The bacterial flagellum plays a crucial role in pathogenesis. However, the mechanism by which the flagellum interferes with host energy metabolism remains unclear. In this study, we confirmed that deletion of the fliC gene resulted in a 30% reduction in the virulence of Pseudomonas plecoglossicida against the large yellow croaker (Larimichthys crocea). Compared to the wild-type strain (WT) infection group, the ΔfliC infection group exhibited a 29.54% decrease in the number of vacuolar degeneration hepatocytes and a 50.83% higher liver glycogen content. Furthermore, infection led to decreased mitochondrial complex V activity, a reduced NAD+/NADH ratio, lower levels of reduced glutathione (GSH), and increased lipid peroxide levels; however, these metabolic perturbations were significantly ameliorated in the ΔfliC group compared to the WT group. Proteomic analysis revealed that the dysregulation of the complement cascade and core carbon metabolic pathways observed in the WT infection group liver was significantly alleviated in the ΔfliC infection group. Additionally, in the ΔfliC infection group, pro-inflammatory genes (such as Tlr5, Tnfα, and Il1β) were downregulated, while lipid metabolism-related genes (such as Acox1, Cpt1a, and Pparα) were upregulated, suggesting the suppression of the Tlr5/NF-κB immune signaling axis and enhanced fatty acid β-oxidation. Collectively, fliC may mediate the disruption of host glucose and lipid metabolism homeostasis through a Tlr5-triggered immunometabolic regulatory axis. In conclusion, this study demonstrates that bacterial flagella modulate host energy metabolism, expanding our understanding of flagellum-mediated pathogenesis.

Visceral white spot disease caused by Pseudomonas plecoglossicida 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 (Epinephelus coioides) 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 luxR expression and attenuated quorum-sensing–associated virulence traits, supporting a possible role for this small RNA in modulating luxR expression and QS-related host immunopathology. These findings identify sRNA0024 as an important contributor to the virulence of P. plecoglossicida and highlight the sRNA0024–luxR module as a potential antivirulence target for controlling visceral white spot disease in aquaculture.

Background: Pseudomonas plecoglossicida ( P. plecoglossicida ) is an important aquatic pathogen whose virulence is closely associated with the type VI secretion system effector ppTse6. Previous studies have demonstrated that ppTse6 disrupts host energy metabolism and contributes substantially to bacterial pathogenicity. However, how environmental nutritional signals regulate ppTse6 expression and influence bacterial adaptation remains poorly understood. This study aimed to determine the role of glucose in regulating ppTse6 expression and glucose-associated physiological traits in P. plecoglossicida . Results: P. plecoglossicida was cultured in minimal salt medium supplemented with 5 mM serine and increasing concentrations of glucose. Phenotypic assays, transcriptomic analysis, quantitative real-time PCR and promoter activity assays were performed to investigate glucose-mediated regulation. Glucose promoted bacterial growth, adhesion, and biofilm formation, with the strongest effects observed at 50 mM. Transcriptomic analysis revealed broad changes in genes associated with secretion systems, flagellar assembly, chemotaxis, biofilm formation, and metabolism. Glucose reduced ppTse6 promoter activity and transcript abundance. Deletion of ppTse6 impaired growth, adhesion, chemotaxis toward host mucus, and biofilm formation under glucose-supplemented conditions, whereas basal swimming motility was unaffected. Conclusions: This study identifies ppTse6 as a glucose-responsive T6SS effector that is transcriptionally repressed by glucose while contributing to bacterial adaptation under glucose-rich conditions. These findings reveal a previously unrecognized connection between carbon availability and T6SS effector regulation, providing new insights into how nutrient availability shapes bacterial fitness and pathogenic potential in P. plecoglossicida.

Copper sulfate (CuSO₄) is widely used in aquaculture for disease prevention and control, which poses significant potential risks to fish health and welfare. This study investigated the response mechanisms of immune-related molecules in the muscle tissue of rainbow trout (Oncorhynchus mykiss) following acute sublethal copper concentrations and during the recovery period. Rainbow trout were exposed to nominal Cu²⁺ concentrations of 0, 75, 150, and 300 μg/L for 4 d, followed by a 2 day depuration period. Integrated biochemical assays, transcriptomics and metabolomics were applied to muscle tissue, focusing on immune relevant pathways. Copper exposure significantly inhibited acetylcholinesterase and Ca²⁺-ATPase activities, indicating neurotoxicity, and induced oxidative stress, as evidenced by altered antioxidant enzyme activities and increased lipid peroxidation. It is worth noting that, during the stress induction process, the pro-inflammatory cytokine TNF-α and genes associated with lipid metabolism (Srebp1, FAS) were continuously up-regulated, and remained slightly higher than in the control group even during the recovery period, leading to the accumulation of saturated fatty acids and inflammatory signaling; this indicates persistent dysregulation of transcription. Muscle copper content paradoxically decreased while total protein increased, suggesting active metal efflux and metabolic reprogramming. Integrated multi-omics identified MYH4 and SCD1 as key nodes, and KEGG and O2PLS analyses showed that copper-induced immunotoxicity was associated with coordinated dysregulation of genes enriched in FoxO, mTOR and PPAR signaling pathways—central regulators of inflammation, protein homeostasis and lipid metabolism. These pathway-level findings are correlative and require causal validation. These findings suggest that short-term exposure to acute sublethal copper concentrations is associated with persistent immunotoxic transcriptional alterations in rainbow trout muscle, characterized by sustained inflammatory signaling and metabolic disruption during the short recovery period. The identified pathways and targets (TNF α, MYH4, SCD1, FoxO/mTOR/PPAR) provide candidate biomarkers for assessing copper-induced immunotoxicity and contribute to understanding the immune risks of copper pollution in aquatic ecosystems.