The Three-Dimensional Structure of CFA/I Adhesion Pili: Traveler’s Diarrhea Bacteria Hang on by a Spring

Abstract

SummaryTo survive the harsh environment of a churning intestinal tract, bacteria attach to the host epithelium via thin fibers called pili (or fimbriae). Enterotoxigenic Escherichia coli expressing CFA/I pili and related pili are the most common known bacterial cause of diarrheal disease, including traveler's diarrhea. CFA/I pili, assembled via the alternate chaperone pathway, are essential for binding and colonization of the small bowel by these pathogenic bacteria. We elucidate unique structural features of CFA/I pili that appear to optimize their function as bacterial tethers in the intestinal tract. Using transmission electron microscopy of negatively stained sample in combination with iterative threedimensional helical reconstruction methods for image processing, we have determined the structure of the CFA/I pilus filament. Our results indicate that strong end-to-end protein interactions and weak interactions between the coils of a sturdy spring-like helix provide the combination of strength, stability, and flexibility required to sustain bacterial adhesion and incite intestinal disease. We propose that CFA/I pili behave like a spring to survive the harsh environment of a churning intestinal tract, thereby persisting long enough for these bacteria to colonize the host epithelium and cause enteric disease. KeywordsAdhesion Pili; Fimbriae; ETEC; Helical Reconstruction; Electron Microscopy and Image Processing Enteric bacterial infection is a significant worldwide health risk, particularly for infants and small children. Enterotoxigenic Escherichia coli (ETEC) are the most frequently isolated cause of community-acquired childhood diarrhea in the developing world, accounting for over 200 million episodes of illness 1; 2 and 380,000 deaths 2 per year, and the most common cause of travelers' diarrhea 3 . Bacterial adhesion is a key initiating step in the infection process, frequently correlated with the ability of bacteria to colonize host tissue. Adhesion is mediated by colonization factor antigens, with Colonization Factor Antigen I (CFA/I) a commonly isolated virulence factor (see, e.g. review by Qadri et al. 4 , and earlier references 5; 6 ). The long, thin surface filaments of CFA/I pili (also called "fimbriae") mediate attachment of ETEC to the small intestine during the earliest pathogenic stages in the host, and are essential for binding and colonization of the small bowel 7 , yet few details of their fine structure are known.* To whom correspondence should be addressed. E-mail: bullitt@bu.edu The views expressed in this article are those of the authors and do not necessarily reflect the official position of the National Institutes of Health, the Department of the Navy, Department of Defense, nor the U.S. Government.Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it i...

The Structure of the CS1 Pilus of Enterotoxigenic Escherichia coli Reveals Structural Polymorphism

“…Thus, the average symmetry of the CS1 pili was determined to be a 113.5°right-handed rotation around the helical axis with an 8.7-Å rise per subunit. These helical parameters are close to those of CFA/I pili (113.6°/8.3 Å [19]). We used IHRSR (38) to determine the CS1 pilus structure.…”

“…The filament symmetries determined here for CS1, 112 o -113°r otation and ϳ9-Å rise, are comparable to those of CFA/I class 5 pili (113.6°/8.3 Å [19]) and those of UPEC P pili and type I pili from the class 1 family (109.6°/7.6 Å for P pili [18]; 106.7/7.2 Å for type I pili [20]). The orientation of the CooA subunits in filament models 1 and 2 is similar to that of CfaB in the CFA/I model (7), while model 3 differs significantly.…”

“…Bullitt and colleagues found that the head-to-tail interactions between the CfaB subunits of the 1-start helix in CFA/I were more extensive than those between subunits on consecutive turns or "layers" of that helix (19). Our structural analysis of the CS1 pili shows that the extent of the head-to-tail interactions depends upon the structural state of the filament.…”

“…Electron microscopy and three-dimensional (3D) image processing of P pili and type I pili from UPEC and CFA/I pili from ETEC, combined with X-ray crystallography of the pilin subunits and subunit complexes, reveal a conserved filament architecture, where subunits are associated in a head-to-tail manner, with the Nte on one subunit, nϩ1, inserted into the hydrophobic groove of the adjacent subunit, n (7,(18)(19)(20). Subunits align with their cylindrical axes almost perpendicular to the filament axis and wrap around the filament with a rotation of ϳ110 o and a rise of ϳ8 Å per pilin monomer for a right-handed 1-start helix.…”

Contribution of the Highly Conserved EaeH Surface Protein to Enterotoxigenic Escherichia coli Pathogenesis

“…The majority of pathogenesis (10)(11)(12) and molecular epidemiology (13) studies, as well as subsequent vaccine development efforts (14,15), have focused primarily on plasmid-encoded fimbrial colonization factors (CFs), which are felt to be critical for colonization of the small intestine.…”