Microscopic Analysis of Bacterial Motility at High Pressure

Abstract

The bacterial flagellar motor is a molecular machine that converts ion flux to the rotation of a helical flagellar filament [1]. Its counterclockwise rotation allows several filaments to join in a bundle and propel the cell forward in solution. Loss of motility can be caused by environmental factors, such as temperature, pH and solvation. Hydrostatic pressure is also a physical inhibitor of bacterial motility, but the detailed mechanism is still unknown. Here, we developed a novel assay that monitored the motility of Escherichia coli cells under various hydrostatic pressure conditions [2]. The fraction and speed of swimming cells decreased with increased pressure. At 80 MPa, all cells stopped swimming, and diffused in solution. After the release of pressure, most cells immediately recovered their initial motility. A rotating tethered cell assay demonstrated that single flagellar motors at 80 MPa rotated with ∼60% of their initial speed, meaning that the motor still generates the torque at high pressure. The discrepancy between free swimming cells and tethered cells could be explained by that applied pressure inhibited the rapid motor rotation and/or change the helical structure of flagella. [1] Sowa et al., Nature. 437: 916-919 (2005). [2] Nishiyama et al., Biophys. J. 96: 1142-1150 (2009).