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

Abstract

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, Corymbia citriodora, in traditional medicine. A closely related eucalypt, C. torelliana, produces a fruit resin with antimicrobial properties that is highly attractive to stingless bees. We tested the antimicrobial activity of extracts from kino of C. citriodora, C. torelliana × C. citriodora, and C. torelliana against three Gram-negative and two Gram-positive bacteria and the unicellular fungus, Candida albicans. All extracts were active against all microbes, with the highest activity observed against P. aeruginosa. We tested the activity of seven flavonoids from the kino of C. torelliana against P. aeruginosa and S. aureus. All flavonoids were active against P. aeruginosa, and one compound, (+)-(2S)-4',5,7-trihydroxy-6-methylflavanone, was active against S. aureus. Another compound, 4',5,7-trihydroxy-6,8-dimethylflavanone, greatly increased biofilm formation by both P. aeruginosa and S. aureus. The presence or absence of methyl groups at positions 6 and 8 in the flavonoid A ring determined their anti-Staphylococcus and biofilm-stimulating activity. One of the most abundant and active compounds, 3,4',5,7-tetrahydroxyflavanone, was tested further against P. aeruginosa and was found to be bacteriostatic at its minimum inhibitory concentration of 200 µg/mL. This flavanonol reduced adhesion of P. aeruginosa 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.

Keywords: Eucalyptus; Pseudomonas aeruginosa; Tetragonula; antibiotic resistance; antimicrobial activity; cytotoxicity; ethnobotany; natural products; stingless bees; traditional medicine.

Figure 1

Structures of seven flavonoids isolated from the kino of Corymbia torelliana.

Figure 2

Antimicrobial activity of 400 μg of kino extract from (A) Corymbia citriodora, (B) C. torelliana × C. citriodora, and (C) C. torelliana against six microorganisms. Zones of inhibition are presented as mean + S.E. (n = 21 trees for C. citriodora and the hybrid; n = 3 trees for C. torelliana). Means among the three kino extracts do not differ significantly (ANOVA, p > 0.05).

Figure 3

Biofilm formation in the presence of kino extracts from Corymbia citriodora (Cc), C. torelliana × C. citriodora (Ct × Cc), and C. torelliana (Ct) by (A) Pseudomonas aeruginosa, (B) Escherichia coli, (C) Salmonella typhimurium, (D) Staphylococcus aureus, and (E) Bacillus cereus. Optical densities at 570 nm wavelength (OD_570nm) are presented as mean + S.E. (n = 3 trees for Cc, Ct × Cc, and Ct; n = 3 for the kino-free control). Means among the three kino extracts do not differ significantly (ANOVA, p > 0.05).

Figure 4

Biofilm formation (OD_570nm) in the presence of 100 μg of 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).

Figure 5

Biofilm formation of Pseudomonas aeruginosa in the presence of 200 µg, 100 µg, and 50 µg (final mass) of 3,4′,5,7-tetrahydroxyflavanone from kino exudate of Corymbia torelliana. Optical densities at 570 nm wavelength (OD_570nm) are presented as mean + S.E. Means with different letters are significantly different (ANOVA and Tukey’s HSD test; p < 0.05; n = 3).