Antimicrobial Effects And Mechanism Of Action Of Selected Plant Essential Oils

The aim of this study was to evaluate the antimicrobial efficacy of selected essential oil (EO) and their combinations against four food-related microorganisms. Ten EOs were initially screened against Escherichia coli, Staphylococcus aureus, Bacillus subtilis and Saccharomyces cerevisiae using agar disc diffusion and broth dilution methods. The highest efficacy against all the tested strains was shown when testing the oregano EO. EOs of basil and bergamot were active against the Gram-positive bacteria, while perilla EO strongly inhibited the growth of S.cerevisiae. The checkerboard method was then used to investigate the antimicrobial efficacy of EO combinations by means of the fractional inhibitory concentration index (FICI). Based on an overall consideration of antimicrobial activity, organoleptic impact and cost, four EO combinations were selected and their MIC values were listed as follows:oregano-basil (0.313-0.313 [μL/mL) for E. coli, basil-bergamot (0.313-0.156μL/mL) for S. aureus, oregano-bergamot (0.313-0.313μL/mL) for B. subtilis and oregano-perilla (0.313-0.156μL/mL) for S. cerevisiae. The chemical components of selected EOs were then analyzed by GC/MS. Phenols and terpenes were the major antimicrobial compounds in oregano and basil EOs. The dominant active components of bergamot EO were alcohols, esters and terpenes. For perilla EO, the major active constituents were mainly ketones.Furthermore, the mechanisms of the antimicrobial action of EO combinations were studied by the measurement of conductivity and the release of cell constituents, the electronic microscopy observations of the cells and the changes in the percentage of principal fatty acids of cells. The results demonstrated that all EO combinations destroyed the integrity of cell membrane, resulting in the death of microorganisms.Finally, we test the ability of nine solubilizers to increase EOs solubility and the effect of selected solubilizer on the antimicrobial activity of EO combinations by checkerboard method. The results indicated that the effect of HP-β-CD on increasing EOs solubility was most obvious. The addition of HP-β-CD could not only decrease the usage of EO combinations significantly, but also enhance the effects of EO combinations on microorganisms.