Rotational tumbling of Escherichia coli aggregates under shear

TL;DR

This study investigates the rheological behavior of Escherichia coli cultures under shear, revealing complex viscosity changes during growth phases and observing aggregate tumbling, supported by a theoretical model of cell attachment and flow.

Contribution

It introduces a new experimental analysis of E. coli rheology under shear and proposes a simple model explaining aggregate tumbling behavior during growth.

Findings

Viscosity increases with cell population during growth.

E. coli aggregates exhibit rotational tumbling under shear.

A theoretical model explains aggregate behavior based on cell attachment.

Abstract

Growing living cultures of Escherichia coli bacteria were investigated using real-time in situ rheology and rheo-imaging 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 stabilises, the viscosity also keeps its maximum constant value, with drops and recoveries, for a long period of time. This complex rheological behaviour, which was 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 was given to the late phase of growth of E. coli populations under shear. Rheo-imaging measurements revealed, near the static plate, a rotational motion of E. coli aggregates, collectively tumbling and flowing in the shear direction. To explain this behaviour, we introduce a simple theoretical model, in which the individual cells are transported by the flow, but remain rigidly attached to the other cells of the aggregate.