Citrobacter freundii Caused Head Ulcer Disease and Immune Response in Juvenile Procambarus clarkii

Red swamp crayfish (Procambarus clarkii) is one of the most important aquaculture species in China. Frequent outbreaks of diseases seriously threatened the sustainable development of the industry. It is necessary to understand the causes of disease and study the mechanism of disease resistance in P. clarkii. In this paper, the pathogenic bacteria causing head ulcers in juvenile P. clarkii were found and identified as Citrobacter freundii, which can cause severe pathological changes in the hepatopancreas and intestines of juvenile P. clarkii. Detection of humoral immune factors revealed that PO activity and lysozyme activity of infected P. clarkii were significantly enhanced at 15 and 20 dpi, respectively. Transcriptome analysis was conducted of hepatopancreas from normal and diseased P. clarkii after C. freundii injection, as well as bacteria-free control of P. clarkii. It was found that DEGs are rich in NF-kappa B, oxidative phosphorylation, JAK/STAT, Leukocyte transendothelial migration, MAPK, and PPAR signaling pathway. These pathways are related to immune modulation, metabolism, and pathogen clearance. Meanwhile, immune-related genes such as Gip, nfyA, psmD13, and FGFR were significantly highly expressed in the normal group, which was verified by qRT-PCR results, suggesting that they may be the key regulatory genes for juvenile P. clarkii resistance to C. freundii. This study will help to elucidate the molecular mechanism of the immune response of P. clarkii to C. freundii. The results are instructive for the prevention and treatment of P. clarkii diseases and for further understanding of the invertebrate immune system.