Chemical Composition, Antibacterial, Antibiofilm and Synergistic Properties of Essential Oils from Eucalyptus globulus Labill . and Seven Mediterranean Aromatic Plants

Essential oils ( EO s) from Eucalyptus globulus Labill . ssp. globulus and from Mediterranean autochthonous aromatic plants – Thymus mastichina L., Mentha pulegium L., Rosmarinus officinalis L., Calamintha nepeta (L.) Savi ssp. nepeta , Cistus ladanifer L., Foeniculum vulgare L., Dittrichia viscosa (L.) Greuter ssp. viscosa – were extracted by hydrodistillation and characterized by GC ‐ FID and NMR spectroscopy. EO s were evaluated for antimicrobial properties against several bacterial strains, using diverse methods, namely, the agar disc‐diffusion method, the microdilution method, the crystal violet assay and the Live/Dead staining for assessment of biofilm formation. Potential synergy was assessed by a checkerboard method. EO s of R. officinalis and C. ladanifer showed a predominance in monoterpene hydrocarbons (> 60%); EO s of C. nepeta , M. pulegium , T. mastichina , E. globulus and F. vulgare were rich in oxygenated monoterpenes (62 – 96%) whereas EO of D. viscosa was mainly composed of oxygenated sesquiterpenes (54%). All EO s showed antimicrobial activity; M. pulegium and E. globulus generally had the strongest antimicrobial activity. EO of C. nepeta was the most promising in hampering the biofilm formation. The combinations D. viscosa / C. nepeta and E. globulus / T. mastichina were synergistic against Staphylococcus aureus . These results support the notion that EO s from the aromatic plants herein reported should be further explored as potential pharmaceuticals and/or food preservatives.