In vitro and in vivo anti-oomycetes activities and mechanisms of linalool against Saprolegnia ferax

Saprolegnia ferax (S. ferax) is one of the most highly pathogenic oomycetes species in freshwater aquatic envi-ronments causing serious saprolegniosis. Linalool is a kind of natural alternative of antibiotics existing in various plants essential oils. In this study, the in vitro and in vivo anti-oomycetes activity and mode of action of linalool against S. ferax CQT1 were investigated. For in vitro test, linalool exhibited mycelium growth and zoospore production inhibition at 0.1% (v/v, 0.862 mg/ml) and 0.025% (v/v, 0.216 mg/ml), respectively. The mycelium was wrinkled, deformed, and shrunken after treatment with linalool. Linalool altered membrane permeability, caused permanent damage, or membrane breakdown. Further, the transcriptomic analysis showed that: (1) Linalool affected the oxidation and metabolism of fatty acids, leading to mitochondrial dysfunction and the inhibition of energy supply. (2) The increase in glycerophospholipid metabolism is probably the cell response to deal with cellular damage. (3) The degradation of branched chain amino acids may inhibit amino acid meta-bolism and result in the dysfunction of protein synthesis. (4) ABC transporters contribute to metabolic resistance to linalool and efflux of linalool outside of cells. For in vivo test, the cumulative survival rate of grass carp treated with linalool was significantly higher than those untreated with linalool, moreover, microscopic histopathology analysis results showed that there was robust mucus cell density in fish skin, the dermis and epidermis were relatively intact after linalool treatment. In all, our findings would provide foundations for the potential use of linalool to prevent and control S. ferax infection in the aquaculture industry.