Differential antibacterial activity of extracts from selected southern African macroalgal thalli

Eight southern African seaweeds (5 Phaeophyta, 2 Rhodophyta and 1 Chlorophy ta), previously shown to exhibit antibacterial activity, were analysed to a ssess the distribution of antibacterial activity in extracts from selected portions of their thalli. Seaweeds were separated into meristem and thallus (stipe, meristem and blade in the case of members of the Laminariales), mi lled, extracted and tested for antibacterial activity against ten Gram posi tive and Gram negative food-associated bacteria by agar diffusion. Extracts of the intercalary meristem of Ecklonia radiata showed significantly great er (P < 0.1) antibacterial activity than corresponding blade and stipe extr acts, whereas Laminaria pallida showed a uniform distribution of antimicrob ial activity throughout its thallus against all test bacteria. For seaweeds showing epical growth, the antibacterial activity of meristem extracts was either significantly greater (P < 0.1) or equal to that of thallus extract s. Thallus extracts never exhibited greater antibacterial activity than mer istem extracts.