Bactericidal and anti-biofilm properties against Streptococcus mutans and Streptococcus sobrinus of rhinacanthin-C isolated from Rhinacanthus nasutus

Abstract

Although chlorhexidine (CHX) is commonly used as a standard antiseptic treatment in the oral cavity, its potent cytotoxicity on human periodontal tissues has been reported. Natural anti-biofilm medications could be therefore used as alternatives. In this study, the anti-bacterial and anti-biofilm activities of rhinacanthin-C (Rh-C) isolated from Rhinacanthus nasutus were determined against Streptococcus spp. (S. mutans and S. sobrinus), responsible for dental caries or tooth decay. Rh-C showed strong inhibitory and bactericidal effects against both tested species. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values for S. mutans were 4 and 8 μM, respectively, and the MIC and MBC for S. sobrinus were 2 and 4 μM, respectively. As expected, 0.12 % (w/v) CHX, being a concentration present in commercial mouth rinses, showed a complete inhibition on biofilm formation and a decrease in the bacterial biomass in the formed biofilms, shown by biofilm formation assay and confocal laser scanning microscopy, respectively. Likewise, Rh-C showed concentration dependent (0-32 µM) anti-adherent property of both streptococci on biofilm formation and a decrease in the bacterial biomass in the formed biofilms. Interestingly, the effective anti-biofilm concentrations of Rh-C did not have cytotoxic effects on human gingival fibroblast (HGF) cell viability, whereas incubation with 0.12 % (w/v) CHX decreased HGF cell viability to approximately 25 %. The results suggest the potential use of natural-based Rh-C (up to 32 µM) as an effective anti-bacterial and anti-biofilm agent in oral topical rinses or toothpastes for prevention of caries.