Antimicrobial activity of eucalypt resin

Motahareh Nobakht, Stephen Trueman, Peter Brooks, Helen Wallace, Mohammad Katouli

University of the Sunshine Coast, Maroochydore DC 4558, Australia

Long-term use of antibiotics is limited by the rapid development of bacterial resistance. Natural compounds from plants can play an important role in the development of novel antibiotics and, therefore, new treatments for infectious diseases. Eucalypt trees exude a polyphenol- and tannin-rich gum (kino) that has been used medicinally in Australia for thousands of years for curing ailments such as bowel inflammation, diarrhoea, sores, skin lesions, scabies, cramps, sore throat and cough. However, kino has not been analysed to identify compounds responsible for biological activity. We screened extracts of kino from three eucalypts against six microorganisms: three Gram-negative bacteria (Escherichia coli, Salmonella typhimurium and Pseudomonas aeruginosa); two Gram-positive bacteria (Staphylococcus aureus and Bacillus cereus); and a unicellular fungus (Candida albicans). We identified one eucalypt species with kino that had high antimicrobial activity and found that its extracts had particularly high activity against Pseudomonas aeruginosa. We isolated and purified nine compounds from the kino of this species, and found that some compounds were active against 14 non-replicate strains of P. aeruginosa obtained from Queensland hospitals. Active compounds were not cytotoxic to Caco-2 (human epithelial colorectal adenocarcinoma) cells. Our results demonstrate the benefits of using an ethnobotanically guided approach to screen natural compounds for use as antimicrobial agents. The importance of these findings for potential antimicrobial functionality in this plant will be discussed.