Infectious diseases are the world’s leading cause of premature deaths in humans and animals. The resistance to antibiotics and the emergence of new infectious diseases has increased the need for additional effective antimicrobial products. Despite numerous publications investigating antimicrobial activity of plant extracts it appears that no effective single product antimicrobial has yet been developed from plants. In many cases, however crude plant extracts have excellent activity and may provide useful products. Plants are frequently selected based on traditional use. Traditional healers usually use aqueous extracts of plants which in our experience generally have very low activities and it may be one of the reasons why no new products were developed from plants. Another approach to select plants for research is to use the taxonomic approach based on the premises that: (1) there is a correlation between active chemical compounds and antimicrobial activity; and (2) species in a family or order may have similar activities if the chemical precursors are inherited from a common ancestor. Future screening programmes could then concentrate on close relatives of species within these promising families and orders. The main aim of this study was to randomly screen leaf extracts of several hundred southern African tree species against important microbial pathogens to determine which taxa have the highest activity and may yield useful products to treat infections in human and animal health markets. A wide selection of plant species improved the possibility of finding promising extracts and has the advantage that active compounds may be discovered from plants that are not used traditionally. To ensure sustainable use only leaves of trees were examined. A spin off of this study would also indicate the susceptibility of different organisms, correlate the antimicrobial activities of the different organisms and determine what minimum inhibitory concentration (MIC) represents a good activity based on investigating many extracts against many microbes. The antimicrobial activity was determined by using a sensitive serial dilution microplate method. Acetone extracts were tested against two Gram-positive bacteria, two Gram-negative bacteria and two fungi, i.e. Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa, Candida albicans and Cryptococcus neoformans. Small and mostly insignificant differences were found between the susceptibility of the microbial pathogens to the extracts. E. faecalis was the most sensitive bacterium and C. neoformans the most sensitive fungal organism. The strongest correlations in activities among the pathogens were between C. albicans and C. neoformans, and among the pathogen classes between Gram-positive and Gram-negative bacteria. The tree extracts analysed in the present study had a wide range of activities against the different pathogens. Twenty six per cent of the extracts inhibited the pathogens at MIC levels of 0.16 mg/ml. This clearly shows that 0.16 mg/ml is not low enough to discriminate between promising species. Some of the extracts inhibited the growth of more than one pathogen while other extracts had selective activities and could be the most promising to follow up. The study identified families and orders with either statistically significantly higher or lower antimicrobial activities. Among the large families, Combretaceae and Fabaceae had high mean activities against all test pathogens. The families Anacardiaceae and Moraceae had high activities against both Gram-positive and Gram-negative bacteria whereas the families Proteaceae and Meliaceae had higher antifungal activities. Among the large orders, Fabales had relatively high activities against all the pathogen classes. Considering that plants in related taxa often contain similar compounds and therefore similar activities, future studies could analyse more representative species in the promising taxa. Many tree species, genera, families and orders, including well-known and lesser known medicinal taxa in southern Africa, were identified with promising activities. To evaluate the potential use of these results, additional cytotoxicity, phytochemical and pharmacological studies should be carried out. The study, although still exploratory, underlined the potential of southern African tree extracts as sources of antimicrobial products. Application of these results within the Phytomedicine Programme has led to patents and products that were as good as commercial products in animal and field trials. We hope that our results will provided a starting point for discovering new products with useful activities. / Thesis (PhD)--University of Pretoria, 2014. / gm2014 / Paraclinical Sciences / Unrestricted