Temperature stress does not affect antimicrobial activity of some South African medicinal plants

Some traditional healers believe that cultivated medicinal plants are not as active as plants collected from nature that may be growing under more environmental stress. This perception has implications for the conservation of popular medicinal plants. We previously found that water stress on plants had a limited effect on antimicrobial activity of its extracts. We now investigated the antimicrobial activity of leaf extracts of seedlings of Leonotis dysophylla Benth. from one batch and clones of Bulbine frutescens (L.) Willd. and Tulbaghia violacea Harv. plant species grown under temperature regimes of 15 and 30 degrees C. The aim was to determine if temperature stress would influence the antimicrobial activity of acetone leaf extracts of these widely used plant species against four bacteria and three fungi. The minimum inhibitory concentration (MIC) and total activity was determined to evaluate the antimicrobial activity of plant extracts. All leaf extracts across both temperature regimes had intermediate antimicrobial activity with MIC values varying from 0.42 to 2.50 mg/ml. With an exception of one plant extract against one fungus, there were no statistically significant differences in the MIC values of plant extracts against bacteria and fungi between the different temperatures. Total activity values that also take into account the quantity extracted were significantly higher particularly for B. frutescens and T. violacea grown under a temperature of 30 degrees C. High temperature might have led to a larger percentage of the plant material being soluble in acetone with little change in the composition of antibacterial compounds. The temperature regimes applied in this study may be optimal for cultivating these plant species, but 30 degrees C may be preferable to 15 degrees C for B. frutescense. These results aswell as results found earlier on the effect of water stress questions the perception that cultivation weakens medicinal properties. Cultivating medicinal plants may lead to herbal medicines with more consistent quality and biological activities. (C) 2019 Published by Elsevier B.V. on behalf of SAAB.