Extracts of Eremophila longifolia inhibit the cariogenic activities of Streptococcus mutans and Streptococcus sobrinus

Plants belonging to the Eremophila genus are an integral part of the traditional medicine of indigenous Australian populations. Extracts of a number of Eremophilia species have demonstrated the presence of bioactivity, including inhibitory effects against Gram positive bacteria. The aim of the present study was to determine the activity of solvent and aqueous extracts of Eremophilia longifolia stem and leaves against Streptococcus mutans and Streptococcus sobrinus. A stem ethanol extract (SEE) demonstrated growth inhibition of the two cariogenic bacteria with a minimum inhibitory concentration (MIC) of 0.5% (w/v). Time-kill experiments indicated bactericidal activity by SEE within a 2 h period. Furthermore, this study assessed the anticariogenic activity of SEE in terms of its effect on glycolytic pH drop, viability of cells within an artificial biofilm and cell attachment to a membrane. In all experiments, SEE demonstrated significant anticariogenic activity compared with controls (p<0.05, Students t-test). SEE also compared favourably with commercial oral healthcare products. Preliminary phytochemical investigations suggested that the active components within SEE were phenolic compounds but unlikely to be flavonoids. Our study advocates SEE as a worthy candidate for further research into alternative chemotherapeutic approaches to dental caries.