Screening for antibacterial activity of Rhinacanthus species used in traditional Ayurvedic medicine in Sri Lanka

Abstract

The antibacterial activity of Rhinacanthus nasutus and R. polonnaruwensis (Family Acanthaceae) used in traditional Ayurvedic medicine in Sri Lanka were tested against standard Gram-negative and Gram-positive bacteria and clinically isolated Gramnegative and Gram-positive bacteria. The study was carried out on six species of bacteria: Escherichia coli, Staphylococcus aureus, S. saprophyticus, Pseudomonas aeruginosa, Salmonella typhii and Shigella flexi. A water extract of the concentration of 0.2 μl/ml of R. nasutus inhibited growth of all standard Gram ..positive bacteria, whereas, 0.2 μl/ml extract of R. polonnaruwensis inhibited the growth of standard Gram-positive S. aureus NCTC 657'1. Clinically isolated S. saprophyticus was inhibited by both R. nasutus (> 0.1 μl/ml) and R. polonnaruwensis (0.2μl/ml). However, none of the tested concentrations of both Rhinacanthus species inhibited the growth of any Gram-negative bacteria.

It can be concluded that both R. nasutus and R. polonnaruwensis studied have antibacterial properties against Gram-positive bacteria. Previous studies show that 75% aqueous ethanolic extract of R. nasutus used was found to have potent anti-bacterial effect on many Grarn positive bacteria, but no inhibitory effect on gram negative bacteria. Rhinacanthin-C, rhinacanthin-O and rhinacanthin-N possessed antifungal, antibacterial, antiviral, anti-inflammatory, anti-allergic and anti-cancerous properties. Out of the two Rhinacanthus species found in Sri Lanka, R. nasutus has higher antimicrobial activity. The plant extract taken by boiling the leaves in water was effective against most standard Gram-positive bacteria and Gram-positive bacteria isolated from wounds of patients.