Selected plant essential oils inhibit biofilm formation and luxS- and pfs-mediated quorum sensing by Escherichia coli O157:H7

Enterohemorrhagic Escherichia coli O157:H7 (EHEC) causes food-borne outbreaks worldwide and the bacteria form antimicrobial-tolerant biofilm. We investigated the abilities of Thymus daenensis and Satureja hortensis essential oils (EOs) to inhibit bacterial growth, biofilm formation and quorum sensing (QS) by E. coli O157:H7. The tested EOs were isolated from plant material by hydrodistillation and analysed under chromatography-mass spectrometry. The antibacterial and anti-biofilm activities of the EOs were determined by microdilution broth and microtitre-plate (MtP) tests, respectively. The QS inhibitory (QSI) potential was examined by inhibition of swimming and swarming motility at sub-MIC levels. Real-time quantitative reverse transcription PCR (RT-qPCR) was used to determine the expression of QS-system-related genes. The MICs for T. daenensis and S. hortensis EOs against EHEC were 3·12 and 6·25 μg ml⁻¹, respectively and the MBCs were 6·25 and 12·5 μg ml⁻¹, respectively. The MtP test showed a significant (P < 0·05) inhibitory and disruptive effect on both EOs for EHEC biofilm formation at MIC/2 (1·56 μg ml⁻¹ for T. daenensis; 3·12 μg ml⁻¹ for S. hortensis) and MIC/4 (0·78 μg ml⁻¹ for T. daenensis; 1·56 μg ml⁻¹ for S. hortensis) concentrations. Gene expression analysis revealed significant down-regulation of luxS and pfs following treatment with MIC/2 concentrations. The results of the present research point to the promising antibacterial, anti-biofilm and anti-QS potential of T. daenensis and S. hortensis EOs against E. coli O157:H7.