Measurements of the Rotation of the Flagellar Motor by Bead Assay

Measuring Bacterial Flagellar Motor Dynamics via a Bead Assay

The bacterial flagellar motor (BFM) is a rotary molecular machine that drives critical bacterial processes including motility, chemotaxis, biofilm formation, and infection. For over two decades, the bead assay, which measures the rotation of a microparticle attached to the flagellum of a surface-attached bacterium, has been instrumental in deciphering the motor's biophysical mechanisms. This technique has not only quantified the rotational speed and frequency of directional switching as a function of the viscous load on the flagellum but has also revealed the BFM's capacity for mechanosensitive speed modulation, adapting to environmental conditions. Yet, many fundamental mechanistic details of the BFM remain to be discovered, and recent breakthroughs in structural biology, providing atomic-level structures of many motor components, further fuel this active area of biophysical research. This chapter presents an in-depth protocol for the bead assay to measure BFM dynamics, emphasizing advanced methodologies for quantifying the intricate mechanics and rich behavior of this molecular machine.

Novel Amiloride Derivatives That Inhibit Bacterial Motility across Multiple Strains and Stator Types

The bacterial flagellar motor (BFM) is a protein complex that confers motility to cells and contributes to survival and virulence. The BFM consists of stators that are ion-selective membrane protein complexes and a rotor that directly connects to a large filament, acting as a propeller. The stator complexes couple ion transit across the membrane to torque that drives rotation of the motor. The most common ion gradients that drive BFM rotation are protons (H⁺) and sodium ions (Na⁺). The sodium-powered stators, like those in the PomA/PomB stator complex of Vibrio spp., can be inhibited by sodium channel inhibitors, in particular, by phenamil, a potent and widely used inhibitor. However, relatively few new sodium motility inhibitors have been described since the discovery of phenamil. In this study, we characterized two possible motility inhibitors, HM2-16F and BB2-50F, from a small library of previously reported amiloride derivatives. We used three approaches: effect on rotation of tethered cells, effect on free-swimming bacteria, and effect on rotation of marker beads. We showed that both HM2-16F and BB2-50F stopped rotation of tethered cells driven by Na⁺ motors comparable to phenamil at matching concentrations and could also stop rotation of tethered cells driven by H⁺ motors. Bead measurements in the presence and absence of stators confirmed that the compounds did not inhibit rotation via direct association with the stator, in contrast to the established mode of action of phenamil. Overall, HM2-16F and BB2-50F stopped swimming in both Na⁺ and H⁺ stator types and in pathogenic and nonpathogenic strains. IMPORTANCE Here, we characterized two novel amiloride derivatives in the search for antimicrobial compounds that target bacterial motility. These compounds were shown to inhibit flagellar motility at 10 μM across multiple strains: from nonpathogenic Escherichia coli with flagellar rotation driven by proton or chimeric sodium-powered stators, to proton-powered pathogenic E. coli (enterohemorrhagic E. coli or uropathogenic E. coli [EHEC or UPEC, respectively]), and finally, sodium-powered Vibrio alginolyticus. Broad antimotility compounds such as these are important tools in our efforts to control virulence of pathogens in health and agricultural settings.

Coupling Ion Specificity of the Flagellar Stator Proteins MotA1/MotB1 of Paenibacillus sp. TCA20

The bacterial flagellar motor is a reversible rotary molecular nanomachine, which couples ion flux across the cytoplasmic membrane to torque generation. It comprises a rotor and multiple stator complexes, and each stator complex functions as an ion channel and determines the ion specificity of the motor. Although coupling ions for the motor rotation were presumed to be only monovalent cations, such as H⁺ and Na⁺, the stator complex MotA1/MotB1 of Paenibacillus sp. TCA20 (MotA1^TCA/MotB1^TCA) was reported to use divalent cations as coupling ions, such as Ca²⁺ and Mg²⁺. In this study, we initially aimed to measure the motor torque generated by MotA1^TCA/MotB1^TCA under the control of divalent cation motive force; however, we identified that the coupling ion of MotA1^TCAMotB1^TCA is very likely to be a monovalent ion. We engineered a series of functional chimeric stator proteins between MotB1^TCA and Escherichia coli MotB. E. coli ΔmotAB cells expressing MotA1^TCA and the chimeric MotB presented significant motility in the absence of divalent cations. Moreover, we confirmed that MotA1^TCA/MotB1^TCA in Bacillus subtilis ΔmotABΔmotPS cells generates torque without divalent cations. Based on two independent experimental results, we conclude that the MotA1^TCA/MotB1^TCA complex directly converts the energy released from monovalent cation flux to motor rotation.