Effects of Citrobacter freundii on sturgeon: Insights from haematological and intestinal-liver immunity

Abstract

Citrobacter freundii is a fish pathogen that can cause severe disease. The aim of this study is to evaluate the histopathology, hematology, and intestinal microbial effects of an experimentally induced C. freundii infection in sturgeons. Histopathological analysis showed lesions in the liver and intestines of infected fish. Haematological parameters indicated that the concentration of white blood cells (WBC) showed a gradual increase after infection and remained at high levels during days 2–14, and the red blood cells (RBC) decreased significantly at 2 DPI (days post-infected) (p < 0.05) and recovered afterwards. Leukocyte categorical counts showed that there was a significant decrease in lymphocytes and a significant increase (p < 0.0001) in monocytes after infection. There were no significant effects of infection on the relative numbers of neutrophils and eosinophils (p > 0.05). Total protein, globulin, albumin, and acid phosphatase content in the serum showed a decreasing trend after infection, then returned to their normal levels at 14 DPI. Creatine kinase activity in the serum was significantly decreased at 14 DPI (p < 0.05). The serum malondialdehyde level showed a rapid decline on day 1 and superoxide dismutase activity rapidly increased after infection. Lactate dehydrogenase, lysozyme, and cortisol activities in the serum showed an increasing trend after infection. The liver concentrations of total protein significantly decreased at 1 DPI, then gradually increased and returned to normal levels at 14 DPI. Alanine aminotransferase activity increased at 1 DPI on day 1 post-infection, then gradually declined and returned to normal levels at 7 DPI. The liver concentrations of acid phosphatase and alkaline phosphatase significantly increased at 7 DPI and returned to normal levels at 14 DPI. Lactate dehydrogenase activity rapidly increased at 1 DPI, and remained high until day 14 (p < 0.05). The levels of superoxide dismutase, reduced glutathione, and catalase in the liver increased significantly at 1 DPI, and then gradually declined. Superoxide dismutase and reduced glutathione remained at low levels until days 7–14 (p < 0.05), and catalase returned to normal levels on day 14. C. freundii infection could increases the richness and diversity of intestinal bacteria in sturgeons to some extent. The predominant bacterial phyla were Fusobacteria and Proteobacteria. The predominant bacterial genera were Cetobacterium and Plesiomonas. PCoA revealed that 14 DPI, the balance of the intestinal flora of surviving sturgeon appears to have been restored. This study revealed that C. freundii infection affected sturgeon histopathology, hematology, and intestinal microbial community composition.