Deletion of sRNA0024 Reduces Virulence of Pseudomonas plecoglossicida and Alleviates Host Immune Injury in Epinephelus coioides

Visceral white spot disease caused by <i>Pseudomonas plecoglossicida</i> severely threatens marine aquaculture, highlighting the need for effective control strategies. To clarify the role of a novel small RNA, sRNA0024, in bacterial pathogenicity, we constructed an sRNA0024 deletion mutant (ΔsRNA0024) and compared its phenotype and virulence with those of the wild-type strain NZBD9. In vitro assays showed that deletion of sRNA0024 did not affect bacterial growth but significantly reduced biofilm formation and adhesion. In vivo infection experiments in orange-spotted grouper (<i>Epinephelus coioides</i>) demonstrated that the ΔsRNA0024 mutant had a 3.8-fold higher 50% lethal dose (LD₅₀), improved host survival, and milder splenic lesions than the wild type. Histopathology and host transcriptome analyses revealed weakened activation of complement-coagulation cascades, neutrophil extracellular traps, leukocyte migration, and inflammatory signaling pathways, indicating a lower-intensity immune response. Bacterial transcriptomics showed that deletion of sRNA0024 was associated with reduced <i>luxR</i> expression and attenuated quorum-sensing-associated virulence traits, supporting a possible role for this small RNA in modulating <i>luxR</i> expression and QS-related host immunopathology. These findings identify sRNA0024 as an important contributor to the virulence of <i>P. plecoglossicida</i> and highlight the sRNA0024-<i>luxR</i> module as a potential antivirulence target for controlling visceral white spot disease in aquaculture.