Discovery of antimicrobial activities of a marine diatom Thalassiosira rotula

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This study investigates the antimicrobial activities of a marine diatom, Thalassiosira rotula against microorganisms including three Gram-stain positive and six Gram-stain negative bacteria and one species of yeast. Well-bioassays were used to evaluate the ability to inhibit the growth of bacteria with extracts derived from algae using different extraction techniques of cell wall rupture and organic solvents. The extracts of T. rotula inhibited the growth of Vibrio harveyi, Staphylococcus aureus, Micrococcus luteus and Bacillus pumilus. The freeze/thaw algal paste extract without solvent extraction showed the highest inhibition against S. aureus and B. pumilus. The hexane:tert-butyl-methyl-ether (H:tBME) extract was the only one inhibiting M. luteus and the chloroform:methanol (C:M) extract was the only one inhibiting Vibrio harveyi. Sonication with beads for 3 min was the most efficient method of releasing the antibiotic substances. In sequential solvent extraction, the vortexed H:tBME extract had a higher ability against S. aureus, B. pumilus, and M. luteus than the sonicated H:tBME extract or the C:M extract. The 1:1 combination of sonicated H:tBME and vortexed H:tBME extracts did not exceed that of the separate extract against S. aureus or B. pumilus. However, the combined sequential C:M and vortexed H:tBME extracts had lower ability against S. aureus than the vortexed H:tBME extract alone, indicating the presence of antagonistic compounds in the C:M extract. This study indicates that T. rotula possess antimicrobial activities but the release of antibiotics depends on physical or chemical rupture of algal cells and extractive solvents.   Key words: Bacteria, algae, antibiotic, organic solvent, sonication, vortex.