The Edwardsiella piscicida type III translocon protein EseC inhibits biofilm formation through sequestering EseE.

The type III secretion system (T3SS) is one of the most important virulence factors of the fish pathogen Edwardsiella piscicida It contains three translocon proteins, EseB, EseC and EseD, required for translocation of effector proteins into host cells. We have previously shown that EseB forms filamentous appendages on E. piscicida 's surface, and these filamentous structures mediate bacterial cell-cell interactions promoting autoaggregation and biofilm formation. In the present study, we show that EseC, but not EseD, inhibits E. piscicida 's autoaggregation and biofilm formation. At 18 h post subculture, a Δ eseC strain developed strong autoaggregation and mature biofilm, accompanied by enhanced formation of EseB filamentous appendages. This is in contrast to weak autoaggregation and immature biofilm formation seen in E. piscicida wild-type strain. EseE, a protein that directly binds to EseC and also positively regulates the transcription of the escC∼eseE operon, was liberated and showed increased levels in the absence of EseC. This led to augmented transcription of the escC ∼ eseE operon, thereby increasing the steady-state protein levels of intracellular EseB, EseD and EseE, and biofilm formation. Notably, the production of intracellular EseB and EseD by Δ eseE and Δ eseC Δ eseE strains was similar, but remarkably lower than that by the wild-type strain at 18 h post subculture. Taken together, we have shown that the translocon protein EseC inhibits biofilm formation through sequestering EseE, a positive regulator of the escC ∼ eseE operon. IMPORTANCE Edwardsiella piscicida , previously known as Edwardsiella tarda , is a Gram-negative intracellular pathogen that mainly infects fish. The type III secretion system (T3SS) plays a pivotal role in its pathogenesis. The T3SS translocon protein EseB is required for the assembly of filamentous appendages on the surface of E. piscicida The interactions between the appendages facilitate autoaggregation and biofilm formation. In this study, we explored the role of the other two translocon proteins EseC and EseD in biofilm formation. We have demonstrated that EseC, but not EseD, inhibits E. piscicida 's autoaggregation and biofilm formation, providing new insights into the regulatory mechanism involved in E. piscicida biofilm formation.