In this study, the bioactivity of some algal extracts were evaluated in vitro against different bacterial strains. Five commonly occurring benthic algae, namely Cystoseira myrica, Digenea simplex, Hormophysa triquetra, Sargassum cervicorne and Sarconema filiforme, were collected from Bahrain’s coastline in July 2007. A total of six chemical extracts were derived from those algal species, four of which were prepared by Soxhlet (petroleum ether, chloroform, ethyl acetate, methanol), aqueous and crude methanol. Their bioactivity was assessed against four testing bacterial strains using the agar well diffusion assay and expressed as the diameter of inhibition zone (mm). Results revealed that the petroleum ether extract of D. simplex exhibited the highest inhibition zone (24.3 mm) against Pseudomonas aeruginosa while the aqueous extract of C. myrica exhibited the lowest inhibition zone (13.3 mm) against Staphylococcus aureus. The Soxhlet extracts of all the algal species were biologically active only against P. aeruginosa. The aqueous extracts showed an inhibitory activity against S. aureus only. The crude methanolic extract was biologically broadly active on a wide range of tested bacteria, P. aeruginosa, Escherichia coli and Bacillus subtilis. The bacterium P. aeruginosa was the most susceptible microbe whereas S. aureus was the most resistant. On the average, The gram-negative bacteria tested were more sensitive than the gram-positive bacteria towards the extracts of the algal species used. The algal species that belong to the red algae class Rhodophyceae were more biologically active than algal species that belong to the brown algae class Phaeophyceae.

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