The CpxR-regulated periplasmic protease DegP coordinates the environmental stress adaptation, motility, and virulence in Edwardsiella piscicida.

Edwardsiella piscicida is a serious aquatic pathogen that infects a diverse spectrum of economically valuable fish species, inflicting substantial economic damage on the aquaculture industry. DegP, a periplasmic protein endowed with dual chaperone-protease activity, is vital for bacteria to preserve homeostasis and acclimate to environmental stresses. Nevertheless, studies about DegP in E. piscicida have been relatively limited. In this study, we determined that the DegP of E. piscicida shared high sequence identity with DegP proteins from diverse bacterial species and comprised conserved functional domains. The azocasein hydrolysis assays showed that the recombinant protein rDegP exhibited significant proteolytic activity in a substrate concentration-dependent manner with the optimal enzyme activity at 48 °C and pH 9.0. To probe the impact of DegP on stress adaptation and virulence, we constructed an in-frame deletion strain, TH1ΔdegP, through homologous recombination. Under normal conditions, TH1ΔdegP exhibited no discernible growth defects. However, TH1ΔdegP caused the significantly decreased survival in various forms of adversity including high temperature, alkalinity, acidity, oxidation stress, and high osmolarity. Additionally, TH1ΔdegP impaired the motility, but had no effect on biofilm formation and cell membrane integrity. Pathogenicity assays revealed that TH1ΔdegP significantly reduced adhesion to FG cells and impaired intracellular proliferation in RAW264.7 macrophages in vitro. Moreover, TH1ΔdegP displayed decreased resistance to tilapia serum, diminished capacity for tissue colonization, and attenuated overall virulence towards tilapia. The complementary strain TH1ΔdegPC successfully restored all the lost capabilities of TH1ΔdegP. To clarify the regulatory mechanism of DegP, the EMSA, β-galactosidase detection and qRT-PCR demonstrated that the Cpx envelope stress system regulated the expression of degP through direct binding of CpxR to the degP promoter. These findings underscore that DegP, a protease regulated by the Cpx system, is indispensable for the adaptation to environmental stresses and virulence in E. piscicida. This study provides a comprehensive elucidation of the multifaceted roles of DegP and enhances our comprehension of its contributions to stress tolerance and pathogenicity in E. piscicida.