Inhibitory Mechanism of the Oregano Essential Oil on the Growth and Biofilm Formation of Streptococcus mutans

Objective: The objective was to determine the in vitro antibacterial effect of the Oregano essential oil (OEO) on Streptococcus mutans (S mutans) and to explore the anti-biofilm effect and mechanism of OEO. Methods: The in vitro antibacterial susceptibility of OEO was detected by the growth curve and colony-forming unit (CFU) counting method. Confocal laser scanning microscope (CLSM) and crystal violet assay were used to explore the anti-biofilm activity of OEO, and the anti-biofilm mechanism was explored from the aspects of extracellular polysaccharides (EPS) synthesis, acidoid, and extracellular DNA changes. OEO was characterized using a gas chromatography-mass spectrometer and cytotoxicity was tested using a cell counting Kit-8 assay. Results: The minimum inhibitory concentration of OEO was 0.25 mg/mL, and the CFU showed the ability of OEO to kill planktonic bacteria. Scanning electron microscopy and CLSM showed that OEO could hinder the development of biofilms, harm bacteria, and eliminate mature biofilms by changing EPS synthesis and pH value. Carvacrol in OEO played a major role in biological antibacterial activity. OEO had low cytotoxicity and good biocompatibility. Conclusions: OEO had excellent antibacterial and anti-biofilm effects on the key virulence traits of S mutans, indicating potential prospects in preventing dental caries.