Type 1 pili are outer membrane organelles formed by 80% of all E. coli strains. They are up to 2 μm long, rod-like appendages covering the bacterial body like spines on a hedgehog (Fig. 1). Type 1 pili expressed by uropathogenic E. coli strains mediate adhesion to specific receptors of the bladder epithelium. They allow colonization of the cell surface as a prelude to infection. Type 1 pili are composed of a rigid rod and a fibrillar tip (Fig. 2a). The minor subunit FimH mediates adhesion and is located at the fibrillar tip as demonstrated by immunolabeling (Fig. 2b). The pilus rod is built from a structural protein, the major subunit FimA. Brinton analyzed parameters describing the helix of the type 1 pilus rod based on paracrystalline arrays. Accordingly, the data predicted one helical repeat to be composed of 25 subunits in 8 turns with a helical pitch of either 2.32 or 2.4 nm.
Here, the 3-D helical structure of the pilus rod has been determined from isolated negatively stained type 1 pili imaged in the annular dark-field mode by scanning transmission electron microscopy (STEM) (Fig. 3a). The power spectra of unbent rods (Fig. 3b) exhibit the strongest layerline at a spacing of (2.42 ± 0.04 nm)−1 representing the pitch of the putative one-start genetic helix (Fig. 3c). The axial distance to its nearest layerline corresponds to the helical repeat (Fig. 3c). Averaging by a linear superposition, as employed for structural analysis of F18 fimbriae, another type of adhering E. coli appendages, confirmed the helical repeat to be 19.31 ± 0.34 nm. Mass-per-length (MPL) determination of unstained type 1 pili by STEM revealed one helical repeat to be composed of 26 ±1 15.8 kDA FimA molecules. All possible helical selection rules were tested by D(Z,k) filtering. Thereby, any data inconsistent with the employed helical selection rule are excluded (Fig. 3d). Hence, the helical selection rule yielding the most power recovery represents the closest selection rule describing the helix. We found l = 8n + 21m to be the optimum helical selection rule. Fig. 3e represents the filtered image. Accordingly, one helical repeat is made of 8 turns containing 27 FimA monomers occurring with an axial rise of 0.72 ± 0.01 nm. D(Z,k) filtering with the helical selection rule assumed by Brinton resulted in significant loss of structure. Hence, we conclude to have refined the previous indexation. The resulting 3-D structure of the type 1 pilus unveils a 6.8 nm wide right handed helical tube harboring an axial cavity (Fig. 4). The tube wall is formed by a coiled fiber comparable to the metal thread of a spring.