Abstract:Sebastes schlegelii, a main fish species in the deep-sea cage culture in northern China, is a near-shore cold-water carnivorous fish with low temperature resistance. However, bacterial diseases such as eye rot, bacterial enteritis, and skin ulcer disease emerged with the expansion of breeding scale and increase in breeding density. Photobacterium damselae subsp. damselae (PDD), a gram-negative pathogen widely distributed in the global marine environment, can infect various marine animals, such as S. schlegelii. The pathogenic cases in China's marine aquaculture industry are gradually increasing, and the pathogenic hosts are diversified, which poses a new threat to the healthy development of China's marine aquaculture industry.
In this study, we focused on the deep-sea aquaculture of S. schlegelii in Bohai and studied the effects and mechanisms of S. schlegelii on PDD infection from the levels of cells, tissues, enzyme activity, and immunity based on histopathological, cellular, physiological and immune responses. S. schlegelii individuals with a body weight of (65.02±3.52) g and length of 15–17 cm were taken as the research object. Fish with normal saline injection composed the control group, whereas fish at 3, 5, and 7 days after PDD infection composed the experimental groups (D3, D5, and D7, respectively). Each group had three replicates of 30 fish each. The breeding experiment was carried out for 1 week. Results showed that PDD infection caused structural lesions of the intestine, spleen, and liver, which led to inflammation and oxidative stress. After infection, an inflammatory response occurred in S. schlegelii, and the number of leukocytes significantly decreased, which verified the inflammatory response. PDD significantly affected the antioxidant activities, such as superoxide dismutase (SOD), catalase (CAT), malonaldehyde (MDA), fish phenol oxidase (PO), and myeloperoxidase (MPO), non-specific immune performance, such as lysozyme (LZM), acid phosphatase (ACP), alkaline phosphatase (AKP), and secreted immunoglobulin (SIgA), and growth indexes, such as α-amylase (α-AL), amylopsin (PAMY), lipase (LPS) of the liver. The SOD activity in the experimental groups was significantly lower than that in the control group, and the lowest activity was (10.26±0.11) U/g in D3 (P<0.05). The MDA content in D3 and D5 was significantly higher than that in the control group (P<0.05). The CAT activity in the experimental groups was significantly lower than that in the control group (P<0.05), and the maximum decrease in enzyme activity in D3 decreased from (112.90±1.56) U/g to (66.81±1.04) U/g. The activity of PO initially increased and then decreased, and it was significantly higher in D3 than in the control group (P<0.05), reaching (110.01±2.39) U/g. MPO activity showed a unimodal shape, which initially increased and then decreased, and the highest in D5 was significantly different from that in the control group (P<0.05). The maximum LZM activity in D3 was (3.41±0.21) U/mL, which was significantly higher than that in the control group, and showed a unimodal shape (P<0.05). The ACP activity in D5 reached a maximum of (280.23±2.91) U/mL, which was significantly different from that in the control group (P<0.05). The AKP activity in D5 reached the lowest (42.37±1.53 U/mL), which was significantly different from that in the control group (P<0.05). The SIgA activity in D5 reached (47.18±0.93) U/g, and no significant difference was found between D7 and the control group (P>0.05). The trends of α-AL and PAMY initially increased and then decreased, and a significant difference was found between D5 and the control group (P<0.05). The LPS concentration initially decreased and then increased, with the highest (137.48±3.13 U/g) in D5, which was significantly different from that in the control group (P<0.05), and the lowest (111.26±2.64 U/g) in D3. PDD injection significantly affected the intestinal pancreatic amylase, lipase, and α-AL activities of S. schlegelii (P<0.05). It also significantly affected the antioxidant performance, non-specific immune performance, and growth indexes in the liver of S. schlegelii. In addition, the expression levels of 11 mRNA genes related to intestinal immunity changed significantly. Tight junction protein genes (CLDN3, CLDN15, and ZO-1), anti-inflammatory factors genes (IL-10, IL-12b), and inflammatory pathway genes (NF-ĸBp65, NLRC3.2, and MyD88) were significantly down-regulated in the experimental groups (P<0.05); in contrast, intestinal inflammatory cytokines related genes (IL-8, IL-15, and IL-1β) were significantly up-regulated (P<0.05). In conclusion, PDD infection can reduce the growth and antioxidant performance of S. schlegelii; damage the tissue structures of the intestine, liver, and spleen; cause tissue lesions; downregulate the relative expression of intestinal tight junction protein and anti-inflammatory factor genes; and upregulate the relative expression of inflammatory factor genes. This study may serve as a scientific basis for the health evaluation of S. schlegelii in the deep-sea environment and development of corresponding impressive technologies.
