α-Cyclodextrin encapsulation enhances antimicrobial activity of cineole-rich essential oils from Australian species of Prostanthera (Lamiaceae)

Abstract

Highly chemically variable cineole-rich essential oils were produced from cultivated specimens of the genus Prostanthera (Lamiaceae), currently taxonomically assigned to P. ovalifolia, P. rotundifolia, P. incisa and P. lasianthos. Essential oils were chemically characterised using GC-MS and NMR. The mean inhibitory concentrations against Gram-positive and Gram-negative bacterial species were measured using a microtitre plate broth dilution assay.  A selection of these oils were further assayed for antimicrobial activity after being encapsulated at a 1:1 molar ratio using α-cyclodextrin. Cineole-rich essential oils are chemically differentiated by the character of the sesquiterpene oxides cis-dihydroagarofuran and kessane; and the sesquiterpene alcohols globulol, prostantherol and ledol. Within the wider context of common essential oils, this selection of essential oils from Prostanthera demonstrated relatively low inhibitory concentrations (high antimicrobial activity), particularly against Gram-positive organisms. When some of these oils were encapsulated in α-cyclodextrin the antimicrobial activity was generally enhanced by two to four-folds. This enhancement may be a result of encapsulation with reduced evaporation during the assay and emulsion formation which may facilitate delivery to bacterial species. The use of cyclodextrins as a feed and formulation additive should be considered within the context of the antimicrobial activity of cineole-rich essential oils from Prostanthera.