Antimicrobial and efflux pumps inhibitory activities of Eucalyptus grandis essential oil against respiratory tract infectious bacteria

Abstract

Essential oils obtained by hydrodistillation from the fresh and dry leaves of Eucalyptus grandis were analyzed by gas chromatography (GC) and gas chromatography mass spectrometry (GC-MS). The main components of the fresh leaf oil were α-pinene (29.6%), p-cymene (19.8%), 1,8-cineole (12.8%) and α-terpineol (6.4%). While, the dry leaf oil had 1,8-cineole (47.4%), limonene (13.3%), α-pinene (7.5%) and spathulenol (7.1%). The antimicrobial activities of the essential oils were tested against respiratory tract infectious microorganisms (Klebsiella pneumoniae, Staphylococcus aureus and Moraxella catarrhalis) using the microdilution-broth methods. The minimum inhibitory concentration and minimum bactericidal concentration values of the oils ranged between (0.31 to 1.25) mg/ml and (0.63 - >5) mg/ml respectively. The minimum bactericidal concentration values caused the release of cytosolic lactate dehydrogenase (membrane damage) which ranges from 8 to 24% in comparison with Triton-X-100. The accumulation of rhodamine 6G in bacterial cells showed that the essential oils were effective as efflux pump inhibitors. The results of this study support the use of the plant in folk medicine.   Key words: Eucalyptus grandis, myrtaceae, essential oil, antimicrobial activity, efflux pump, R6G.