Rotational tumbling of Escherichia coli aggregates under shear

Growing living cultures of Escherichia coli bacteria are investigated using real-time in situ rheology and rheoimaging measurements. In the early stages of growth (lag phase) and when subjected to a constant stationary shear, the viscosity slowly increases with the cell's population. As the bacteria reach the exponential phase of growth, the viscosity increases rapidly, with sudden and temporary abrupt decreases and recoveries. At a certain stage, corresponding grossly to the late phase of growth, when the population stabilizes, the viscosity also keeps its maximum constant value, with drops and recoveries, for a long period of time. This complex rheological behavior, which is observed to be shear strain dependent, is a consequence of two coupled effects: the cell density continuous increase and its changing interacting properties. Particular attention is given to the late phase of growth of E. coli populations under shear. Rheoimaging measurements reveal, near the static plate, a rotational motion of E. coli aggregates, collectively tumbling and flowing in the shear direction. This behavior is interpreted in the light of a simple theoretical approach based on simple rigid body mechanics.

Figure 1

E. coli cultures steady-state shear viscosity growth curve $\eta \left(t\right)$ measured at a constant shear rate of 10 ${\mathrm{s}}^{-1}$ (black) and in a culture medium (gray) (representative curves). Dashed lines separate distinct growth phases, the lag phase, exponential phase, and late phase, determined by ${\text{OD}}_{620\text{nm}}$ and population colony-forming units (cfu/ml) measurements. All measurements are performed at $37{}^{\circ }\mathrm{C}$. The inset shows schematic details of the image acquisition setup at an intermediate radius-plate position.

Figure 2

Each row corresponds to a four-image sequence (1 image/s) extracted from rheoimaging measurements of an E. coli culture, in order to illustrate the aggregates' rotational motion (the rotating aggregates are indicated by the white arrows and a full rotation may be seen for each sequence). The images were collected during steady-state shear, at a constant shear rate of 10 ${\mathrm{s}}^{-1}$, in the late phase of growth. Each image is $225\mu \mathrm{m}$ wide ($640\times 480$ pixels). Measurements were performed at $37{}^{\circ }\mathrm{C}$. A full video may be seen in [8].