  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.
