Antibacterial and antifungal activities of 10 Kenyan Plectranthus species in the Coleus clade

Abstract

Background Information: Plectranthus L’Hér. is an economically important genus with horticultural, medicinal and food uses. Most Plectranthus species are used in traditional medicine and have attracted the interest of researchers who have studied them in attempt to explore the bioactivities of their phytoconstituents. Materials and Methods: The current study investigated the antimicrobial activities of 10 Kenyan Plectranthus species through disc diffusion and broth dilution method. Results: Results indicated that, dichloromethane/methanol (1:1) total leaf extracts from Plectranthus barbatus displayed the highest antimicrobial activity compared to the other nine Plectranthus species with minimum inhibitory concentration (MIC) values of 25, 40, 100, 50, and 100 mg/ml against methicillin resistant Staphylococcus aureus (MRSA), Bacillus cereus, Escherichia coli, Candida albicans, and Aspergillus niger, respectively. At a concentration of 200 mg/ml, the antibacterial activity of total leaf extracts of P. barbatus (MIC value = 25 mg/ml) and Plectranthus lanuginosus (MIC value = 40 mg/ml) against MRSA was not significantly different from positive control drug; amoxicillin. Similarity, at a concentration of 200 mg/ml, total leaf extracts from Plectranthus ornatus (MIC value= 50 mg/ml) and P. barbatus (MIC value = 50 mg/ml) exhibited antifungal activity against C. albicans which was not significantly different from that of the positive control; ketoconazole. Conclusion: The study reports for the first time, the antimicrobial activity of Plectranthus pseudomarrubioides, Plectranthus edulis, Plectranthus aegyptiacus, Plectranthus Otostegioides, and Plectranthus lanuginosus. The study has demonstrated broad bacteriostatic activity of P. barbatus and thus recommends further studies on this plant aimed at discovery of novel antimicrobial agents.