Force/Extension Curves of Fimbriae
(A) Force/extension curve when two fimbriae attach to a mannose-coated tip retracting at 2 μm/s. After the first fimbria snaps off the tip, the second fimbria continues to be attached and is pulled until it detaches. The pulling force in the first constant-force region is close to twice that in the second constant-force region, and two distinct break events are observed, indicating that individual fimbriae are being probed.
(B) Reversible uncoiling and coiling of a single fimbria. After pulling a fimbria for a set distance, the pulling direction was reversed and the fimbria was observed to coil back at a nonzero force. After a second reversal to the initial pulling velocity, the force returned to its original level. These data show that both uncoiling and coiling are sequential and reversible. The spring constant of the cantilever used in these pulls was 7.9 pN/nm, and the pulls were done on two different bacteria. Most of the >5,000 successful constant velocity pulls we observed are qualitatively similar to those in Figures 1B or 2A with two (shown here) or more fimbriae. The main difference was in the length of the constant-force region as expected from the variability of the length of fimbriae.