Chemical composition and antimicrobial properties of essential oils of three Australian Eucalyptus species

This study investigated the chemical composition and antimicrobial properties of the essential oils of three common Australian Eucalyptus specie, namely E. oilida, E. staigeriana and E. dives. Essential oils were obtained by steam distillation of fresh leaves of the plant and their chemical composition was determined by gas chromatography coupled with mass spectrometry. Antimicrobial activity of the essential oils was tested by the agar disc diffusion method against a range of foodborne pathogens and spoilage bacteria and yeast. Yield of essential oils ranged from 2.13 to 3.12% on a fresh weight basis, with E. oilida and E. staigeriana giving the highest and lowest yield, respectively. A total of 24 compounds were identified from the essential oil of E. dives, with the dominant compounds being piperitone (40.5%), '-phellandrene (17.4%), p-cymene (8.5%) and terpin-4-ol (4.7%). For E. staigeriana, 29 compounds were identified with 1,8-cineole (34.8%), neral (10.8%), geranial (10.8%), '-phellandrene (8.8%) and methyl geranate (5.2%) being the dominant ones. In contrast, a single compound, (E)-methyl cinnamate, accounted for 99.4% of the essential oils of E. oilda, although 19 compounds were identified. The essential oils displayed a variable degree of antimicrobial activity with E. staigeriana oil showing the highest activity. In general, Gram-positive bacteria were found to be more sensitive to the essential oils than Gram-negative bacteria. S. aureus was the most sensitive strain while P. aeruginosa was the most resistant. The particularly high antimicrobial activity of E. staigeriana essential oil, especially against S. aureus, suggests that it may have a promising prospect for applications in food and potentially pharmaceutical products.