Eight southern African seaweeds (5 Phaeophyta, 2 Rhodophyta and 1 Chlorophyta), previously shown to exhibit antibacterial activity, were analysed to assess 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), milled, extracted and tested for antibacterial activity against ten Gram positive and Gram negative food-associated bacteria by agar diffusion. Extracts of the intercalary meristem of Ecklonia radiata showed significantly greater (P < 0.1) antibacterial activity than corresponding blade and stipe extracts, whereas Laminaria pallida showed a uniform distribution of antimicrobial activity throughout its thallus against all test bacteria. For seaweeds showing apical growth, the antibacterial activity of meristem extracts was either significantly greater (P < 0.1) or equal to that of thallus extracts. Thallus extracts never exhibited greater antibacterial activity than meristem extracts.
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