Screening of aqueous extracts of medicinal herbs for antimicrobial activity against oral bacteria

BACKGROUND: Dental caries is considered to be a preventable disease, and various antimicrobial agents have been developed for the prevention of dental diseases; however, many bacteria show resistance to existing agents. In this study, 14 medicinal herbs were evaluated for antimicrobial activity against five common oral bacteria as a screen for potential candidates for the development of natural antibiotics. METHODS: Aqueous extracts of medicinal herbs were tested for activity against Enterococcus faecalis, Actinomyces viscosus, Streptococcus salivarius, Streptococcus mutans, and Streptococcus sanguis grown in brain heart infusion (BHI) broth. A broth microdilution assay was used to determine the minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC). A disk diffusion assay was performed by inoculating bacterial cultures on BHI agar plates with paper disks soaked in each of the medicinal herb extracts. Inhibition of the synthesis of water-insoluble glucans by S. mutans was also investigated. RESULTS: The aqueous extracts of many of the 14 medicinal herbs demonstrated antimicrobial activity against the five types of pathogenic oral bacteria. The extracts of Sappan Lignum, Coptidis Rhizoma, and Psoraleae Semen effectively inhibited the growth of oral bacteria and showed distinct bactericidal activity. The extracts of Notoginseng Radix, Perillae Herba, and Psoraleae Semen decreased the synthesis of water-insoluble glucans by the S. mutans enzyme glucosyltransferase (GTase). The present study is the first to confirm the antimicrobial activity of the extract of Sappan Lignum against all five species of oral bacteria strains. CONCLUSION: These results suggest that certain herbal medicines with proven antimicrobial effects, such as Sappan Lignum and Psoraleae Semen, may be useful for the treatment of dental diseases.