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.