Mechanisms of Flagellar Switch Activation

Abstract

Bacteria are amongst the most resilient living organisms that rapidly and robustly adapt to different types of environments. Their remarkable success in colonizing almost all parts of the planet is aided in part by their ability to swim. Escherichia coli swim with the aid of rotary motors that rotate extracellular filaments called the flagella. The flagellar motor carries a switch complex that reverses the direction of rotation of the motor between clockwise (CW) and counterclockwise (CCW). Switching is necessary for the cell to navigate towards favorable chemical environments – a process called chemotaxis. The key to successful chemotaxis is homeostasis in switching, which is enabled by multiple mechanisms. In this work, we have focused on explaining the unknown mechanisms that activate the switch. We employed single-cell level assays in combination with imaging and particle-tracking algorithms to probe these activation mechanisms. With these tools, we have found that a major signaling molecule in the human gastrointestinal tract, indole, promotes switching by directly interacting with the motor. Based on our findings, we propose that indole-based activation of the switch possibly predated the coupling of the chemotaxis network with the flagellar motor. Next, we investigated the mechanisms by which the motor switches its direction of rotation. Specifically, we tested a recently proposed model in which a part of the switch complex, a homo-oligomeric ring called FliG, dramatically expands and contracts to facilitate switching. Our preliminary findings do not support this model. Finally, we probed if the morphology of the cell influenced the adaptability of the flagellar switch, finding a weak degradation in the latter when cells became longer. We expect our work to provoke exciting research undertakings in the future to probe the role of metabolites and antibiotics in chemotaxis and bacterial infections.