Mechanical Forces to Trigger Morphological Changes in Motile Bacteria

Bacterial swarming is an important mechanism for flagellated bacteria to effectively cover large distances on soft surfaces, such as tissues. Swarming is likely initiated when the flagella sense higher viscous loads such as those experienced near solid boundaries. However, the underlying assumption that soft and porous surfaces cause significantly high loads on the flagella remains untested. Here, we developed an approach to estimate the hydrodynamic drag as a function of separation from soft interfaces. In one type of testing, optical trapping experiments were employed to estimate the diffusivities of tiny objects close to soft interfaces, using a novel flow-geometry. The method was validated by comparing results for transverse diffusion coefficients for single spherical particles near a no-slip solid boundary with predictions from Faxen’s law. The data were in good agreement with published models. In a second type of testing, the surface-induced load on the flagellar motor was interpreted from the rates of swarming in two different strains that developed differential hydrodynamic thrust. These results are anticipated to help estimate the magnitude of mechanical forces necessary to initiate the transition of a bacterial cell to the swarmer state.