Monday, 14 April, 2014
Reproduce or rather conquer the world? Surprisingly also bacteria have to face this question. Theoretical biophysicists of the NIM cluster now calculated how the organisms should decide to optimally maintain their species.
The bacterium Bacillus subtilis is quite adaptable. It moves about in liquids and on agar surfaces by means of flagella. Alternatively, it can stick to an underlying substrate. Actually, the bacteria proliferate most effectively in this stationary state, while motile bacteria reproduce at a notably lower rate.
In order to sustain and extend the colony, bacteria primarily require sufficient nutrients. Moving slowly would mean that nutrients are soon used up. But adventurous bacteria that decide to move out fast in search of the land of Cockaigne may end up feeling lonely.
Up to now such experiments have only been performed on homogeneous bacterial cultures consisting of genetically identical cells. The LMU theorists are the first to calculate the behavior of heterogeneous Bacillus subtilis populations. For this purpose, they assign a fixed migration rate to each bacterium, which is inherited by its progeny. This rate determines the proportion of time the organisms spend in a motile state, as opposed to being engaged in nutrient uptake and proliferation. In the simulated scenario, the bacteria are, for instance, seeded at the center of a plate containing nutrients. They gradually consume the nutrients available locally and are forced to colonize unpopulated territory.
Fighting for the best spots
The model reveals that, initially, there is intense competition for the limited resources at the edge of the occupied territory. During this phase, individuals that reproduce fast and occupy as much space as possible are at an advantage. But they are soon superseded by colonies of generalists, which devote equal time to migration and proliferation. These gradually spread out, forming sectors as shown in the Figure.
The Munich theorists explain this behavior in terms of the invasion speed of so-called Fisher fronts. This speed is maximal in the case of a balance between growth and motility. Initially, colonies that steer a middle course form individual sectors, until they eventually occupy the entire invasion front.
Range Expansion of Heterogeneous Populations. Matthias Reiter, Steffen Rulands, Erwin Frey. Phys. Rev. Lett. 112, 148103 (2014). Published online: 11 April 2014
Prof. Erwin Frey
Arnold-Sommerfeld-Center for Theoretical Physics
Phone: +49 (0)89 / 2180 - 4538
Matthias Reiter: Reiter.Matthias(at)physik.uni-muenchen.de
Dr. Steffen Rulands: Steffen.Rulands(at)physik.uni-muenchen.de