Extracts of Ceramium rubrum, Mastocarpus stellatus and Laminaria digitata inhibit growth of marine and fish pathogenic bacteria at ecologically realistic concentrations

Microbially-derived diseases cause economic losses in aquaculture on global scale. Since many marine invertebrates and plants have evolved potent defence mechanisms against microbial epibionts, such as bacteria, viruses and fungi, there is increasing interest in these organisms as a source for natural antifouling and antibacterial substances. In this context, marine macroalgae are considered as a promising source for new antimicrobial agents. In this study, we investigated antibacterial effects of hexane and methanol extracts of the macroalgae Mastocarpus stellatus, Laminaria digitata and Ceramium rubrum on 12 marine and 7 prominent fish pathogenic bacteria at ecologically realistic concentrations of macroalgal secondary metabolites. Antibacterial activity was determined with a highly sensitive growth inhibition assay that records the fluorescence intensity of stained bacterial DNA. The assay allowed the detection of bacterial growth inhibition at and below the tissue level concentration of algal metabolites. Tests were carried out with algae collected in November 2006 and in March 2007 to investigate seasonal variation of antibacterial activities. Methanol extracts of C. rubrum at 10 mg dry weight per ml and hexane extracts of L. digitata at 31 mg dry weight per ml evoked strong antibacterial activities and inhibited almost all tested bacteria. There was no clear distinction between antibacterial effects of algae collected in November 2006 and March 2007. The growth inhibition of bacteria by algal extracts was frequently higher than 100%. This result indicated that bacteria must have been lysed during the exposure to algal extract components. Therefore, the extracts presumably evoked not only a bacteriostatic but also bacteriolytic mode of action. The assays revealed different susceptibilities of the bacterial phyla under investigation to algal extracts. Gram-positive marine Bacillaceae were generally more susceptible than Gram-negative marine Vibrionaceae. Listonella anguillarum, Pseudomonas anguilliseptica and Aeromonas salmonicida were the most susceptible fish pathogenic bacteria.