Cock I Ea,b*
aBiomolecular and Physical Sciences, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Brisbane, Queensland 4111, Australia.
bEnvironmental Futures Centre, Nathan Campus, Griffith University, 170 Kessels Rd, Nathan, Brisbane, Queensland 4111, Australia.
Introduction: Australian Leptospermum species had roles as traditional bush medicines for Australian Aborigines, including uses as antiseptic agents. Despite this ethnobotanical usage, the antimicrobial properties of Leptospermum spp. have not been rigorously studied. Methods: The antimicrobial activity of methanolic extracts of Leptospermum bracteata and Leptospermum juniperium leaves and flowers were investigated by disc diffusion and growth time course assays against a panel of bacteria and fungi. Toxicity was determined using the Artemia franciscana nauplii bioassay. Results: L. bracteata leaf and flower extracts inhibited the growth 36% and 50% of the bacteria tested respectively. Both Gram-positive and Gram-negative bacteria were equally affected by L. bracteata leaf extract. Of the 11 Gram-negative bacteria tested, 4 (36%) were inhibited by L. bracteata leaf extract compared with 1 of the 3 Gram-positive bacteria tested (33%). The L. bracteata flower extract was more selective with Gram-positive bacteria being more susceptible. Of the 14 Gram-negative bacteria tested, 4 were inhibited by L. bracteata flower extract (29%) compared with 100% of the Gram-positive bacteria tested. L. juniperium extracts were less effective antibacterial agents than were the L. bracteata extracts. L. juniperium leaf extract inhibited the growth of 36% of the bacteria tested compared to only a single bateria (7%) being inhibited by the flower extract. Gram-positive bacteria (100% inhibited) were more susceptible to L. juniperium leaf extract than were Gram negative bacteria (18% inhibited). No growth inhibitory activity was seen for any Leptospermum extract towards any of the fungi tested. The antibacterial activity of L. bracteata flower and L. juniperium leaf extracts was further investigated by growth time course assays which showed significant growth inhibition in cultures of Bacillus cereus, Bacillus subtilis, Pseudomonas fluorescens and Aeromonas hydrophilia within 1 h. Conclusions: The low toxicity of these Leptospermum extracts and their inhibitory bioactivity against a panel of bacteria validate Australian Aboriginal usage of Leptospermum bracteata and Leptospermum juniperium as antiseptic agents and confirms their medicinal potential.
Keywords: Leptospermum bracteata, Leptospermum juniperium, Australian plants, antibacterial activity, medicinal plants, toxicity.