Figure 1.

Quantifying the nanomechanics of type IV pili on living bacteria using chemical force microscopy. (a,b) AFM deflection images of wild-type P. aeruginosa bacteria recorded in buffer (a) and in air (b), showing that surface appendages can only be visualized in air. Adhesion force histogram (c) and rupture length histogram (d) obtained by recording force curves in M63 medium between a hydrophobic tip and the polar region of a wild-type P. aeruginosa cell (n = 1024 curves). Adhesion values in (c) correspond to the largest adhesion forces observed in each curve, while rupture lengths in (d) correspond to the last rupture events. All curves were obtained using a contact time of 100 ms, a maximum applied force of 250 pN, and approach and retraction speeds of 1.0 μm s^–1. Similar data were obtained using three different tips and three cells from different cultures. (e,f) Stretching individual pili yields constant force plateaus: (e) typical plateau curves composed of a region at zero force followed by a progressive, nonlinear increase in the force (red arrows) to reach the constant force regime, and (f) histogram of the average plateau forces (n = 90 curves from three experiments). As illustrated in the right panel, force plateau signatures are believed to result from force-induced conformational changes within the pili. (g,h) Stretched pili also show single linear force peaks, indicating that they behave as nanosprings: typical linear force peak signatures (g) and histogram of maximum adhesion forces (n = 61 curves from three experiments) (h) as well as quantification of spring-like properties, estimation of pilus spring constant k_p (h; inset). Superimposition of 10 curves shows that spring-like properties are highly reproducible.