative and two Gram-positive bacteria and the unicellular fungus, . All extracts were active against all microbes, with the highest activity observed against . We tested the activity of seven flavonoids from the kino of against and . All flavonoids were active against , and one compound, (+)-(2S)-4',5,7-trihydroxy-6-methylflavanone, was active against . Another compound, 4',5,7-trihydroxy-6,8-dimethylflavanone, greatly increased biofilm formation by both and . The presence or absence of methyl groups at positions 6 and 8 in the flavonoid A ring determined their anti- and biofilm-stimulating activity. One of the most abundant and active compounds, 3,4',5,7-tetrahydroxyflavanone, was tested further against and was found to be bacteriostatic at its minimum inhibitory concentration of 200 µg/mL. This flavanonol reduced adhesion of cells while inducing no cytotoxic effects in Vero cells. This study demonstrated the antimicrobial properties of flavonoids in eucalypt kino and highlighted that traditional medicinal knowledge and ecological cues can reveal valuable natural compounds.