Swimming bacteria often direct their movement to follow chemical gradients. This process is called chemotaxis. Most of the chemicals bacteria can sense, like sugars, oxygen or amino acids, are also consumed by them. Accordingly, chemotaxis is often a collective phenomena in which many cells move together along self-generated gradients.
For localized cells growing in soft-agar, this process leads to an expansion dynamics where a subpopulation of pioneering cells expands outwards. Expansion is easily traceable by the observation of a ’swarming ring’ (see photo).
Together with Terence Hwa and other members of his lab we have analyzed this expansion dynamics for the bacterium Escherichia coli. Combining experiments with simulations and a theoretical analysis we have uncovered how the interplay of sensing, consumption, swimming and growth leads to both, a fast expansion of pioneering cells, and an effective colonization of occupied territories.
We call this dynamics navigated range expansion to contrast it with the Fisher-Kolmogorov dynamics. The latter is an unguided form of expansion (no chemotaxis, only random diffusion like movement) and the canonical scenario of expansion dynamics discussed in the ecology literature.
Chemotaxis and the directed movement of cells along self-generated gradients leads to a huge fitness advantage of navigated range expansion. This becomes apparent when directly comparing both forms. For expanding E.coli populations this is seen in the following movie.
Scanning different growth conditions and attractant levels, our results further suggest that Escherichia coli is optimizing its swimming and sensing behavior to ensure a fast expansion dynamics into nutrient replete habitats. Particularly, E.coli swims when growth conditions are good, and also senses and degrades attractants with low nutritional value.
Together with Chenli Liu we have also studied the consequences of competition for this expansion dynamics. Combining evolution experiments with modeling we show that there exists a habitat-size dependent optimal expansion speed to colonize habitats.
For further information on the context of our studies and a short summary have a look at the nice perspective article from Henry Mattingly and Thierry Emonet from the Emonet lab.
A press release from UCSD for both papers is also available.