Effects of Saprolegnia parasitica on pathological damage and metabolism of Epithelioma papulosum cyprini cell

Saprolegniasis is a common fungal disease in aquaculture. It will form white flocculent hyphae on the skin of fish, and the hyphae may grow inward and penetrate into muscle tissue, which will reduce the immunity of the body and eventually lead to death. However, there are still some gaps in the mechanism of the fish body surface against the invasion of Saprolegnia. This study explored the defense mechanism of Epithelioma papulosum cyprini cell (EPC) in the process of Saprolegnia parasitica infection from the perspective of pathogenic bacteria and host cells, so as to provide a theoretical basis for further exploring the mechanism of host resistance to S. parasitica invasion. The EPC cell was used as the research object. The EPC cells were treated with 1 × 10⁶ CFU/mL of S. parasitica for 0, 6, 12, 24, 48 and 72 h. Cell viability and cell membrane damage were detected, and the non-specific immune enzyme activity in the cells was detected. Based on the above research, the apoptosis genes and antioxidant genes in the cells were detected to analyze the effect of S. parasitica on the metabolism of the EPC cells. The results showed that with the prolongation of the co-culture time of S. parasitica and cells, the cell viability gradually decreased and the cell membrane integrity was destroyed, but at the same time, the activity of non-specific immune enzymes increased to resist the infection of S. parasitica. In addition, the detection of EPC apoptosis gene casp3a and CTSD showed that the relative content of casp3a gene increased significantly at 24 h and reached the maximum value of the culture time (P < 0.05). The content of CTSD gene increased significantly at 12 h and reached the maximum value (P < 0.05). The results of antioxidant immune genes serpinh1a and gpx1a were opposite to the structure of apoptotic genes. The content of serpinh1a and gpx1a genes decreased significantly at 12 h (P < 0.05), but with the prolongation of culture time, the content increased significantly at 24 h and 48 h (P < 0.05). After stimulation of EPC cells by S. parasitica, the differential metabolites were mainly concentrated in Lipids, Compounds with biological roles and Phytochemical compounds. The KEGG pathway mainly focused on ABC transporters, Glycerophospholipid metabolism, Cysteine and methionine metabolism, Glycine, serine and threonine metabolism, Purine metabolism. In general, S. parasitica can affect cell activity, destroy the cell membrane of EPC cells, and cause apoptosis. However, EPC cells can also resist the invasion of S. parasitica by regulating their own non-specific immunity and their own metabolites, thereby protecting the body from the infection of S. parasitica.