Potent antimicrobial compounds from the Australian medicinal plant 'Eremophila duttonii' (F. Muell.) (Myoporaceae)

Abstract

The Australian medical ethnobotanical literature reveals a number of citations concerning the arid adapted genus 'Eremophila' describing traditional uses suggestive of antimicrobial activity, e.g. in the topical treatment of minor wounds, ocular and otonasopharangeal complaints [1]. In broad based screening programmes examining antibacterial activity of native indigenous medicinal plants, extracts of the species 'Eremophila duttonii' (F. Muell.) (Myoporaceae) have been consistently shown to produce the greatest levels of activity amongst all plants studied, both by this group and elsewhere [2;3]. The genus is characterised phytochemically by the acumulation of structurally and stereochemically unusual terpenoids, unique to the plant kingdom [1]. Here we report on the isolation and identification of three compounds from a petroleum extract of 'Eremophila duttonii' exhibiting antibacterial activity against 'Staphylococcus aureus' and 'Candida albicans'. Active compounds were detected and isolated using a combination of TLC, bioautography and flash column chromatography. Structural assignments for active compounds were performed using 2-dimensional ¹³C and ¹H NMR spectroscopy. Major active compounds were identified as the serrulatane diterpenes, (serrulat-14-en-7,8,20-triol [I] and serrulat-14-en-3,7,8,20-tetraol [II]) and a novel furanosesquiterpene (11-hydroxy ngaione [III]). Minimum inhibitory concentrations (MIC) of the isolated compounds were determined for inhibition of three gram positive bacteria commonly associated with dermal and upper respiratory tract infections. [I] produced the lowest MIC (23μg/ml) against 'Streptococcus pneumoniae'.