The Edwardsiella piscicida type III effector EseJ suppresses the expression of type 1 fimbriae leading to decreased bacterial adherence to host cells

ABSTRACT The type III secretion system (T3SS) of Edwardsiella piscicida plays a crucial role in its pathogenesis. Our previous study indicated that the T3SS effector protein EseJ inhibits the bacterium’s adhesion to epithelioma papillosum cyprini (EPC) cells, while the mechanism of the inhibition remains elusive. In this study, we revealed that EseJ negatively regulates the fimA gene, as demonstrated by comparative transcription analysis of Δ eseJ and wild-type (WT) strains. As well, the dramatically increased production of FimA was detected in the absence of EseJ compared to that by the WT strain. The adherence of the Δ eseJ strain decreased far below that of the WT strain in the absence of FimA, demonstrating that FimA plays a pivotal role in the hyperadhesion of the Δ eseJ strain. Adherence analysis with a strain with truncated eseJ demonstrated that the C-terminal region of EseJ (Gly1191 to Ile1359) is necessary to inhibit the transcription of the type 1 fimbrial operon. Binding between the EseJ fragment from amino acid residues 1191 to 1359 and the DNA fragment upstream of fimA was not detected, indicating that EseJ might indirectly regulate the type 1 fimbrial operon. Our study reveals that EseJ controls E. piscicida adherence to EPC cells by negatively regulating the type 1 fimbrial operon.