Establishment of a protocol for testing antimicrobial activity in southern African macroalgae

Six southern African seaweeds, two Chlorophyta, Phaeophyta and Rhodophyta, were used to test for antibacterial activity against Bacillus subtilis, Staphylococcus aureus, Acinetobacter Iwoffi and Escherichia coli. Extracts were prepared using a range of organic solvents (acetone, chloroform, diethyl ether, 80% ethanol and methanol), by soaking ground seaweed samples in the extraction solvent, evaporating to dryness, and redissolving in the respective solvent. In addition, milled and ground samples were boiled in 80% ethanol for 4 h. Crude extracts were tested for antibacterial activity using three application techniques, viz paper discs, direct spotting and wells, and onto three types of agar plates, viz seeded agar, bacterial lawns and overlay agar. Inhibition zone diameters were measured and analysed by multifactorial analysis of variance. Boiling milled seaweeds in 80% ethanol, with application of the extracts into 6.1 mm wells in overlay agar constituted the test protocol which yielded the highest activity indices.