Antimicrobial activity and stability of rhinacanthins-rich Rhinacanthus nasutus extract

Rhinacanthins-rich Rhinacanthus nasutus (RRn) extract was prepared and standardized to contain total rhinacantins not less than 70% w/w. In this study, antimicrobial activities of the RRn extract as well as rhinacanthin-C against Streptococcus mutans, Propionibacterium acnes, Helicobacter pylori, Staphylococcus aureus, S. epidermidis and Candida albicans were evaluated by microdilution assay. It was found that the RRn extract exhibited potent bactericidal activity against S. mutans with MIC and MBC of 4 μg/ml, and potent bacteriostatic activity against S. epidermidis, P. acnes and S. aureus with the MICs of 8–16 μg/ml. However, the RRn extract was not active against C. albicans at concentration up to 2000 μg/ml. The antimicrobial activities of the RRn extract was almost equal to those of rhinacanthin-C. Stability evaluations of the RRn extract through a period of 4 months found that the RRn extracts were stable when kept in a well-closed container protected from light and stored either under 4±2 °C or 30±2 °C. The RRn extracts that were exposed to light were not stable after 1 week of storage. Under accelerated conditions at 45 °C with 75% relative humidity, the RRn extracts were not stable after 8 weeks of storage. Study on the effect of pH on the stability of the aqueous methanolic solution of the RRn extract found that the solutions were not stable at pH 5.5, pH 7.0, and pH 8.0. However, the rhinacanthin solution at pH 5.5 was more stable than those at pH 7.0 and pH 8.0.