Identification of a Pseudomonas putida-like Strain and Its Association with the Immune Response in Hybrid Sturgeon (Huso dauricus × Acipenser schrenckii)

Pseudomonas putida is a conditional pathogenic bacterium and affects a variety of aquatic animals, including fish, amphibians and reptiles. However, studies focusing on the immune responses post-P. putida infection of sturgeon remain limited. In this study, a Pseudomonas putida-like strain (YFI-327) was isolated and identified from diseased hybrid sturgeon (Huso dauricus × Acipenser schrenckii). Firstly, the morphological, physicochemical characteristics of the bacterium, as well as the sequences of the 16s rRNA and gyrB genes, were analyzed. Then, the histopathological analysis showed that the diseased hybrid sturgeon exhibited extensive necrosis in the liver, spleen, gut and gill. Transcriptomic analysis of the spleen samples from hybrid sturgeon infected with the P. putida-like strain revealed 3337 DEGs, including 2161 upregulated genes and 1176 downregulated genes. Differentially expressed genes analysis indicated that 84 key components of the antigen processing and presentation pathway, hematopoietic cell lineage pathway and NOD-like receptor signaling pathway were upregulated, whereas 20 key components of the complement and coagulation cascades pathway were downregulated, which suggests a potential involvement of these pathways in the responses to P. putida-like infection. Moreover, the intestinal microbiota results showed a trend toward increased richness and diversity after infection, together with an apparent shift in dominant microorganisms from Firmicutes to Proteobacteria (phylum level) and Candidatus arthromitus to Xylophilus (genus level). The results also revealed that the serum biochemical parameters were altered after P. putida-like infection. These results provide evidence that the P. putida-like strain was a potential pathogen of hybrid sturgeon, and the infection might pose a serious threat to sturgeon aquaculture.