Impedance measurements to study the antimicrobial activity of essential oils from Lamiaceae and Compositae

A wide range of essential oils from sage. mint, hyssop, camomile and oregan o were tested for their inhibitory effects against nine strains of Gram-neg ative bacteria and six strains of Gram-positive bacteria. Three principles were used in describing the antimicrobial effects of the essential oils: th e overall antimicrobial activity determined by use of an impedometric metho d, the bactericidal effect determined as colony forming units after exposur e to the essential oils, and the number of apparent dead cells determined a fter further enrichment. The data obtained indicate that while the essentia l oils of sage, mint., hyssop and camomile had generally a bacteriostatic a ctivity, the essential oil from oregano appeared to be bactericidal at conc entrations above 400 ppm, probably because of high contents in phenolic com pounds, For the other essential oils, the chemical analysis was unable to e xplain the antimicrobial effect. The bacteriostatic activity was more marke d against Gram-positive bacteria; in contrast. the bactericidal activity wa s greatest against Gram-negative bacteria. The most sensitive strain was Es cherichia coli O157:H7 and, of the Grain-positive species even at the lowes t oil concentrations, Listeria innocua was the most sensitive. The data obt ained from the study of the bactericidal effect of oregano essential oil in dicated that the major part of the species was irreversibly inactivated, i. e. they could not be revived by enrichment. (C) 2001 Elsevier Science B.V. All rights reserved.