Chemotactic Patterns without Chemotaxis

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Entry: Chia Wei Hsu, AP 225, Fall 2010

M.P. Brenner, Chemotactic Patterns without Chemotaxis, Proc. Natl. Acad. Sci, 107, 11653–11654 (2010).

Summary

This commentary addresses the study by Cates et al (ref 1) where an effective model is used to describe the pattern formation in chemotaxis. In particular, the effective model considers not the mechanisms of the chemotaxis but the averaged effect of it, thus greatly reducing the complexity of the modeling. Brenner uses this study as an example to show the promise of using effective models to understand biological systems.


Pattern Formation in Chemotaxis

Chemotaxis is the phenomenon where bacteria and other organisms direct their motion based on the environment. For example, bacteria can swim up the food concentration gradient to get to a region with more food, or swim down the poison concentration gradient to avoid poisoning.

The chemotaxis phenomenon results in pattern formation. Is has been found that (ref 2) when chemotactic bacteria swim through a small tube of rich medium, the bacteria form a dense band that moves at constant velocity. Subsequent theoretical works concluded that in order to qualitatively reproduce this behavior, detailed knowledge of the nutrient consumption and the production/depletion of chemo-attractant is required. It is disturbing that such a simple collective behavior requires such a detailed level of understanding. This motivates the research for a simpler description.

Effective Model

The model of Cates et al (ref 1) boldly ignores the direct interaction between bacteria and attractant fields. Rather, they assume a density-dependent swim speed <math>v=v(\rho)</math>, where <math>\rho</math> is the density of bacteria. This swim speed is assumed to decrease with increasing <math>\rho</math>. Thus, there is a net drift in the direction of increasing bacteria density. Since bacteria density tends to be higher in regions with more attractant, this models creates the same effect: that bacteria drift toward high attractant concentration.


Connection to Soft Matter

References

[1] Cates ME, Marenduzzo D, Pagonabarraga I, and Tailleur J, "Arrested phase separation in reproducing bacteria: A generic route to pattern formation," Proc Natl Acad Sci USA 107, 11715–11720 (2010).

[2] Adler J, "Chemotaxis in bacteria," Science 153, 708–716 (1966).