Enhancing copper toxicity against Saprolegnia parasitica, a devastating oomycete pathogen in aquaculture

Saprolegnia parasitica is an oomycete pathogen responsible for saprolegniasis diseases that result in large production losses in the catfish and salmon aquaculture industry. Recently, copper sulfate has emerged as a cost-effective anti-Saprolegnia treatment therapy, although local water quality can alter copper sulfate potency. The current study is aimed at boosting the anti-saprolegnia activity of copper by enhancing its intracellular toxicity with ionophores at low doses. S. parasitica was cultured in chemically defined 2222X growth media containing micromolar concentrations of copper sulfate and potential ionophore small molecules. From targeted screening experiments, the chemical agents tetraethylthiuram disulfide (TDD), ciclopirox olamine (CLP), 2-mercaptopyridine N-oxide (MPO), 5-chloro-8-hydroxy-7-iodoquinoline (CHI), 5,7-dichloro-8-hydroxyquinoline (DHQ) and 8-quinolinol (8QN) were identified to inhibit S. parasitica growth in a copper-dependent manner. Consistent growth inhibition with these ionophores is observed in both liquid and solid support growth formats. The addition of the exogenous copper-chelator, bathocuproine sulfate (BCS), reversed the toxicity of copper-ionophore treatment against S. parasitica for TDD, CLP, MPO, and 8QN but not CHI and DHQ. To probe the mechanism of copper toxicity upon acute exposure to MPO, the most effective ionophore identified in our screen, global protein oxidation levels were assessed. This investigation suggests that Cu-MPO’s S. parasitica growth inhibition may be targeted and does not involve widespread changes in protein oxidation levels. Together, this study demonstrates that low doses of ionophores, in combination with low levels of copper, display enhanced toxicity towards S. parasitica versus Cu-sulfate treatment alone.