Glucose promotes bacterial fitness while simultaneously repressing a major virulence effector ppTse6

Abstract

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 . 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. 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.