Red sea bream (Pagrosomus major) is one of the most popular farmed marine teleost fish species. Fish cell lines are becoming important research tool in the aquaculture field, and they are suitable models to study fish virology, immunology and toxicology. To obtain a Pagrosomus major cell line for biological studies, a continuous cell line from brain of red sea bream (designated as RSBB cell line) was established and has been successfully subcultured over 100 passages. The RSBB cell line predominantly consisted of fibroblast-like cells and multiplied well in M199 medium supplemented with 10% fetal bovine serum at 28 °C. Karyotyping analysis indicated that the modal chromosome numbers of RSBB cells was 48. After transfection with pEGFP-N1, RSBB cells showed bright green fluorescence with a transfection efficiency approaching 8%. For toxicology study, it was demonstrated that metal Cd could induce cytotoxic effects of RSBB cells, accompanied with a dose-dependent MTT conversion capacity. Morphologically, cells treated with metal Cd produced rounding, shrinking and detaching and induced both cell apoptosis and necrosis. For virology study, the RSBB cells were highly susceptible to Nervous necrosis virus (NNV) and Singapore grouper iridovirus (SGIV) with steady titers (i.e., 10^8.0~8.3 TCID₅₀ mL⁻¹ and 10^7.0~7.2 TCID₅₀ mL⁻¹ respectively). Furthermore, an obvious cytopathic effect (CPE) could be observed in RSBB cells infected with Infectious spleen and kidney necrosis virus (ISKNV) and Siniperca chuatsi rhabdoviruses (SCRV). Meanwhile, all the infections were confirmed by polymerase chain reaction. The new brain cell line developed and characterized from red sea bream in this study could be used as an in vitro model for fish studies in the fields of toxicology and virology.

Aeromonas schubertii is a pathogen that severely affects aquatic animals, including the snakehead, Channa maculata. Lytic bacteriophages have been recognized as effective alternatives to antibiotics for controlling bacterial infections. However, there have been no reports of A. schubertii phages as far as we know. In this study, a lytic bacteriophage SD04, which could effectively infect A. schubertii, was isolated from pond water cultured with diseased snakehead. The SD04 phage formed small, round plaques on Petri dishes. Electron microscopy revealed a hexagonal head and a contractile tail. Based on its morphology, it may belong to the Myoviridae family. Two major protein bands with molecular weights of 50 and 38 kilodaltons were observed after the phage was subjected to SDS-PAGE. The phage showed a large average burst size, high specificity, and a broad host range. When stored at 4 °C, phage SD04 had high stability over 12 months and showed almost no variation within the first six months. All fish were healthy after both intraperitoneal injection and immersion administration of SD04, indicating the safety of the phage. After treatment with SD04, Channa maculata in both phage therapy groups and prevention groups showed high survival rates (i.e., 83.3 ± 3.3% and 100 ± 1.3%, respectively). Phage therapy inhibits bacterial growth in the liver, the target organ of the infected Channa maculat. The experimental results indicate the potential use of phage SD04 for preventing A. schubertii infection in Channa maculata.

The disease caused by Largemouth bass ranavirus (LMBV) is one of the most severe viral diseases in largemouth bass (Micropterus salmoides). It is crucial to evaluate the genetic resistance of largemouth bass to LMBV and develop markers for disease-resistance breeding. In this study, 100 individuals (45 resistant and 55 susceptible) were sequenced and evaluated for resistance to LMBV and a total of 2,579,770 variant sites (SNPs-single-nucleotide polymorphisms (SNPs) and insertions–deletions (InDels)) were identified. A total of 2348 SNPs-InDels and 1018 putative candidate genes associated with LMBV resistance were identified by genome-wide association analyses (GWAS). Furthermore, GO and KEGG analyses revealed that the 10 candidate genes (MHC II, p38 MAPK, AMPK, SGK1, FOXO3, FOXO6, S1PR1, IL7R, RBL2, and GADD45) were related to intestinal immune network for IgA production pathway and FoxO signaling pathway. The acquisition of candidate genes related to resistance will help to explore the molecular mechanism of resistance to LMBV in largemouth bass. The potential polymorphic markers identified in this study are important molecular markers for disease resistance breeding in largemouth bass.

Infectious spleen and kidney necrosis virus (ISKNV) has brought huge economic loss to the aquaculture industry. Through interfering with the viral replication and proliferation process that depends on host cells, its pathogenicity can be effectively reduced. In this study, we investigated the role of asparagine metabolites in ISKNV proliferation. The results showed that ISKNV infection up-regulated the expression of some key enzymes of the asparagine metabolic pathway in Chinese perch brain (CPB) cells. These key enzymes, including glutamic oxaloacetic transaminase 1/2 (GOT1/2) and malate dehydrogenase1/2 (MDH1/2) associated with the malate-aspartate shuttle (MAS) pathway and asparagine synthetase (ASNS) involved in the asparagine biosynthesis pathway, were up-regulated during ISKNV replication and release stages. In addition, results showed that the production of ISKNV was significantly reduced by inhibiting the MAS pathway or reducing the expression of ASNS by 1.3-fold and 0.6-fold, respectively, indicating that asparagine was a critical limiting metabolite for ISKNV protein synthesis. Furthermore, when asparagine was added to the medium without glutamine, ISKNV copy number was restored to 92% of that in the complete medium, indicating that ISKNV could be fully rescued from the absence of glutamine by supplementing asparagine. The above results indicated that asparagine was a critical factor in limiting the effective replication of ISKNV, which provided a new idea for the treatment of aquatic viral diseases.

Background/Objectives: Largemouth bass rhabdovirus (Micropterus salmoides rhabdovirus, MSRV) disease causes high mortality in largemouth bass farming. Therefore, vaccine development is critical for largemouth bass prevention against MSRV. Methods: An attenuated strain, denoted as MSRV-0509, was selected through intraperitoneal injection and immersion challenge assays, followed by plaque purification. The biological characteristics of MSRV-0509, including optimal inoculation dose, replication kinetics, thermostability, pH resistance, chloroform tolerance, and storage viability, were determined via viral titration. Spatiotemporal distribution patterns in largemouth bass post-intraperitoneal injection or immersion infection were quantified by qPCR. Immunoprotective efficacy was evaluated through intraperitoneal and immersion vaccination. Mechanistic insights were explored via relative qPCR and serum neutralization assays. Safety was assessed by single-dose overdose immunization and virulence reversion experiments. Results: An attenuated strain MSRV-0509 was screened through a challenge assay, exhibiting complete avirulence in largemouth bass compared to the virulent strain SCRV-T6. MSRV-0509 demonstrated optimal replication at low MOI (0.0001) in CPB cells, with peak titers (10^8.3 TCID₅₀/mL) at 96 h post-infection. The virus showed susceptibility to high temperatures, lipid solvents and acidic conditions, with prolonged stable storage viability at −80 °C. Tissue distribution revealed the spleen as the primary target after intraperitoneal injection, while immersion restricted infection to gills, with rapid clearance by 3–6 dpi. Vaccination trials identified 5 × 10² TCID₅₀/fish via intraperitoneal injection and 10^6.0 TCID₅₀/mL via immersion as effective immunizing doses, providing 100% relative survival post-challenge. Immune gene expression and serum neutralization showed Th1 and Th2 activation via intraperitoneal injection (elevated IL-12, IFN-γ, IL-10, IgM), whereas only the Th1 response was activated after vaccine immersion. No abnormality and mortality were observed in single overdose vaccination and virulence reversion experiments, confirming that MSRV-0509 was safe. Conclusions: These results proved that MSRV-0509 could be a promising vaccine candidate to protect largemouth bass from MSRV disease.

The role of ferroptosis—a novel iron-dependent programmed cell death pathway—in infectious spleen and kidney necrosis virus (ISKNV) infection remains poorly understood. Here, we demonstrate that ISKNV infection induces ferroptosis in CPB cells. Following ISKNV challenge, CPB cells exhibited hallmark morphological alterations including mitochondrial shrinkage, increased membrane density, and cristae reduction. Biochemical assays confirmed significant time-dependent elevations in ferroptosis markers: malondialdehyde (MDA, 1.7-fold), reactive oxygen species (ROS, 3.14-fold), and ferrous iron (Fe²⁺, 1.42-fold) compared to controls (p < 0.05). Mechanistic studies revealed that ISKNV downregulated glutathione peroxidase 4 (GPx4) while upregulating acyl-CoA synthetase long-chain family member 4 (ACSL4), as validated by quantitative real-time PCR (qRT-PCR) and immunoblotting. Ferroptosis induction with erastin enhanced ISKNV replication, whereas inhibition with liproxstatin-1 suppressed viral yield. These findings establish that ISKNV exploits ferroptosis to facilitate its replication, and pharmacological blockade of this pathway significantly suppresses viral propagation, providing a new strategy and intervention approach for controlling ISKNV infection.