Abstract:The antimicrobial activity of different molecular-weight fractions of phlorotannins from Sargassum thunbergii kuntze was screened by plate growth inhibition assay. The results indicated that all the fractions of phlorotannins can inhibit the growth of tested microorganisms including Vibrio parahaemolyticus, Vibrio harveyi, Vibrio nereis, Staphylococcus aureus, Escherichia coli, Vibrio anguillarum, Vibrio alginolyticus and Micrococcus tetragenus. Among them, the antibacterial activity of the lowest molecular-weight fraction I (Mr < 5.0×103) of phlorotannins was the best with the minimum inhibition concentration (MIC) values against the tested microorganisms above as 900，900，900，1 800，1 800，1 800，1 800 and 3 600 μg/ml respectively. Additionally, phlorotannins from S.thunbergii kuntze showed excellent thermal stability, and the best antimicrobial activity to the tested bacteria was in pH 3～4.
