The antimicrobial potential of Exocarpos cupressiformis Labill. (Santalaceae): a bioassay-guided screening and fractionation of Australia’s ethnobotanically significant flora

Australia’s First Peoples medicine practices were suppressed after colonisation, leading to their decline in use. The tree genus Exocarpos (ballarts) was used widely as a curative (styptic, poultice and astringent) and is still used in smoking ceremonies for purification. The current study aims to use a bioassay-guided screening approach to evaluate 46 plant taxa of ethnobotanical interest and provide a candidate for follow up fractionation; use bioassay guided fractionation to purify the active compound; and elucidate the identity and measure the antimicrobial potential of the active component. Disc Diffusion Assays guided screening and fractionation, where 31 of 46 taxa demonstrated some antimicrobial activity. Exocarpos cupressiformis Labill. (cherry ballart) showed a particular propensity for activity and was chosen for further fractionation. TLC-bioautography was then used to inform column chromatography to purify the antimicrobial fraction. QQQ-MS was used in scan and product ion mode with NMR (1H, 13C, HMBC, HSQC, COSY, NOESY) to fully assign structure. Minimum inhibitory concentrations (MICs) were measured by an agar dilution method. Ultimately, aromadendrin-(1→7)-α-L-rhamnopyranoside, a known antimicrobial flavonoid, was a key component responsible for the activity of the extract and was measured to have a MIC of 32 mg L–1 (Streptococcus pyogenes) and 64 mg L–1 (Streptococcus dysgalactiae). This provides a mechanism by which some of the medicinal uses of E. cupressiformis by Australian First Peoples can be explained.