Antimicrobial activity of limonene enantiomers and 1,8-cineole alone and in combination

Two common essential oil constituents, 1,8-cineole and limonene, were assessed for antimicrobial activity, using the minimum inhibitory concentration (MIC) microtitre plate method against eight organisms. The limonene enantiomers, i.e. (+), (-) and the racemate, singularly and in combination (1:1) with 1,8-cineole, were investigated to establish possible interactions. The MIC values were in the ranges 3-27 mg/ml for (+)-limonene; 2-27 mg/ml for (-)-limonene and 23 mg/ml for 1,8-cineole, depending on the pathogen studied. The combinations, when investigated in a 1:1 ratio, mostly indicated reduced activity. Using various ratios of limonene and 1,8-cineole, the specific interaction was further investigated against Staphylococcus aureus (Gram-positive), Pseudomonas aeruginosa (Gram-negative) and a yeast, Cryptococcus neoformans. A figurative representation of the results using isobologram construction indicated that, depending on the ratio and specific enantiomer, an additive, synergistic or antagonistic interaction may be observed. Copyright (c) 2007 John Wiley & Sons, Ltd.