The aim of this study was to investigate the effect of Thymus daenensis L., Satureja hortensis L., and Origanum vulgare L. essential oils (EOs) on the planktonic growth, biofilm formation, quorum sensing (QS), and competence system (CS) of Streptococcus pneumoniae. The anti-biofilm activity of EOs was determined by Microtiter-Plate Test (MtP) and scanning electron microscope (SEM). The QS and CS inhibitory activities were determined on the pre-grown biofilm by gene expression analysis using quantitative real-time RT-PCR. Using gas chromatography-mass spectrometry analysis, the major components of the tested EOs were detected. The MtP and SEM detected a significant inhibitory effect of the three EOs on biofilm formation at sub-minimum inhibitory concentrations (MICs). The most anti-biofilm activity was seen for T. daenensis. LuxS and pfs genes (genes involved in QS) downregulated the following treatment with MIC/2 of Thymus and Satureja EOs. Thymol, carvacrol, p-cymene, pulegone, and 1,8-cineole were the major components of the tested EOs. The used EOs seem to be good candidates for preventing biofilm formation and subsequent colonization of S. pneumoniae. This study introduced T. daenensis and S. hortensis as new anti-biofilm and QS inhibitor agents with a natural origin.
