Flagella: multipurpose structures in EPEC

Abstract

Enteropathogenic E. coli (EPEC) is the most important cause of infant diarrhea in the developing world. The hallmarks of EPEC infection are the formation of attaching and effacing (AE) lesions on intestinal epithelial cells (in these lesions the cytoskeleton is rearranged, the microvilli are effaced, and there is the formation of pedestal-like structures that cup the bacteria individually); and adherence-forming microcolonies on epithelial cells (localized adherence). The genes necessary for the formation of the AE lesions are encoded within a chromosomal pathogenicity island named the locus of enterocyte effacement (LEE). The LEE encodes a type III secretion system, an adhesin and a receptor for this adhesin. Concerning localized adherence, the major player described so far is the type IV fimbriae bundle-forming pilus (BFP). Epidemiological studies have always grouped EPEC within a restricted number of O-antigen serogroups and H flagellar antigen types. Now, Giron et al. [1xThe flagella of enteropathogenic Escherichia coli mediate adherence to epithelial cells. Giron, J.A. et al. Mol. Microbiol. 2002; 44: 361–379Crossref | PubMed | Scopus (235)See all References][1] demonstrate that the EPEC flagella function as an adhesin, aiding in the formation of the microcolonies characteristic of localized adherence.The authors demonstrated that purified EPEC flagella bind to epithelial cells and show a functional and expression relationship between the expression of the flagella, BFP and LEE-encoded type III secretion (several mutants impaired in type III secretion and BFP production were also impaired for flagella expression). This work also suggests a role for the presence of epithelial cells in flagella expression, given that flagella expression was highly induced in their presence. Overall, this study shows for the first time that the flagella of EPEC are directly involved in the adherence of these bacteria and supports a molecular relationship between the two existing type III secretion pathways of EPEC (LEE-encoded type III secretion and flagella).These studies are particularly exciting because flagellar H antigen is one of the surface and epidemiological markers that identifies EPEC as a diarrheagenic class of E. coli. Thus, a functional relationship between EPEC flagella and pathogenesis has been demonstrated and it will be interesting to know how the expression of these elements is synchronized during infection.