The Type III Translocon Protein EseC Inhibits Biofilm Formation by Sequestering EseE.

The type III secretion system (T3SS) is one of the most important virulence factors of the fish pathogen 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 the surface of , 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 the autoaggregation and biofilm formation of At 18 h postsubculture, a Δ strain developed strong autoaggregation and mature biofilm formation, accompanied by enhanced formation of EseB filamentous appendages. This is in contrast to the weak autoaggregation and immature biofilm formation seen in the wild-type strain. EseE, a protein that directly binds to EseC and also positively regulates the transcription of the operon, was liberated and showed increased levels in the absence of EseC. This led to augmented transcription of the operon, thereby increasing the steady-state protein levels of intracellular EseB, EseD, and EseE, as well as biofilm formation. Notably, the levels of intracellular EseB and EseD produced by the Δ and Δ Δ strains were similar but remarkably lower than those produced by the wild-type strain at 18 h postsubculture. Taken together, we have shown that the translocon protein EseC inhibits biofilm formation through sequestering EseE, a positive regulator of the operon., previously known as , 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 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 the autoaggregation and biofilm formation of , providing new insights into the regulatory mechanism involved in biofilm formation.