Substrate secretion by different EHEC secretion systems during their interaction with epithelial cells

Enterohemorrhagic <i>Escherichia coli</i> (EHEC) causes severe foodborne illness in humans. EHEC harbors five types of secretion systems (SSs) and unlike the type three secretion system (T3SS), the other SSs are less explored. Some substrates secreted by these SSs have been described; however, how these SSs collectively participate in EHEC-epithelial cell interaction during infection remains unknown. Here, we optimized protein secretion by four of the EHEC SSs in the absence and presence of epithelial cells, since the T6SS is not expressed <i>in vitro</i>. The secretion of substrates through the EHEC SSs followed a hierarchical pattern when bacteria encountered epithelial cells. Epithelial cells increase the protein secretion by EHEC and mutants in SSs (Western-blot and proteomics). Remarkably, mutants in one particular SS affect protein secretion by other SSs. Analysis of the EHEC secretome in the absence/presence of cells vs different SS mutants showed that epithelial cells increase the abundance of specific substrates; the lack of one SS affects the secretion by other SSs; epithelial cells diminish the effects caused by mutating some SS; bioinformatics analyses identified new potential substrates for each SS. Western blot analysis validated the interdependence in substrate secretion between SSs. Consequently, a complex interaction exists between SSs during epithelial cell infection, affecting the epithelial cells-bacteria interaction and the EHEC pathogenesis.