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 ΔeseJ 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 ΔeseJ 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 ΔeseJ 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.

Sepsis is a systemic inflammatory response syndrome caused by infection, with high incidence and mortality. Therefore, it is necessary to carry out an effective anti-infection treatment. In this work, we designed and synthesized red blood cell (RBC) membrane-coated PLGA nanoparticles named γ3-RBCNPs, which target the highly expressed intercellular adhesion molecule-1 (ICAM-1) at the site of infection through the γ3 peptide on its surface and kill the Klebsiella pneumoniae through ciprofloxacin encapsulated in its core. In addition, the homogenous RBC membrane coated on the surface of the nanoparticles helps them avoid immune surveillance and prolong the circulation time of the drug in the body. We found that the γ3-RBCNPs target human umbilical vein endothelial cells (HUVECs) activated by TNF-α in vitro and the infected lung of mice in the sepsis model very well. In vitro evaluation suggested that γ3-RBCNPs have a low risk of acute hemolysis and are less likely to be engulfed by macrophages. In vivo evaluation showed that γ3-RBCNPs has a long half-life and good bio-safety. More importantly, we confirmed that γ3-RBCNPs have the good antibacterial and anti-infection ability in vivo and in vitro. Our research provides a new strategy for the nano-drug treatment of Klebsiella pneumoniae-induced sepsis.