Bacteria on a surface grow like a continuous culture

Bacteria are usually studied in well-mixed environments such as in shaken tubes or chemostats. However, bacteria often live on surfaces and migrate in space while they grow. The growth laws of such planar bacterial populations have been less studied. Here we employ a novel method for quantifying growth and gene expression in space and time and find that motile bacteria expand outward and continuously leave a portion of the population behind. The advancing bacteria grow and keep their density constant, similar to growth in a chemostat.

HFSP Long-Term Fellow Daniel Koster and HFSP Program Grant holder Uri Alon and colleagues
authored on Tue, 18 December 2012

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, an open question is: what is the relationship between growth, motility and spatial position, or what growth model describes such bacterial colonies. To answer these questions, 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.

Figure: Bacteria in nature often thrive on surfaces and reach nutrients by chemotaxis. Quantitative measurements of their growth show that a population of bacteria that migrates on a surface grows at constant density, similar to bacterial growth in a continuous culture such as a chemostat. Image copyright: Daniel Koster and Tremani ( Journal of Molecular Biology, 2012 Dec 7;424 (3-4): 180-91, D. Koster et al.).

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, which 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 relatively higher levels of the chemotaxis and galactose metabolism genes fliC, fliL and galE than those that stay behind in the center of the plate.

Bacterial biology has been frequently tested in chemostat conditions. This study suggests that the chemostat is more than a useful tool for holding bacteria at constant conditions and that the chemostat may be a good model for advancing motile populations. In a chemostat, the bacteria are held at the boundary between exponential and stationary phases—a phase that is passed very rapidly in batch culture but, based on the present study, may be relevant to spatially growing populations.

Reference

Surface Growth of a Motile Bacterial Population Resembles Growth in a Chemostat. Daniel A Koster, Avi Mayo, Anat Bren and Uri Alon.  Journal of Molecular Biology, 2012 Dec 7;424(3-4):180-91. doi:10.1016/j.jmb.2012.09.005.

Pubmed link