The Response Regulator PhoP is Essential for Virulence and Environmental Adaptation in Pseudomonas plecoglossicida
Pseudomonas plecoglossicida is an important pathogenic bacterium in aquaculture. It can cause serious diseases in many economically important fish species and leads to significant economic losses. The PhoP/PhoQ two-component system is crucial for bacterial adaptation to environmental stress and virulence regulation, but its function in P. plecoglossicida is still unclear. In this study, we constructed a phoP deletion mutant and a complemented strain to investigate the role of PhoP in bacterial physiology and pathogenicity. Phenotypic analysis showed that Delta phoP had significantly impaired growth, motility, and biofilm formation. Under simulated host stress conditions, such as low magnesium and acidic pH, the mutant showed more severe growth defects and motility suppression. Using a hybrid grouper infection model, Delta phoP was found to have much lower virulence, and the cumulative host survival rate increased by 28.41% compared to the wild-type strain. Transcriptomic analysis identified 1151 differentially expressed genes. These genes were mainly related to ribosome assembly, flagellar and pilus synthesis, outer membrane integrity, stress response, and antibiotic resistance. Further electrophoretic mobility shift assays confirmed that the PhoP protein can directly bind to the promoter regions of tolC (a key part of multidrug efflux pumps) and clpB (a stress chaperone protein). In summary, this study is the first to systematically show the function of PhoP as a global transcriptional regulator in P. plecoglossicida. It plays a central role in the pathogenic process by coordinating basic physiology, environmental adaptation, and virulence expression. This provides a theoretical basis for further understanding its pathogenic mechanisms and developing new prevention and control strategies.