Medicinal plant: Dye combinations – Impact on antimicrobial potency and toxicity

Abstract

Indigenous medicinal plants have been shown to be effective in treating skin conditions in southern Africa. In the past, antimicrobial dyes have also been used to treat dermatological infections with great success. The aim of this study was to investigate the antimicrobial properties of a combination of selected southern African medicinal plant species and dyes against infectious dermatological pathogens. Medicinal plants, dyes and their combinations were antimicrobially explored, with particular interest towards ascertaining the inhibitory activity via minimum inhibitory concentration (MIC) assays and cidal activities via minimum bactericidal/fungicidal (MBC or MFC) assays. The studies were conducted against various strains (reference, clinical and resistant) of Staphylococcus aureus, Pseudomonas aeruginosa and Candida albicans. The toxicity of the most active plants, dyes and combinations were also investigated employing the brine shrimp lethality assay. Seven of the tested plants, exhibited moderate antimicrobial activities (>160 - ≤ 1000 µg/ml) against various strains of S. aureus, P. aeruginosa and C. albicans. The best observed antibacterial activities were from Aristea ecklonii roots (130 µg/ml) and Calodedrum capense (125 µg/ml) against the reference strain of S. aureus ATCC 25923. Most of the selected dyes exhibited good antimicrobial activities with the best activity observed for malachite green (0.5 µg/ml) against S. aureus (clinical and methicillin resistant strain ATCC 43360) and C. albicans (resistant strain). The combination of the dye malachite green with selected plant species demonstrated the most enhanced synergy with ΣFIC values as low as 1.0 × 10−6. Two plant species (A. ecklonii leaves and roots as well as C. capense) demonstrated toxicity (LC50 values 166 µg/ml, 99 µg/ml and 1005 µg/ml, respectively) and most of the dyes exhibited some toxicity. In combination the toxicity was reduced. This study highlights the critical role that plants can play in bringing back previously neglected therapies (dyes) which have the potential, when combined, to combat skin infections and thus be considered for future possible treatments.