The presence of substances of the Shewanella strain 0409, isolated from grouper intestine, gives antiviral activity against Betanodavirus in cell cultures and protection of fish

Abstract

Nervous necrosis virus (NNV), belonging to the genus Betanodavirus, causes mass mortality of many species of marine and freshwater fish at the larval and juvenile stages. Biocontrol is a potential strategy for the prevention and control of diseases associated with NNV infection. A bacterial strain, 0409, was isolated from the microbiome of grouper intestines, and the bacterial culture supernatant (CS0409) was found to suppress NNV RNA synthesis and reduce NNV titers. The aim of the present study was to elucidate how the bacterial strain 0409 antagonized NNV in vitro and in vivo. The 16S rDNA of strain 0409 had high sequence identity to Shewanella spp. The anti-NNV substances in CS0409 were heat stable, resistant to protease and nuclease activity, widely pH-tolerant, hydrophilic, and had molecular weights of <3 kDa. CS0409 exhibited anti-NNV effects, but not by neutralizing NNV or blocking the viral receptor on GF-1 cells. The anti-NNV activity of CS0409 was present when NNV-infected GF-1 cells were treated with CS0409 prior to 2 h post infection (hpi) and had to be cultured in CS0409-supplemented medium throughout the entire course of the experiment. Once CS0409 was removed from the culture medium of infected cells, the anti-NNV effect was abrogated. Moreover, CS0409 could induce cBB cells to express Mx gene, which could inhibit NNV RNA synthesis. Furthermore, groupers that were fed with strain 0409 for 2 months showed lower morbidity and mortality after NNV challenge compared with the mock-fed groupers. Therefore, strain 0409 has the potential to be a beneficial bacterium for biocontrol against NNV.