Antimicrobial Activity of Extracts from Six Green Algae from Tanzania

The antimicrobial activity of the extracts of six marine green algae from Tanzania was screened against three bacterial species Viz.; Staphylococcus aureus (SA), Bacillus subtilis (BS), Escherichia coli (EC), and a yeast, Candida albicans (CA) using a disk assay method. A brine shrimp bioassay using newly hatched Artemia salina larvae was used for cytotoxicity studies of extracts from three algal species. Of the six species tested, Valonia aegrophila extract was most active against all the tested organisms, and its extract was even more active against CA, SA and EC than Penicillin G at a concentration of 2.5 flg hole-I. The extracts of Halimeda opuntia and H. tuna showed mild activity against all organisms. The extract of Ulva pertusa was more active against SA and BS but less active against EC and was not active against CA. The extract of Caulerpa mexicana was inactive against all the tested organisms. Occasional development of antimicrobial resistance colonies within the inhibition zones were seen in H. opuntia and H. tuna extracts when they were assayed against CA and EC. After 24 months storage, the extract of Valonia aegrophila exhibited halved antifungal activity and a slight decrease against EC, but increased activity against SA and BS. The crude extract of U. pertusa had lethal effects and killed 50% of the shrimp larvae when LC50 was 116 µg ml -1 , whereas extracts of Call1erpa raccl1losa and Valoniaa aegrophila LC50 occurred far above 1000 µg ml 1