Surface Drag and Swarming in Motile Bacteria

Escherichia coli swim in liquid medium with the aid of flagellar filaments that switch erratically between clockwise (CW) and counter-clockwise (CCW) directions of rotation. When grown on semi-soft agar substrates, cells undergo subtle morphological changes and transition into a swarming state (a type of 2D group-motility). Presumably, the transition is initiated by mechanical forces arising from contact with the surface. In E. coli, the cell lengths double during swarming. Longer cells counter the increase in surface drag by increasing the number of filaments to generate additional thrust. We are interested in determining the interplay between cell lengths, filament forms, the direction of flagellar rotation and hydrodynamic interactions with the substrates during swarming.

We will present our latest findings that suggest that CCW-biased mutants fail to swarm but motility can be rescued by simply wetting the agar surface, in agreement with earlier reports. However, swarming in CW-biased mutants cannot be rescued by increasing surface wetness. To determine the underlying reason for this difference, we employed biophysical characterization assays and measured the torque generated by individual flagellar motors in the CCW and CW mutants. Results suggested anisotropic torque generation in the two directions, consistent with our previous observations in a different species [1]. However the anisotropy by itself was inadequate in explaining the inhibition of swarming in the CW-mutants. Fluorescent-labeling experiments were carried out to determine the predominant polymorphic flagellar forms in the two types of mutants. We will discuss the insights revealed by the fluorescent experiments and the resulting effects on the thrust developed by each cell. The implications of these phenotypes in swarm-inititation will also be discussed.

[1] P. P. Lele, T. Roland, A. Shrivastava, Y. Chen, H. C. Berg, “Directional dependence of stall torques in C. crescentus motors”, Nature Physics (2016), 12, 175-178.