Combretum woodii (Combretaceae) leaf extracts have high activity against Gram-negative and Gram-positive bacteria

Abstract

Dried ground leaves of Combretum woodii were extracted with 10 different solvents (hexane, diisopropyl ether, diethyl ether, methylene dichloride, ethyl acetate, tetrahydrofuran, acetone, ethanol, methanol and water) to determine the best extractant for subsequent isolation and characterization of antibacterial compounds. With the exception of the water extract, which had no antibacterial activity, the other extracts were bioactive with at least one of them exhibiting minimum inhibitory concentration values of 0.04 mg/ml against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli or Enterococcus faecalis. Intermediate polarity solvents extracted c. 10% of the dry mass compared to c. 3% with the more polar or non-polar solvents. These solvents also had higher antibacterial activity than more polar or non-polar extractants. Ethyl acetate was the best extractant with an average minimum inhibitory concentration (MIC) value of 0.08 mg/ml for the four pathogens followed by acetone and methylene dichloride with values of 0.14 mg/ml. The average MIC values for the positive controls were 0.13 (ampicillin) and 0.12 mg/ml (chloramphenicol). By taking the quantity extracted from the leaf powder into consideration, the total activity, a measure of potency, was highest for methylene
dichloride (1309 ml/g) followed by acetone (1279 ml/g) extracts. The antibacterial activity was high enough to consider the use of extracts for  clinical application and to isolate and characterise antibacterial compounds from the extracts. Based on the Rf values of the antibacterial compounds
determined by bioautography, the antibacterial compound was not a polyphenol or a tannin.