Methanolic and ethyl acetate extracts from eight different seaweeds collected from the red sea Hurghada, Egypt (June, 2009) were screened for their antibacterial activities against both gram positive bacteria (Staphylococcus aureus NCIMB 50080 and Bacillus cereus) and gram negative bacteria (Escherichia coli NCIMB 50034, Enterococcus feacalisNCIMB 50030, Salmonella sp. and Pseudomonase aeruginosa). The antibacterial activities were expressed as zone of inhibition and minimum inhibitory concentrations (MIC). The seaweeds belong to Phaeophyaceae (Cystoesira myrica, Cystoesira trinodis, Padina gymnospora, Sargassum dentifolium and Sargassum hystrix); Rhodophaceae (Actinotrichia fragilis) and Chlorophyceae (Caulerpa racemosa and Codium fragile). Ethyl acetate extracts of C. racemosa, C. fragile and P. gymnospora; methanolic extracts of P. gymnospora and C. fragile showed higher antibacterial activities than other members of the tested algae. The most resistant bacteria was E. feacalis against both solvents extracts of S. dentifolium, C. myrica and A. fragilis while, Salmonella sp. and P. aeruginosa were resistant to methanolic extracts of C. racemosa, S. dentifolium and A. fragilis. On the other hand, B. cereus, S. aureus and E. coli were the most sensitive to all seaweed extracts. Our conclusion confirmed that susceptibility of gram positive bacteria to the algal extracts (zone of inhibition up to 19 mm) was more than those of gram negative bacteria (zone of inhibition up to 14 mm). The activities of ethyl acetate extracts were higher than those of methanolic extracts and the most powerful inhibitory extract was ethyl acetate extract of C. racemosa.
Key word: Antibacterial, ethyl acetate extracts, marine algae, methanolic extracts, minimum inhibitory concentration, seaweeds.
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