Essential oils are well-known for their antimicrobial activity against different plant and human pathogenic microorganisms. The results of the most commonly used antimicrobial assays are very different; sometimes their reliability is questionable, therefore standardized methods need to be used to solve this problem. The present study aims at the phytochemical characterization of some essential oils (thyme, lavender, eucalyptus, spearmint and cinnamon) that are important from the therapeutic and economic aspects and the optimized microbiological investigation of the effect of essential oils on human and plant pathogenic microorganisms. The chemical composition of the essential oils was analysed with thin-layer chromatography (TLC) and their composition was controlled by gas chromatography (GC). The antibacterial effect was investigated using the TLC-bioautographic method. The solvents applied in TLC developing systems were also tested. Our results showed that toluene, ethyl acetate, ethanol and chloroform as solvents used in the assay had no inhibiting effect on the test bacteria. The antibacterial activity of thyme, lavender and cinnamon oils and their main components (thymol, carvacrol, linalool, eugenol) was observed in the case of two plant pathogenic bacteria (Xanthomonas campestris pv. vesicatoria and Pseudomonas syringae pv. phaseolicola) and some human pathogens (Staphylococcus epidermidis, S. saprophyticus and two strains of S. aureus, including one methicillin-resistant strain). On the whole, the antibacterial activity of essential oils can be related to their most abundant components, but the effect of the minor components should also be taken into consideration. Direct bioautography is more cost-effective and compares better with traditional microbiological laboratory methods (e.g. disc-diffusion, agar-plate technique).

The aim of the present study was the chemical characterization of some traditionally used and therapeutically relevant essential oils (thyme, eucalyptus, cinnamon bark, clove, and tea tree) and the optimized microbiological investigation of the effect of these oils on clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible S. aureus (MSSA). The chemical composition of the oils was analyzed by TLC, and controlled by gas chromatography (GC) and gas chromatography/mass spectrometry (GC/MS). The antibacterial effect was investigated using a TLC-bioautographic method. Antibacterial activity of thyme, clove and cinnamon oils, as well as their main components (thymol, carvacrol, eugenol, and cinnamic aldehyde) was observed against all the bacterial strains used in this study. The essential oils of eucalyptus and tea tree showed weak activity in the bioautographic system. On the whole, the antibacterial activity of the essential oils could be related to their most abundant components, but the effect of the minor components should also be taken into consideration. Direct bioautography is more cost-effective and better in comparison with traditional microbiological laboratory methods (e.g. disc-diffusion, agar-plate technique).