Aggregative adherence fimbria II, a second fimbrial antigen mediating aggregative adherence in enteroaggregative Escherichia coli

Abstract

Enteroaggregative Escherichia coli (EAEC) has been implicated as an agent of pediatric diarrhea in the developing world. We have shown previously that EAEC adheres to HEp-2 cells by virtue of a plasmid-encoded fimbrial adhesin designated aggregative adherence fimbria I (AAF/I), the genes for which have been cloned and sequenced. However, not all EAEC strains express AAF/I. Using TnphoA mutagenesis, we have characterized a novel fimbria (designated AAF/II) which mediates HEp-2 adherence of the human-pathogenic strain 042. AAF/II is 5 nm in diameter and does not bind AAF/I antiserum, as determined by immunogold transmission electron microscopy. TnphoA identified a gene (designated aafA) which bears significant homology to aggA, the fimbrial subunit of AAF/I (25% identity and 47% similarity at the amino acid level). When hyperexpressed and purified by polyhistidine tagging, the AafA protein assembled into 5-nm-diameter filaments which bound anti-AAF/II antiserum. The cloned aafA gene complemented a mutation in the aggA gene to confer fimbrial expression from the AAF/I gene cluster, manifesting phenotypes characteristic of AAF/II but not AAF/I. The aafA mutant did not adhere to human intestinal tissue in culture, suggesting a role for AAF/II in intestinal colonization. By using DNA probes for AAF/I and AAF/II derived from fimbrial biosynthesis genes, we show that AAF/I and AAF/II are each found in only a minority of EAEC strains, suggesting that still more EAEC adhesins exist. Our data suggest that AAF adhesins represent a new family of fimbrial adhesins which mediate aggregative adherence in EAEC.

Pathogenesis of Afa/Dr Diffusely AdheringEscherichia coli

“…In many instances adhesins and invasins are located on the bacterial surface in extended hair-like appendages named pili or fimbriae or in amorphous outer membrane-associated structures termed afimbrial sheaths (404). The Afa/Dr family of adhesins contains representatives having fimbrial (37,43,90,115,316,442), afimbrial (241, 243-245, 251, 455, 470), and nonfimbrial (340) architectures (Table 1).…”

“…Only the human adhesins AfaE-I, AfaE-III, Dr, Dr-II, and F1845 have been fully explored with regard to their genetic organization, receptor recognition, and involvement in Afa/Dr DAEC pathogenicity. In addition, it was noted that the EAEC adhesins AAF-I, AAF-II (90), and AAF-III (37) are probably more distantly related members of the Afa/Dr family of adhesins (91). In particular, despite similar genetic organizations of the gene clusters involved in the biogenesis of these three adhesins and Afa/Dr adhesins, it remains important to explore whether or not EAEC adhesins recognized the Afa/Dr receptors, type IV collagen, DAF (CD55), and/or carcinoembryonic antigen-related cellular adhesion molecules (CEACAMs), which play a pivotal role in Afa/Dr DAEC pathogenesis.…”

Pathogenesis of Human Diffusely Adhering Escherichia coli Expressing Afa/Dr Adhesins (Afa/Dr DAEC): Current Insights and Future Challenges

“…Moreover, Montiero et al (74), in a study investigating the presence of dispersin in pathogenic and nonpathogenic intestinal E. coli isolates, observed the presence of agg3C-positive E. coli strains despite the absence of expression of the pilin-encoding gene agg3A and have suggested that the agg3C primers may have cross-reacted with an Afa/Dr usher-encoding gene(s). Indeed, the biogenesis of the well-characterized EAEC adhesins AAF-I, -II, and -III and Hda (75)(76)(77)(78) involved a periplasmic chaperone, an outer membrane usher protein, a major adhesin subunit, and a capping subunit (79)(80)(81). Afa/Dr DAEC and EAEC strains are cause of diarrheal illness in young children.…”

“…The major families of adhesive proteins (145) include the classical chaperone/usher pathway-dependent fimbrial adhesins (146,147), the alternate chaperone/usher pathway-dependent E. coli surface pili (148), the extracellular nucleation precipitation-dependent curly or thin aggregative fimbrial adhesins (149), the type I secretion system-dependent afimbrial adhesins (150), the type III secretion system-dependent integral outer membrane proteins (151), the polymerization-assembled type IV pili (152), and the type V secretion system-dependent nonfimbrial trimeric autotransported adhesins (153). The Afa/Dr family of adhesins contains fimbrial (29,32,75,76,154,155) or afimbrial (25-27, 30, 37, 44, 64, 84, 156) adhesins (Table 1). These adhesins are encoded by genes present in operons containing five major genes, including highly conserved genes A to D, encoding accessory proteins, and more divergent genes E, encoding the adhesin subunits ( Fig.…”

“…Moreover, despite a similar genetic organization with the gene clusters triggering the biogenesis of Afa/Dr adhesins, the EAEC adhesins AAF-I (77), AAF-II (76), AAF-III (75), and Hda (78) are distant pathogenic factors of the Afa/Dr family of adhesins ( Table 1). The four major characteristics of EAEC pathogenesis (79-81) are as follows: (i) adherence to the intestinal mucosa via adhesins (18,(75)(76)(77)(78), (ii) the formation of typical "stacked-brick" microcolonies as each bacterium interacts with others, (iii) production of enterotoxins and cytotoxins, and (iv) the development of a severe mucosal inflammation. Boisen et al (78), analyzing this superfamily of adhesins, have proposed a pertinent phylogram composed of three distinct clusters.…”