Polar features in the flagellar propulsion of E. coli bacteria

TL;DR

This study investigates the complex flagellar dynamics of E. coli bacteria, revealing two distinct run states with different propulsion features using advanced holographic microscopy.

Contribution

It uncovers the existence of two asymmetric propulsion states in E. coli, providing detailed 3D motion analysis and hydrodynamic comparisons between them.

Findings

Most cells have two run states with different forces and torques.

Bundle conformations differ between forward and backward runs.

Hydrodynamic properties vary between the two run states.

Abstract

E. coli bacteria swim following a run and tumble pattern. In the run state all flagella join in a single helical bundle that propels the cell body along approximately straight paths. When one or more flagellar motors reverse direction the bundle unwinds and the cell randomizes its orientation. This basic picture represents an idealization of a much more complex dynamical problem. Although it has been shown that bundle formation can occur at either pole of the cell, it is still unclear whether this two run states correspond to asymmetric propulsion features. Using holographic microscopy we record the 3D motions of individual bacteria swimming in optical traps. We find that most cells possess two run states characterised by different propulsion forces, total torque and bundle conformations. We analyse the statistical properties of bundle reversal and compare the hydrodynamic features of forward and backward running states. Our method is naturally multi-particle and opens up the way towards controlled hydrodynamic studies of interacting swimming cells.