Evaluation of Antibacterial Activity of Rhinacanthus Species in Sri Lanka

Abstract

Rhinacanthus nasutus (L.) Kurz is a valuable medicinal plant belonging to the family Acanthaceae that has many applications in the Ayurvedic system of medicine in Sri Lanka. Rhinacanthus polonnaruwensis Cramer is a more recently discovered species endemic to Sri Lanka, but its medicinal properties have not been recorded so far. The objective of the present study was to screen the antibacterial activity of leaf extracts of R. nasutus and R. polonnaruwensis against clinically isolated Gram-negative and Gram-positive bacteria. The study was carried out on six bacterial species, Escherichia coli, Staphylococcus aureus, S. saprophyticus, Pseudomonas aeruginosa, Salmonella typhi and Shigella flexi. Polar extracts of R. nasutus and R. polonnaruwensis were obtained by grinding the leaves with sterilized water and boiling the leaves in distilled water to obtain a decoction. A decoction of the concentration of 0.2 ml/ml of R. nasutus inhibited growth of all standard Gram-positive bacteria; Staphylococcus aureus and MRSA, whereas 0.2 ml/ml of decoction of R. polonnaruwensis inhibited growth of Staphylococcus aureus. Clinically isolated Staphylococcus saprophyticus was inhibited by both decoctions of Rhinacanthus species. None of the tested concentrations of the two Rhinacanthus species inhibited the growth of any Gram-negative bacteria; Pseudomonas aeruginosa, Escherichia coli, and Shigella flexi. In this study, we demonstrated that the leaf extracts of both R. nasutus and R. polonnaruwensis were effective at inhibiting Gram-positive bacteria. However, the decoction of R. nasutus was found to be more effective against the tested Grampositive bacteria than R. polonnaruwensis.