Essential oils show specific inhibiting effects on bacterial biofilm formation

Essential oils are promising natural ingredients for the food industry due to their preservative and antimicrobial effects. We analysed the inhibiting effects of thyme, oregano and cinnamon essential oil at sublethal concentrations on biofilm formation of three biofilm forming bacterial strains. These strains of the genera Acinetobacter, Sphingomonas and Stenotrophomonas were isolated from authentic biofilms in the food industry during a previous study. Minimal inhibitory concentrations (MIC's) for growth and biofilm forming activity were tested in a 96-well microtiter plate assay. For two out of three strains we found an inhibiting effect of essential oils on biofilm formation below the minimal inhibitory concentration (MIC) for growth of these strains. In contrast, for one strain inhibition of growth and inhibition of the biofilm formation by the essential oils are initiated at the same concentration. Thyme oil was capable to inhibit the development of a biofilm at sublethal concentrations at 0.001% (w/v). This oil seems to be a more efficient specific inhibitor compared with the other tested essential oils against the biofilm formation of all tested isolates. Controls showed that the detergent used, Tween 20, was not responsible for this effect. Structural changes of the biofilms after exposition to sublethal concentrations of essential oils were confirmed by fluorescence microscopy. (C) 2013 Elsevier Ltd. All rights reserved.