Antimicrobial and Gene Expression Modulation Effects of Ethanolic Extract of Propolis on <i>Streptococcus mutans</i> Adherence and Biofilm Formation

This study aimed to investigate the effect of an ethanolic extract of propolis (EEP) on the expression of genes associated with the adherence of Streptococcus mutans ( S. mutans ) to the tooth surface. A total of 10 S. mutans isolates, obtained from individuals with and without dental caries, were included in the study and identified using biochemical methods. The biofilm-forming ability was assessed using the microtiter plate method. Then, the antibacterial effect of the EEP was evaluated through the agar well diffusion and broth microdilution assay. Real-time polymerase chain reaction as employed to investigate the impact of sub-MIC (minimum inhibitory concentration) of EEP on the expression of gtfB, gtfC, gbpB, and ftf genes. In the standard microtiter plate method, 30% of the isolates produced strong biofilms ( n = 3), 50% produced moderate biofilms ( n = 5), and 20% produced weak biofilms ( n = 2). The EEP demonstrated significant antimicrobial activity against S. mutans , with concentrations ranging from 30% to 100% (1.2–4 mg/mL) producing a notable increase in inhibition zone size compared with the control ( p &lt; 0.05), as measured by the well diffusion method. The MIC for all strong biofilm-producing isolates was found to be 1.2 mg/mL (30% EEP concentration). Gene expression analysis revealed a reduction in the expression of all target genes in the presence of EEP. Among the genes studied, gtfB exhibited the most substantial reduction with a 4.63-fold decrease ( p &lt; 0.001), while ftf showed the smallest decrease at 2-fold ( p &lt; 0.05). These findings suggest that the EEP has a notable antimicrobial effect, particularly against biofilm-producing strains. Moreover, the observed downregulation of key genes involved in biofilm formation and adhesion ( gtfB, gtfC, ftf, and gbpB ) highlights the potential of propolis as a natural agent for combating S. mutans infections and inhibiting biofilm formation.