Abstract:
New measures to control infections in humans and other animals are continuously being
sought because of the increasing resistance of bacteria to antibiotics. In a wide tree screening
survey of the antimicrobial activity of extracts of tree leaves (www.up.ac.za/phyto), Ochna
pulchra, a small tree found widely in southern Africa, had good antibacterial activity. We
therefore investigated the antibacterial activity of acetone leaf extracts of some other available
Ochna spp. Antibacterial activity and the number of antibacterial compounds in acetone leaf
extracts of Ochna natalitia, Ochna pretoriensis, O. pulchra, Ochna gamostigmata and Ochna serullata
were determined with a tetrazolium violet serial microplate dilution assay and bioautography
against Staphylococcus aureus, Escherichia coli, Enterococcus faecalis and Pseudomonas aeruginosa,
bacteria commonly associated with nosocomial infections. The percentage yields of the extracts
varied from 2.5% to 8%. The minimum inhibitory concentrations of the five species ranged
from 40 μg/mL to 1250 μg/mL. E. coli was sensitive to all the extracts. The O. pretoriensis
extract was the most active with minimum inhibitory concentrations of 0.065 mg/mL and
0.039 mg/mL against E. coli and E. faecalis, respectively. The O. pretoriensis extract also had
the highest total activities of 923 mL/g and 1538 mL/g, indicating that the acetone extract
from 1 g of dried plant material could be diluted to 923 mL or 1538 mL and would still kill
these bacteria. Based on the bioautography results, the two most active species, O. pretoriensis
and O. pulchra, contained at least 10 antibacterial compounds with similar Rf values. Some
of these antibacterial compounds were polar and others were non-polar. Variation in the
chemical composition of the species may have some taxonomic value. The order of activity of
the species to the bacteria were O. pretoriensis > O. pulchra > O. gamostigmata > O. serullata > O.
natalitia. Even before toxicity and bioavailability issues are considered, some Ochna spp. leaf
extracts have the potential to be used in treating skin infections.