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The previous studies have indicated that Photobacterium damselae subsp. damselae (Pdd) was the pathogen of black rockfish (Sebastes schlegeli) with seriously skin ulcer symptom. This study aimed to explore the spleen transcriptome response of S. schlegeli at 24 h (P24 group) and 48 h (P48 group) after Pdd infection and characterize expression profile of immune‐related genes. The fish in the control group were injected with equal volume of PBS. Results showed that there were 2,139 (88.42%), 510 (21.08%) and 242 (10%) differentially expressed genes in PBS vs P24, PBS vs P48 and P24 vs P48 respectively. As the time increases, the number of differential genes gradually decreases. GO annotation indicated that the immune‐related function category contributed the most in the PBS vs P24 and PBS vs P48 groups, including immune response, immune system process, immune effector process and regulation of immune system process. KEGG enrichment analysis showed that these genes were located on 277, 184 and 164 signalling pathways in the PBS vs P24, PBS vs P48 and P24 vs P48 groups respectively; among them, 30, 26 and 16 pathways were related to immune system, involving 344, 119 and 24 genes. P24 group involved more immune‐related signal pathways and genes than P48 group. RT‐qPCR validation indicated that the expression patterns of 10 genes at different time points were well agreed with RNA‐Seq analysis. This study provided a better understanding of the molecular responses to photobacteriosis in S. schlegeli. [ABSTRACT FROM AUTHOR]

Photobacterium damselae subsp. damselae (PDD) is a Gram-negative bacterium that can infect a variety of aquatic organisms and humans. Based on an epidemiological investigation conducted over the past 3 years, PDD is one of the most important pathogens causing septic skin ulcer in deep-sea cage-cultured Sebastes schlegelii in the Huang-Bohai Sea area and present throughout the year with high abundance. To further understand the pathogenicity of this species, the pathogenic properties and genome of PDD strain SSPD1601 were analyzed. The results revealed that PDD strain SSPD1601 is a rod-shaped cell with a single polar flagellum, and the clinical symptoms were replicated during artificial infection. The SSPD1601 genome consists of two chromosomes and two plasmids, totaling 4,252,294 bp with 3,751 coding sequences (CDSs), 196 tRNA genes, and 47 rRNA genes. Common virulence factors including flagellin, Fur, RstB, hcpA, OMPs, htpB-Hsp60, VasK, and vgrG were found in strain SSPD1601. Furthermore, SSPD1601 is a pPHDD1-negative strain containing the hemolysin gene hlyAch and three putative hemolysins (emrA, yoaF, and VPA0226), which are likely responsible for the pathogenicity of SSPD1601. The phylogenetic analysis revealed SSPD1601 to be most closely related to Phdp Wu-1. In addition, the antibiotic resistance phenotype indicated that SSPD1601 was not sensitive to ceftazidime, pipemidic, streptomycin, cefalexin, bacitracin, cefoperazone sodium, acetylspiramycin, clarithromycin, amikacin, gentamycin, kanamycin, oxacillin, ampicillin, and trimethoprim-sulfamethoxazole, but only the bacitracin resistance gene bacA was detected based on Antibiotic Resistance Genes Database. These results expand our understanding of PDD, setting the stage for further studies of its pathogenesis and disease prevention. [ABSTRACT FROM AUTHOR]