Highly conserved type 1 pili promote enterotoxigenic E. coli pathogen-host interactions

Abstract

Enterotoxigenic Escherichia coli (ETEC), defined by their elaboration of heat-labile (LT) and/or heat-stable (ST) enterotoxins, are a common cause of diarrheal illness in developing countries. Efficient delivery of these toxins requires ETEC to engage target host enterocytes. This engagement is accomplished using a variety of pathovar-specific and conserved E. coli adhesin molecules as well as plasmid encoded colonization factors. Some of these adhesins undergo significant transcriptional modulation as ETEC encounter intestinal epithelia, perhaps suggesting that they cooperatively facilitate interaction with the host. Among genes significantly upregulated on cell contact are those encoding type 1 pili. We therefore investigated the role played by these pili in facilitating ETEC adhesion, and toxin delivery to model intestinal epithelia. We demonstrate that type 1 pili, encoded in the E. coli core genome, play an essential role in ETEC virulence, acting in concert with plasmid-encoded pathovar specific colonization factor (CF) fimbriae to promote optimal bacterial adhesion to cultured intestinal epithelium (CIE) and to epithelial monolayers differentiated from human small intestinal stem cells. Type 1 pili are tipped with the FimH adhesin which recognizes mannose with stereochemical specificity. Thus, enhanced production of highly mannosylated proteins on intestinal epithelia promoted FimH-mediated ETEC adhesion, while conversely, interruption of FimH lectin-epithelial interactions with soluble mannose, anti-FimH antibodies or mutagenesis of fimH effectively blocked ETEC adhesion. Moreover, fimH mutants were significantly impaired in delivery of both heat-stable and heat-labile toxins to the target epithelial cells in vitro, and these mutants were substantially less virulent in rabbit ileal loop assays, a classical model of ETEC pathogenesis. Collectively, our data suggest that these highly conserved pili play an essential role in virulence of these diverse pathogens.

Author summary

Enterotoxigenic Escherichia coli (ETEC) infections contribute substantially to death and morbidity due to diarrheal illness and are associated with serious sequelae including malnutrition, stunted growth, and intellectual impairment among young children in developing countries. Effective engagement of intestinal epithelial cells is essential for ETEC pathogenicity. Consequently, pathovar specific plasmid-encoded adhesin structures known as colonization factors (CFs) have been a principal target for vaccines. However, tremendous inter-strain variation in the carriage of gene clusters encoding different CFs and significant antigenic diversity of the CF adhesins has posed a challenge to vaccine development. In contrast, type 1 pili are encoded by the fim operon located in the chromosome of most ETEC strains and are highly conserved. While type 1 pili are known to play a critical role in virulence of extraintestinal pathogenic E. coli, the present studies represent the first detailed examination of the contribution of these pili in ETEC pathogenesis. Here we demonstrate that ETEC type 1 pili are essential for optimal interactions with intestinal epithelia and that they play a critical role in virulence. These data may inform additional approaches toward development of broadly protective vaccines for these pathogens of global importance.