QUORUM SENSING INHIBITORS FROM MICRO-ALGAE COMMONLY USED IN AQUACULTURE Natrah F.M.I.12, Tom Defoirdt', Mireille Mardel', Wiyoto Wiyoto', Patrick Sorgeloos1 and Peter Bossier1 1 Laboratory of Aquaculture & Artemia Reference Center, Ghent University, Rozier 44, 9000 Gent, Belgium 2 Department of Aquaculture, Faculty of Agriculture, University Putra Malaysia, 4B400, Serdang, Selangor, Malaysia E-mail : natrah@put ra.upm.edu. my Quorum sensing, bacterial ce Il-to-ce 11 communication with small signal molecules such as acyl- homoserine lactones, regulates the virulence of many (aquatic) pathogenic bacteria (Schauder and Bassler, 2001; Waters and Bassler, 2005). Therefore, interfering with quorum sensing is currently being explored as a novel biocontrol strategy to fight bacterial infections. In this study, the effects of different marine and freshwater micro-algal strains on acyl-homoserine lactone-regulated phenotypes of three reporter strains were investigated. Two freshwater micro-algae inhibited violacein production of quorum sensing reporter strain Chromobacterium violaceum CV026. Further tests using Escherichia coli JB52B showed that micro-algal extracts inhibited or stimulated quorum sensing, depending on the algal strain. One freshwater and 5 marine algae showed quorum sensing inhibitory activity, whereas two algae stimulated quorum sensing-regulated gene expression. Micro- algal strains that showed inhibitory activity in the previous assays also inhibited acyl-homoserine lactone-regulated bioluminescence in the aquaculture pathogen Vibrio harveyi. The growth of all reporter strains was found to be unaffected by the micro-algal samples. The most promising micro- algal strain was found to be Chlorella saccharophila CCAP211/48, as its extracts inhibited quorum sensing-regulated gene expression in all three reporter strains. This study revealed that micro-algae are able to act as a biocontrol agent against pathogenic bacteria in aquaculture. References SchauderSand B.L. Bassler. 2001.The languages of bacteria. Genes and Development 15:1468- 1480. Waters C.M. and B.L. Bassler. 2005. Quorum sensing: ce Il-to-ce 11 communication in bacteria. Annual Review Cell and Developmental Biology 21:319-346. - 58 -