In vitro screening of 35 compounds against Saprolegnia spp.

Introduction: Oomycetes belonging to the genus Saprolegnia cause high mortality rates in freshwater fish culture. The lack of alternative treatment with effectiveness comparable to the banned malachite green (MG), urge the identification of new molecules active against this pathogen. This work focused on an in vitro screening aimed at assessing the effectiveness of 35 compounds against Saprolegnia spp. in order to select molecules that could be of major interest for further in vivo investigations and applications in aquaculture. Methodology: In vitro trials were carried out on two Saprolegnia parasitica strains and one S. delica strain. Tests were performed according to existing protocols to determine the Minimum Inhibitory Concentration (MIC) in agar and the Minimum Lethal Concentration (MLC) after one hour of contact in water. Compounds were provided diluted in DMSO at a concentration of 10 mM (with the exception of Dequalinium Chloride, diluted at 2 mM) that were further diluted to working concentrations of 0.00001; 0.0001; 0.001; 0.01; 0.1 and 0.25 mM. Pure DMSO was also screened at the same amount present in different concentrations, until 1mM, in order to check its own inhibitory or lethal effects. Each strain was tested in triplicate. Results: Triplicates were consistent among each other. DMSO showed no inhibitory effects against Saprolegnia at concentrations from 0.00001 to 0.25 mM, but was able to inhibit the oomycete at higher concentrations also when used alone. MICs were determined for 15 out of 35 compounds examined. The lowest MIC was defined for zinc pyrithione (0.01 mM). Seven compounds had a MIC of 0.1 mM, seven of 0.25 mM, while the other 20 were ineffective at the tested concentrations. Conclusion: Although it was not possible to define MICs for several compounds at the tested concentrations, most of the molecules showed the ability to slow down the radial growth and/or to inhibit the aerial mycelium. These compounds, although not showing a high efficacy against Saprolegnia in in vitro trials, could be effective in decreasing the invasive capacity of the oomycete in the host, therefore representing good candidates for further in vivo studies.