Molecular Mechanisms of Sulforaphane-Mediated Biofilm and Virulence Regulation of Streptococcus mutans
TL;DR
SFN is a bacteriostatic, multi-target anti-virulence agent against clinical S. mutans, simultaneously disabling biofilm scaffolding, acidogenesis and oxidative-stress defense without killing the cell — a profile favorable for caries control with low resistance-selection pressure.
Abstract
Dental caries is the most prevalent biofilm-mediated disease worldwide, and Streptococcus mutans is its principal etiological agent. Because broad-spectrum antiseptics disturb the commensal flora and may drive resistance, anti-virulence strategies based on safe natural products are increasingly sought. Sulforaphane (SFN), a cruciferous isothiocyanate, has documented bacteriostatic activity, but its effect on clinical S. mutans isolates and on the molecular determinants of cariogenicity is poorly defined. Twenty-five clinical S. mutans isolates from carious lesions and dental plaque were challenged with SFN (Sigma-Aldrich) in 0.5% DMSO. Antibacterial activity was assessed by agar-well diffusion and broth microdilution (MIC/MBC) with chlorhexidine as positive control and triphenyltetrazolium chloride as viability indicator. Biofilm formation and the anti-biofilm effect were quantified by the crystal-violet microtiter assay. Expression of gtfC (EPS synthesis), ldh (acidogenicity) and sodA (oxidative-stress defense) was measured by qRT-PCR at sub-MIC (625 µg/mL) using 16S rRNA as reference gene (2-ΔΔCt method). SFN showed dose-dependent activity (zone of inhibition 17.8 ± 0.55 mm at 5%). The MIC was 1250 µg/mL for all isolates; no killing occurred up to 2500 µg/mL (MBC > 2500 µg/mL; MBC/MIC > 2), indicating a bacteriostatic, anti-virulence action. Eighteen isolates were strong, 3 moderate and 4 weak biofilm producers; SFN suppressed biofilm formation dose-dependently (p < 0.0001). At sub-MIC, SFN down-regulated gtfC by ≈99.5% (≈195-fold), sodA by ≈97.2% (≈35-fold) and ldh by ≈97.0% (≈34-fold) (all p < 0.01). gtfC/sodA expression was strongly correlated (r = 0.72), and PCA cleanly separated treated from untreated profiles (PC1 = 70.8%). SFN is a bacteriostatic, multi-target anti-virulence agent against clinical S. mutans, simultaneously disabling biofilm scaffolding, acidogenesis and oxidative-stress defense without killing the cell — a profile favorable for caries control with low resistance-selection pressure.