Seven plants generally used for traditional oral care namely, Barleria albostellata, Cotyledon orbiculata, Dichrostachys cinerea, Heteropyxis natalensis, Carpobrotus edulis, Zanthoxylum capense and Dodonaea viscosa were investigated for antimicrobial activity and safety. Four pathogenic microorganisms, Actinomyces israelii, Streptococcus mutans, Prevotella intermedia and Candida albicans, were selected that represented the diversity of microbial flora in the oral cavity.
No evidence could be found in the literature on the activity of the selected plant extracts against A. israelii, P. intermedia and S. mutans. Only H. natalensis exhibited activity against the Gram-positive microorganisms, A. israelii and S. mutans; minimum inhibitory concentration (MIC) was found to be 0.88 mg/ml and 1.82 mg/ml respectively. The MIC against the Gram-negative bacteria, P. intermedia was found to be 3.13 mg/ml. Dichrostachys cinerea exhibited activity towards a drug-sensitive stain of C. albicans (MIC of 10.71 mg/ml) and against a drug-resistant (polyene and azole resistant) strain of C. albicans (MIC of 10.42 mg/ml). Dichrostachys cinerea was the least toxic to both the Kidney epithelial cells of the African Green Monkey (Vero) and Human laryngeal epidermoid carcinoma cells (HEp-2) cell lines with 50% inhibitory concentrations (IC50) of 204 ± 0.13 μg/ml and 224 ± 0.1 μg/ml respectively. Heteropyxis natalensis was selected for further study as it exhibited moderate cytotoxicity (IC50 of 33.66 ± 0.04 μg/ml) on HEp-2 cells and the best antibacterial activity as compared to the other plant extracts investigated in this study. When H. natalensis was incorporated in a synergistic combination with the essential oils Melaleuca alternifolia (Tea tree) and Mentha piperita (peppermint); a fourfold reduction in the MIC of A. israelii was exhibited.
Gingivitis, the infection of the gums, induces inflammation. To attract the white blood cell, leukocytes, to the site of infection; a chemokine known as Interluekin-8 (IL-8) is released. These cytokine, IL-8, levels were not reduced when the extract of H. natalensis was utilized to prevent the interaction of A. israelii with the epithelial cells, HEp-2. A Scanning Electron Microscopy (SEM) study to determine bacterial adhesion in the presence of H. natalensis indicated that the plant extract interferes with pellicle formation and glucan binding of S. mutans to the enamel surface of the tooth.
Five known compounds were identified from the ethanolic extract of H. natalensis leaves and twigs. The compounds were identified as Aurentiacin A (1), Cardamomin (2), 5-hydroxy-7-methoxy-methylflavanone (3), Quercetin (4) and 3,5,7-trihydroxyflavan (5). The MICs of the compounds 1 and 4 were found to be 0.063 mg/ml and 1.0 mg/ml respectively against A. israelii. Compounds 2 and 5 exhibited no inhibitory activity at 1.0 mg/ml (the highest concentration tested) against A. israelii. This is the first report of the isolation of the five compounds and their activity against A. israelii.