Yersinia ruckeri strain SC09 restrains innate immunity to promote infection process in rainbow trout (Oncorhynchus mykiss) via the type IV secretion system (T4SS).

In the process of continuous combat between pathogens and fish, bacteria have evolved a series of unique strategies to evade the defenses of the natural immune system. The bacterial type IV secretion system (T4SS), as an important multi-protein complex, plays an important role in the process of bacterial infection of the host. However, the T4SS effector protein and its function during bacterial infection remain to be elucidated. A typical T4SS is present in the integrative and conjugative element ICE (r2) of Yersinia ruckeri SC09. In this study, we found that SC09-T4SS mediated the virulence of bacteria and improved the viability of bacteria in eukaryotic cells. SC09-T4SS also mediated the inhibition of pro-inflammatory signals, thereby assisting in the immune escape of bacteria during infection. These effects of SC09-T4SS were due to the secretion of molecules such as STIR-1, STIR-2, and STIR-3. We believe that SC09-T4SS secretion systems are common in bacteria, and our research may provide a general rule of bacterial pathogenicity. • The SC09-T4SS assists Yersinia ruckeri immune escape. • STIR-1, STIR-2, STIR-3 are secreted by SC09-T4SS. • VirB3–4 is an important component of SC09-T4SS. [ABSTRACT FROM AUTHOR]