BACKGROUND: The study randomly screened 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 the animal health market.
METHODS: We determined the antibacterial and antifungal activity of 714 acetone leaf extracts of 537 different tree
species against Enterococcus faecalis, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Candida
albicans and Cryptococcus neoformans. A sensitive serial dilution microplate method was used.
RESULTS: Several extracts had MICs as low as 0.02 mg/ml. We analysed 14 out of the 38 tree orders where we
determined the activity of more than 8 different tree species representing 89% of all species examined. There were
statistically significant differences in some cases. Celastrales, Rosales and Myrtales had the highest activity against
Gram-positive bacteria, the Myrtales and Fabales against the Gram-negative bacteria and the Malvales and Proteales
against the fungi. Species present in the Asterales followed by the Gentiales and Lamiales had the lowest activities
against all the microorganisms tested. Fabales species had the highest activities against all the microorganisms
tested. There was substantial selectivity in some orders. Proteales species had very high activity against the fungi
but very low activity against the bacteria. The species in the Celastrales and Rosales had very low antifungal activity,
low activity against Gram-negative bacteria and very high activity against Gram-positive bacteria.
CONCLUSION: Against all classes of microorganisms, the four orders containing species with the highest average
antimicrobial activities also contained several species with low activities against different pathogens and vice versa.
These results therefore should be used with circumspection in selecting tree orders that would yield the highest
probability of finding species with promising activities. Nevertheless there was a twofold increase in probability of
finding extracts with interesting antifungal activity from orders with high mean activity than from orders with low
mean activity. The probability increased to threefold and fivefold for Gram-positive and Gram-negative bacteria