Activity of Ligustrum robustum (Roxb.) Blume extract against the biofilm formation and exopolysaccharide synthesis of Streptococcus mutans

Ligustrum robustum (Roxb.) Blume is utilized as a traditional Chinese herbal tea with various health benefits and protective effects. Streptococcus mutans is an important cariogenic oral bacteria species. The present study aimed to assess the influence of Ligustrum robustum extract (LRE) on the biofilm formation of S. mutans and the mechanism of its action, as well as to identify its chemical components. For chemical identification, HPLC-MS and nuclear magnetic resonance were applied and four identified phytochemicals were reported (Ligurobustoside B, Ligurobustoside N, Ligurobustoside J, and Ligurobustoside C). The dose-dependent (0.5 to 2.0 μg/μL) antimicrobial toxicity of LRE against S. mutans biofilm formation and exopolysaccharide (EPS) synthesis was evaluated by confocal laser scanning microscopy (CLSM), Crystal violet stain, and CFU counting. The microstructure of S. mutans biofilm treated with LRE was investigated both on glass coverslips and ex vivo bovine dental enamel by scanning electron microscopy (SEM). Moreover, LRE downregulated the expression of S. mutans glucosyltransferase-encoding genes gtfB, gtfC, and gtfD, and the quorum sensing (QS) factors comD and comE, suggesting its toxic mechanism. In addition, the result of CCK-8 test on human oral cells revealed an acceptable biocompatibility of LRE. These findings indicated the possible application of this daily consumed herbal tea for caries prevention.