Spatially controlled activity of light-driven E. coli suspensions

Suspensions of swimming bacteria, such as Escherichia coli, are widely used as model active colloids. But the swimming speed of bacteria normally cannot be easily tuned. However, E. coli can be genetically modified to swim only when illuminated with green light, potentially giving biological active colloids with an externally tuneable speed v.

We studied several mutants of E. coli for which their swimming speed can be controlled by the intensity of incident green light and we specially engineered mutants which adjust their speed rapidly in response to changes in intensity. By projecting intensity patterns of light onto a suspension of such bacteria we are then able to spatially control the activity of these suspensions over large length-scales (~1 mm).

In order to characterise our active suspensions we introduce a spatially resolved version of Differential Dynamic Microscopy. This allows us to extract key parameters, e.g. swimming speed v, relative changes in local cell density ?/?0 and the fraction of non-motile cells ?. We compare our quantitative findings with theoretical predictions that the local density is inversely proportional to the swimming speed.