Persistence Infection of TGEV Promotes Enterococcus faecalis Infection on IPEC-J2 Cells

Transmissible gastroenteritis virus (TGEV) is a coronavirus causing diarrhea with high incidence in swine herds. Its persistent infection might lead to epithelial-mesenchymal transition (EMT) of swine intestinal epithelial cells, followed by subsequent infections of other pathogens. Enterococcus faecalis (E. faecalis) is a member of the enteric microorganisms and an opportunistic pathogen. There is no report of secondary E. faecalis infection to TGEV, even though they both target to the intestinal tracts. To investigate the interactions between TGEV and E. faecalis, we set up an in vitro infection model by the swine IPEC-J2 cells. Dynamic changes of cell traits, including EMT and cell motility, were evaluated through qPCR, Western blot, electronic microscopy, scratch test, Transwell migration test and invasion test, respectively. The adhesion and invasion tests of E. faecalis were taken to verify the impact of the preceding TGEV infection. The cell morphology and molecular marker evaluation results showed that the TGEV persistent infection induced EMT on IPEC-J2 cells; increased cellular motility and invasion potential were also observed. Spontaneously, the expression levels of fibronectin (FN) and the membrane protein integrin-α5, which are dominant bacterial receptors on IPEC-J2 cells, were upgraded. It indicated that the bacteria E. faecalis adhered to IPEC-J2 cells through the FN receptor, and then invaded the cells by binding with the integrin-α5, suggesting that both molecules were critical for the adhesion and invasion of E. faecalis to IPEC-J2 cells. Additionally, it appeared that E. faecalis alone might trigger certain EMT phenomena, implying a vicious circle might occur. Generally, bacterial and viral co-infections are frustrating yet common in both human and veterinary medicines, and our observations on enteric TGEV and E. faecalis interactions, especially the diversity of bacterial invasion strategies, might provide new insights into the mechanisms of E. faecalis pathogenicity.