Study on molecular basis of species-specific adhesion of enteropathogenic Escherichia coli to host cells.

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Enteropathogenic Escherichia coli (EPEC) is a causative agent of diarrhea in humans. Localized adherence of EPEC onto intestinal mucosa was reproduced in an in vitro adherence assay with cultured human epithelial cells. We found that the efficiency of EPEC adherence to a mouse-derived colonic epithelial cell line, CMT-93, was remarkably lower than its adherence to human-derived intestinal cell lines, such as Intestine-407, or Caco-2. Although EPEC did adhere to some cell lines derived non-human species, fixing the cells with formalin to inactivate one or more formalin-sensitive factors allowed us to observe species-specific differences in EPEC adherence. In contrast to these results, an EPEC mutant that is defective in bundle-forming pili (BFP) production adhered as efficiently to CMT-93 cells as to Caco-2 cells. Furthermore, a purified BfpA-His6 fusion protein showed higher affinity for Caco-2 cells than for CMT-93 cells, and inhibited EPEC adherence. These results indicated that BFP plays an important role in the cell-type-dependent adherence. On the other hand, we determined the role of BFP in EPEC adherence. BFP is necessary for autoaggregation and formation of microcolony. BFP expressed by EPEC on epithelial cells was shown to disappear in accordance with expansion of microcolony. Bacterial dispersal and release of BFP from EPEC aggregates were induced by contact with host cellular membrane extract. Consequently, BFP-expressing EPEC adhered directly to the surface of cells, in preference to attaching to pre-formed microcolonies on the cells. These results suggested that BFP mediate initial attachment of EPEC through direct interaction with host cell rather recruitment of unattached bacteria to microcolony on the cell.