Influence of annual rainfall on antibacterial activity of acetone leaf extracts of selected medicinal trees

•Tree species growing under different rainfall conditions had different antibacterial activities.•Terminalia sericea, Sclerocarya birrea and Combretum collinum leaf extracts were examined.•MICs and total activity were determined by serial dilution using tetrazolium violet growth indicator.•Generally lower rainfall led to higher antibacterial activity, but not in all cases.•Question whether stress increases antibacterial activity requires controlled conditions work.

With the increasing demand for medicinal plants there is a danger that some populations may be destroyed by over collection. Cultivation of medicinal plants may address this problem, but many traditional healers believe that cultivated plants are not as active as plants growing in nature. There is uncertainty around the extent to which environmental stress affects the biological activity of plant extracts. We determined the effects of different annual rainfalls on antibacterial activity of tree leaf extracts of Terminalia sericea, Combretum collinum and Sclerocarya birrea growing under annual rainfalls of ≥ 870 mm, 650 mm and ≤ 480 mm in the Lowveld, South Africa. The minimum inhibitory concentration of acetone leaf extracts was determined by a serial microplate dilution technique using tetrazolium violet as growth indicator on four nosocomial bacteria Enterococcus faecalis, Staphylococcus aureus, Escherichia coli and Pseudomonas aeruginosa. MICs of leaf extracts of different trees varied between 60 and 1460 μg/ml. Variations were for T. sericea (60–820 μg/ml), for C. collinum (70–820 μg/ml) and for S. birrea (70–1460 μg/ml). Total activity values of the same extracts ranged between 48 ml/g and 539 ml/g. Antibacterial activity and total activity of plant extracts against test bacteria generally significantly (P ≤ 0.05) increased under lower rainfall. In some cases there were no clear correlations. There were varying sensitivities of different bacteria of leaf extracts of plants grown under different annual rainfalls. This variation suggests that possible water stress caused by different rainfalls did not exclusively affect antibacterial activity of plant extracts. It is possible that other factors such as genetic, edaphic, microclimate, herbivory and pathogens could also have had an effect. More experiments using clonal plant material under well controlled environmental conditions are required to provide a more definitive answer.