Role of bacterial quorum sensing and micro-algae in fish and crustacean larviculture

Bacterial disease is one of the major problems in aquaculture. Pathogenic bacteria such as V. harveyi and Aeromonas spp. infect various host species. Antibiotics remain by far the easiest and fastest solution to cure bacterial disease. However, the overuse of antibiotics has lead to resistant bacteria, causing the problem to become even more severe. As a result, antibiotics are not able anymore to cure infections in many cases. In this work, disruption of bacterial communication or quorum sensing (QS) is investigated as a novel approach to control bacterial infections in aquaculture. The first part of the research studied the effects of V. harveyi quorum sensing on virulence factor regulation and the impact of quorum sensing disruption on the virulence of V. harveyi and Aeromonas spp. towards giant river prawn (M. rosenbergii) and burbot (Lota lota) larvae, respectively. It was shown that QS negatively regulates the production of phospholipase, while caseinase and gelatinase are positively regulated. Meanwhile, hemolysin and lipase were found to be independent of QS. Furthermore, HAI-1 and AI-2 quorum sensing were found to be important for the virulence of V. harveyi towards prawn larvae. The CAI-1 system, on the other hand, had no effect on virulence of the bacterium in this host system. Similarly, AHL quorum sensing regulates the virulence of A. hydrophila towards burbot larvae. Meanwhile, QS antagonists (Ntetradecanoyl-L-homoserine lactone and cinnamaldehyde) protected burbot from A. salmonicida. In the second part of the research, the ability of micro-algae to interfere with QS was investigated through screening the effect of algal extracts on different bacterial QS reporter strains, including V. harveyi. The screening results showed that extracts from different micro-algal strains were able to interfere with QS regulated gene expression in the reporter strains. The interference activities were dependent on the QS signal type (unsubstituted, oxo-substituted and hydroxyl-substituted). The most interesting micro-algal strain was found to be C. saccharophila CCAP211/48 as it was able to inhibit QS-regulated gene expression in all three reporters used in this study. In another study, freshwater micro-algae (C. saccharophila CCAP211/48 and C. reinhardtii CCAP11/45) were also able to protect burbot larvae from Aeromonas disease. In conclusion, the results presented in this study showed that there is a link between QS and the production of virulence factors and pathogenicity towards aquatic animal larvae. Quorum sensing disruption through QS antagonists and micro-algae are promising as novel methods t