A fatty acid from the diatom Phaeodactylum tricornutum is antibacterial against diverse bacteria including multi-resistant Staphylococcus aureus (MRSA).

Pathogenic bacteria, such as multidrug-resistant Staphylococcus aureus (MRSA), which are not susceptible to most conventional antibiotics, are causing increased concern in healthcare institutions worldwide. The discovery of novel antibacterial compounds for biomedical exploitation is one avenue that is being pursued to combat these problematic bacteria. Marine eukaryotic microalgae are known to produce numerous useful products but have attracted little attention in the search for novel antibiotic compounds. Cell lysates of the marine diatom, Phaeodactylum tricornutum Bohlin, have been reported to display antibacterial activity in vitro, but the compounds responsible have not been fully identified. In this paper, using column chromatography and reversed-phase high-performance liquid chromatography, we report the isolation of an antibacterial fatty acid. Mass spectrometry and (1)H-nuclear magnetic resonance spectroscopy revealed it to be the polyunsaturated fatty acid, eicosapentaenoic acid (EPA). We show that EPA is active against a range of both Gram-positive and Gram-negative bacteria, including MRSA, at micromolar concentrations. These data indicate that it could find application in the topical and systemic treatment of drug-resistant bacterial infections.