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Six organic extracts prepared with different solvents (methanol, acetone, hexane, chloroform and dichloromethane-methanol) and aqueous extract of 27 species of marine algae belonging to the Chlorophyta and Phaeophyta were studied for antibacterial and antifungal activities against pathogenic microorganism: eight Gram-positive bacteria: Bacillus cereus, Bacillus thuringiensis, Bacillus subtillus, Clostridium sporogenes, Staphylococcus aureus, Mycobacterium smegmatis, Streptococcus faecalis and Bacillus sp, two Gram-negative bacteria: Escherichia coli and Pseudomonas sp and against fungi: Candida tropicalis and Cryptococcus neoformans. The best activity was observed in methanolic extract followed by acetonic extract and that prepared with methanol–dichloromethane. Of the 27 species tested, those belonging to Phaeophyceae were the most active in comparison with Chlorophyceae. The Gram-positive bacteria presented a sensibility superior to the Gram-negative and S. aureus ssp. aureus was the more sensitive
Key words: Macroalgae, algal extracts, antibacterial activity, antifungal activity, pathogenic microorganism.