FIG. 7.

Drawing showing possible ways in which pili can tether bacteria to a surface. In panel A, the tip touches the surface and long pili from bacteria near the tip apex or near the tip base are folded and adhere with their distal end to the mica surface. (B) On tip retraction, pili straighten out, and the shortest pili exerts a force on the cantilever, thus producing a force signature in the force spectrum. (C) On further retraction, some part of the bacterium-pili-substrate complex breaks (most likely the pili-substrate bond) and the cantilever returns to its neutral position. On further retraction, the above process will be repeated with the next longest pili. (D) An alternate model, discussed in the text, of the form of the tether that anchors bacteria to the substrate. Here, several turns of short pili fibers are unraveled and the constituent pilin molecules are denatured, thus forming a long amino acid chain with much lower persistence length than whole pili.