Inhibition of listeriolysin O and phosphatidylcholine-specific production in Listeria monocytogenes by subinhibitory concentrations of plant essential oils.

Successful infection by Listeria monocytogenes is dependent upon a range of bacterial extracellular proteins including a cytolysin termed listeriolysin O and phosphatidylcholine-specific phospholipase C. Five plant essential oils--bay, clove, cinnamon, nutmeg and thyme--significantly reduced the production of listeriolysin O by L. monocytogenes. The greatest change was observed after culture with oil of thyme, which reduced haemolysis to 52.1 haemolytic units (HU)/ml compared with 99.8 HU/ml observed with the control. Oil of clove was the only oil that also significantly reduced phosphatidylcholine-specific phospholipase C activity. These changes were observed despite the oils causing no change to the final bacterial concentration or total extracellular protein concentration.