Inhibition of antibacterial activity of essential oils by tween 80 and ethanol in liquid medium.

The emulsifying agents used to disperse essential oils in culture media can interfere in the estimation of essential oils antimicrobial activity; we showed in this study that 0.2% Agar suspension was sufficient to obtain a stable dispersion of oregano and clove essential oils in liquid media comparable to the dispersions obtained with tween 80 (0.25%) or ethanol (0.2%). The dispersion with agar was as homogenous as a true solution in absolute ethanol. Furthermore, minimal inhibitory concentration and minimal lethal concentration for different bacterial species in presence of agar were significantly lower than those observed in presence of tween 80 or ethanol. This demonstrates the fact that solvents and detergents often used in antimicrobial studies significantly decreases the antibacterial activity of essential oils.