Interaction effects of Micropterus salmoides and Edwardsiella piscicida under ammonia nitrogen and nitrite stress.

This study investigated the interactive effects of ammonia nitrogen (AN) and nitrite stress on Micropterus salmoides during infection with Edwardsiella piscicida (E. piscicida). The results indicated that combined stress of 2 mg/L nitrite initially induced a short-term immune enhancement in the host. This enhancement was characterized by the activation of genes associated with antigen recognition (CD40, TLR-2, IgM, and IFN-γ), innate immunity genes (MsP-2 and HSP70), inflammatory factors (IL-8 and TNF-α), and autophagy (VAMP8 and STX17) in the gills, liver, and head kidney. However, this immune response gradually weakened with prolonged exposure. Conversely, a high concentration of AN (24.0 mg/L) significantly suppressed the expression of most immune-related genes, leading to an imbalance in the regulation of autophagy and apoptosis, oxidative damage, and exacerbated histopathological changes, including hepatocyte necrosis, renal hemorrhage, and granuloma formation. These findings indirectly suggest that such combined stress may promote a vicious cycle favoring pathogen persistence, although direct bacterial proliferation data are lacking. Under high nitrite stress (20.0 mg/L), the modulation of immune genes exhibited tissue-specific differences, resulting in severe pathological damage, including lamellar fusion in the gills, spleen necrosis, and liver vacuolization. Further enzyme activity analysis revealed that fish with a single E. piscicida infection enhanced their anti-infection and anti-oxidation capabilities by increasing MPO and SOD activities across various tissues, whereas the combined stress of high AN (24.0 mg/L) inhibited these activities. Under high nitrite stress, ACP and MPO activities in various tissues, as well as serum SOD activity, were significantly lower than those observed in the single infection group. The findings suggest that IFN-γ, MsP-2, and HSP70 play crucial roles in both anti-infection and stress responses.