Antimicrobial activity of extracts from selected southern African marine macroalgae

Extracts from 56 southern African seaweeds, from the Divisions Chlorophyta (green), Phaeophyta (brown) and Rhodophyta (red), were screened against 12 bacteria, two yeasts and two moulds for antimicrobial activity. Seaweed extracts were prepared by boiling powdered samples in 80% ethanol. Extracts were pipetted into wells in overlay agar plates seeded with the test microorganisms. Generally, the bacteria were more susceptible to the antimicrobial effects of seaweed extracts than the yeasts and moulds. Of the test strains, more Gram-positive than Gram-negative bacteria were inhibited by the extracts, with Bacillus subtilis being the most susceptible. The extracts from the Phaeophyta (brown algae) exhibited the largest relative activity, against all the test strains. Of the seaweeds tested here, the brown algae Zonaria subarticulata (Lamouroux) Papenfuss showed the highest amount and broadest spectrum of antimicrobial activity against the microorganisms tested in this study.