Motorization of bacterial pili is key to generate traction forces to achieve cellular function. The Tad (or Type IVc) pilus from Caulobacter crescentus is a widespread motorized nanomachine crucial for bacterial survival, evolution and virulence. An unusual bifunctional ATPase motor drives Tad pilus retraction, which helps the bacteria to land on target surfaces. Here, we use a novel platform combining a fluorescence-based screening of piliated bacteria and atomic force microscopy (AFM) force-clamp spectroscopy, to monitor over time (30 s) the nanomechanics and dynamics of the Tad nanofilament retraction under a high constant tension (300 pN). We observe striking transient variations of force and height originating from two phenomena: active pilus retraction and passive hydrophobic interactions between the pilus and the hydrophobic substrate. That the Tad pilus is able to retract under high tensile loading – at a velocity of ∼150 nm s⁻¹ – indicates that this nanomachine is stronger than previously anticipated. Our findings show that pilus retraction and hydrophobic interactions work together to mediate bacterial cell landing and surface adhesion. The motorized pilus retraction actively triggers the cell to approach the substrate. At short distances, passive hydrophobic interactions accelerate the approach phenomenon and promote strong cell-substrate adhesion. This mechanism could provide a strategy to save ATP-based energy by the retraction ATPase. Our force-clamp AFM methodology offers promise to decipher the physics of bacterial nanomotors with high sensitivity and temporal resolution.

中文翻译：

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AFM 力钳光谱捕获 Tad 菌毛回缩的纳米力学

细菌菌毛的电动化是产生牵引力以实现细胞功能的关键。来自Caulobacter crescentus的 Tad（或 IVc 型）菌毛是一种广泛使用的电动纳米机器，对细菌的生存、进化和毒力至关重要。一个不寻常的双功能 ATPase 马达驱动 Tad 菌毛收缩，这有助于细菌落在目标表面上。在这里，我们使用一种新的平台，结合基于荧光的菌毛细菌筛选和原子力显微镜 (AFM) 力钳光谱，以随着时间的推移 (30 s) 监测 Tad 纳米丝在高恒定张力下收缩的纳米力学和动力学(300 pN)。我们观察到源自两种现象的力和高度的显着瞬态变化：主动菌毛回缩和菌毛与疏水底物之间的被动疏水相互作用。Tad 菌毛能够在高拉伸载荷下收缩——速度约为 150 nm s ^-1– 表明该纳米机器比之前预期的要强大。我们的研究结果表明，菌毛回缩和疏水相互作用共同介导细菌细胞着陆和表面粘附。电动菌毛收缩主动触发细胞接近基材。在短距离内，被动疏水相互作用加速了接近现象并促进了细胞与底物的强粘附。这种机制可以提供一种通过收缩 ATPase 来节省基于 ATP 的能量的策略。我们的力钳 AFM 方法有望以高灵敏度和时间分辨率破译细菌纳米马达的物理学。