## 4 Discussion
[PERSON] and [PERSON] (1997) using the broth microdilution method, report the MIC for tea tree oil against MRSA at concentration of 0.08% (_v/v_), which was less than that in our study, however the findings cannot be compared because no data from the sensitivity tests are provided [48]. [PERSON] et al. (1996) report the tea tree oil MIC and MBC against _S. aureus_ to be 0.12-0.5% (_v/v_) and 0.25-2% (_v/v_) respectively [37], whereas [PERSON] (2000) reports that the tea tree oil MIC against MRSA was 0.25% (_v/v_) without providing the sensitivity test details [49]. Both studies, presented similar results with our findings (Table 3), but again there are differences in the methodological approaches. In the present study the MIC values reported for origanum oil, thyme oil and tea tree oil against _S. aureus_ (MRSA, reference strains) in general are not different from those reported by other researchers, but still there is the methodological diversity difference. Concerning the reported MIC values for basil oil in our study they were much different denoting much less effective antimicrobial activity against drug-resistant clinical isolates. In the aforementioned literature the methods used for the determination of the antimicrobial activity of the essential oils, included agar well diffusion assays, paper disk diffusion assays, agar dilution assays and broth microdilution and macrodilution methods. Even surfactants such as Tween 20 (polyoxyethylene 2-sorbitan monolaurate) or Tween 80 (polyson80) are variable in different papers, even though they are used only to enhance oil solubility without contributing to the antimicrobial activity. Regarding the employed essential oils, even when identical names are reported it is uncertain whether their main constituents are exactly the same. Several factors such as the plant growth stage, environmental and ecological conditions, harvesting and processing conditions, and plant endogenous factors can affect the composition of the extracted essential oil [39]. Nevertheless, our results like most of the findings reported by other researchers indicate that essential oils are likely to act as alternative antimicrobial agents against some multi-drug resistant Gram-positive isolates. It has to be emphasized that Gram-positive bacteria are not considered to be as resistant as the Gram-negatives [50]. To a large extent this trait is attributed to differences in their cell wall structure which is less complex in Gram-positives than in Gram-negatives, allowing the penetration of antimicrobial compounds, such as the phenolic compounds (e.g., thymol, carvacrol, eugenol) of the essential oils [18; 51]. The cell wall of the Gram-positive bacteria permits hydrophobic molecules to get into the cells and act on the cell wall and the cytoplasm. Apparently, the potential antimicrobial activity of the essential oils' compounds derives from their ability to disrupt the bacterial cell wall and the cytoplasmic membrane, consequently leading to cell lysis and leakage of intracellular compounds [52]. Taking into consideration that an intact cell wall is vital for the bacterium survival, any changes in the permeability of the cell wall and cytoplasmic membrane affect the bacterial propagation. In the presence of antibacterial compounds in their environment, bacteria are forced to alter the synthesis of fatty acids and membrane proteins in order to survive by modifying the permeability of their cell wall [53; 54]. Essential oils particularly those rich in phenolics have the ability to penetrate the phospholipids layer of the bacterial cell wall, bind to proteins and block their functions. Existing data on the essential oils antimicrobial activity show that their potency depends on several biochemical and structural mechanisms that take place at various sites of the bacterial cell and more research in this direction is needed to explore thoroughly the antimicrobial potential of plant essential oils. H.S.