Fish farming has becoming increasingly important due to overexploitation of wild stocks. However, common fish pathogens such as Saproleginia parisitica and Saprolegnia diclinia have increased since the ban on the antimicrobial malachite green in 2002. S. parasitica and S. diclinia are both oomycetes, fungus-like filamentous eukaryotic microorganisms that infect both fish and their eggs. The overall objective of the EU-funded SAPRO (Sustainable approaches to reduce oomycete (Saprolegnia) infections in aquaculture) project was to create a training network for future researchers that would develop sustainable methods for treating Saprolegnia. The training network investigated the ecological, molecular, immunological and biochemical factors underlying pathogen-host interactions and trained 10 early stage researchers and 3 experienced researchers. Project partners identified target enzymes for disease control and analysed the cell wall composition of several pathogenic oomycetes using quantitative proteomics. Of the 1655 plasma membrane proteins that were identified, several were targeted for disease control studies. For this, compounds were identified that affected the growth of S. parasitica, screening then selected the compounds that specifically affected targeted proteins. New morphological markers were also developed and used to identify species and populations. A new pathogenic species of salmonid eggs (S. australis) was found and its resistance to the antimicrobial treatment bronopol was determined. In addition, the use of these molecular markers was examined in two case studies in Spain and the Baltic countries. The creation of a unique collection of Saprolegniales cultures based on 1 000 specimens and a DNA bank containing more than 3 000 extractions of Saprolegnia represented most of the biodiversity of this group. This allowed the first molecular taxonomy to be conducted using molecular operational taxonomic units (MOTUs), enabling faster and more precise identification of economically important species of Saprolegnia. Project partners also investigated the pathology and infectivity of Saprolegnia species in fish and fish eggs. This provided essential information on possible modes of resistance to the Saprolegniosis, which could allow breeding for resistance. Furthermore, researchers studied the link between the immune status of the host (Atlantic salmon and rainbow trout) and the ability of Saprolegnia to establish a successful infection. Researchers also addressed disease management, seeking beneficial microorganisms from salmon egg cultures that could suppress the growth of Saprolegnia. These included specific commensal Actinobacteria, with the genus Frondihabitans (Microbacteriaceae), which inhibited attachment of Saprolegnia to salmon eggs. SAPRO’s success will provide new sustainable means to mitigate Saprolegniosis and possibly other diseases. A video about the project is available online.
Saprolegnia parisitica, Saprolegnia diclinia, aquaculture, SAPRO, oomycete, Saprolegnia australis, molecular operational taxonomic units, Actinobacteria