Surface growth of a motile bacterial population resembles growth in a chemostat

The growth behavior in well-mixed bacterial cultures is relatively well understood. However, bacteria often grow in heterogeneous conditions on surfaces where their growth is dependent on spatial position, especially in the case of motile populations. For such populations, the relation between growth, motility and spatial position is unclear. We developed a microscope-based assay for quantifying in situ growth and gene expression in space and time, and we observe these parameters in populations of Escherichia coli swimming in galactose soft agar plates. We find that the bacterial density and the shape of the motile population, after an initial transient, are constant in time. By considering not only the advancing population but also the fraction that lags behind, we propose a growth model that relates spatial distribution, motility and growth rate. This model, that is similar to bacterial growth in a chemostat predicts that the fraction of the population lagging behind is inversely proportional to the velocity of the motile population. We test this prediction by modulating motility using inducible expression of the flagellar sigma factor FliA. Finally, we observe that bacteria in the chemotactic ring express higher relative levels of the chemotaxis and galactose metabolism genes fliC, fliL and galE than those that stay behind in the center of the plate.