The biomechanical properties of E. coli pili for urinary tract attachment reflect the host environment

Abstract

Uropathogenic Escherichia coli express pili that mediate binding to host tissue cells. We demonstrate with in situ force measuring optical tweezers that the ability of P and type 1 pili to elongate by unfolding under exposure to stress is a shared property with some differences. The unfolding force of the quaternary structures under equilibrium conditions is similar, 28 +/- 2 and 30 +/- 2 pN for P pili and type 1 pili, respectively. However, type 1 pili are found to be more rigid than P pili through their stronger layer-to-layer bonds. It was found that type 1 pili enter a dynamic regime at elongation speeds of 6 nm/s, compared to 400 nm/s for P pili; i.e., it responds faster to an external force. This possibly helps type 1 to withstand the irregular urine flow in the urethra as compared to the more constant urine flow in the upper urinary tract. Also, it was found that type 1 pili refold during retraction at two different levels that possibly could be related to several possible configurations. Our findings highlight functions that are believed to be of importance for the bacterial ability to sustain a basic antimicrobial mechanism of the host and for bacterial colonization.

FIGURE 1

Schematic energy landscape diagram of the interactions between neighboring units elongated along the long axis of the pilus rod (the reaction coordinate, denoted by x). State A represents the closed layer-to-layer bond, state B the head-to-tail interaction that makes up the backbone of the rod. The position of the maximum of the energy landscape curve between the states A and B are referred to as the transition state, and is denoted by t. The uppermost curve represents the energy landscape for a pilus not exposed to any force, whereas the lower curve refers to the case when the pilus is exposed to a force equal to the unfolding force of the quaternary structure of the rod, represents the distance from the minimum of state A (in the presence of a force) to the transition state, whereas is the distance from the transition state to the minimum of state B. represents the bond opening length along the reaction coordinate. The energies and are those of the energy barrier and the difference between state A and B, respectively. State C represents an alternative configuration of the head-to-tail bond.

FIGURE 2

Panels A and B show a typical force-versus-elongation response of a single P pili and type 1 pili during unfolding (black curve) and refolding (gray curve) at an elongation speed of 0.1 μm/s.

FIGURE 3

Comparison of the force-versus-elongation response of a type 1 pilus extended at a speed of 0.1 and 0.5 μm/s, respectively. It is clearly seen that the unfolding of region II is in dynamic regime whereas region III is unaffected by the difference in elongation speed.

FIGURE 4

Typical force-versus-time responses for relaxation of P and type 1 pili from an initial force of ∼50 pN to steady state. An initial force of 50 pN was set through elongation at a fixed speed, 40 μm/s for P pili and 0.3 μm/s for type 1 pili. When the elongation was stopped the pili relaxed to steady state. Type 1 pili reached steady state in ∼7 s and P pili in ∼0.2 s. The dashed line is a fit of Eq. 7 over the force interval 35–50 pN. The solid line corresponds to a numerical simulation of Eq. 6 using the assessed parameters. The overlap of the two curves justifies that 35 pN is high enough to neglect refolding.

FIGURE 5

Force-versus-elongation speed where the elongation speed was derived through using the parameters assessed via the fit in Fig. 4. The solid curve is a simulation of the force data, based on Eq. 6.

FIGURE 6

A refolding sequence of a type 1 pilus that shows a force drop, which almost always is permanent whenever it occurs.