Ecotin as a novel virulence factor: Enhancing Edwardsiella piscicida survival and pathogenicity

Abstract

Edwardsiella piscicida is a highly pathogenic and stress-resistant bacterium that poses significant threats to the aquaculture industry. Ecotin (EcoT) is a multifunctional serine protease inhibitor that plays important roles in the pathogenicity of various bacteria. Its ability to inhibit host proteases and protect bacteria from immune attacks makes it a significant factor in bacterial infections. However, the role of EcoT in bacterial virulence remains largely unexplored. This study presents the first comprehensive elucidation of the diverse and critical roles of EcoT in E. piscicida. Our findings revealed that the mutation of ecoT significantly increases the production of bacterial extracellular polysaccharides (EPS) and enhances bacterial biofilm production. Under the strong acid stress, the expression of ecoT was significantly induced. Consistently, the mutation of ecoT significantly reduced bacterial capability to consume H⁺ ions and impaired its survival under strong acid stress conditions. Infection experiments demonstrated that ecoT deletion diminished the bacterium's resistance to host serum-mediated killing, proliferation within phagocytes, dissemination in immune tissues, and overall virulence. Based on these findings, we have formulated a hypothesis concerning the mechanism of action of EcoT. Under acidic conditions within the host, E. piscicida increases the levels of the periplasmic protein EcoT. EcoT enhances bacterial resistance to acidic stress, thereby facilitating bacterial survival within host cells. Concurrently, EcoT reduces EPS production and inhibits host protease activity, aiding the bacteria in evading the host's immune response. Our results suggest that EcoT functions as a novel virulence factor in E. piscicida, underscoring its significance in bacterial biofilm formation, stress tolerance, and pathogenicity.