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The Mechanical Properties of E. coli Type 1 Pili Measured by Atomic Force Microscopy Techniques

Eric Miller ^*, Tzintzuni Garcia , Scott Hultgren ^* and Andres F. Oberhauser 

^* Department of Molecular Microbiology, Washington University School of Medicine, St. Louis, Missouri 63110; and Sealy Center for Structural Biology, Department of Neuroscience and Cell Biology and Department of Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, Texas 77555

Correspondence: Address reprint requests to Andres F. Oberhauser, University of Texas Medical Branch, Galveston, TX 77555. Tel.: 409-772-1309; Fax: 409-772-1301; E-mail: afoberha{at}utmb.edu.

The first step in the encounter between a host and a pathogen is attachment to the host epithelium. For uropathogenic Escherichia coli, these interactions are mediated by type 1 and P adhesive pili, which are long (1 µm) rods composed of more than 1000 protein subunits arranged in a helical structure. Here we used single-molecule atomic force microscopy to study the mechanical properties of type 1 pili. We found that type 1 pili readily extend under an applied force and that this extensibility is the result of unwinding the pilus rod's helical quaternary structure. The forced unraveling is also reversible, with helical rewinding taking place under considerable forces (60 pN). These data are similar to those obtained on P pili using optical tweezers, indicating that these are conserved properties of uropathogenic E. coli pili. We also show that our data can readily be reproduced using Monte Carlo simulation techniques based on a two-state kinetic model. This model provides a simple way to extrapolate the mechanical behavior of pili under a wide range of forces. We propose that type 1 pilus unraveling is an essential mechanism for absorbing physiological shear forces encountered during urinary tract infections and probably essential for adhesion and colonization of the bladder epithelium.

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