The population dynamics of bacteria in physically structured habitats and the adaptive virtue of random motility
Yan Weia,b, Xiaolin Wangc, Jingfang Liuc, Ilya Nememand,e, Amoolya H. Singha,e, Howie Weissc, and Bruce R. Levina,1
-Author Affiliations
aDepartment of Biology,
bGraduate Program in Population Biology, Ecology, and Evolution,
dDepartments of Physics and Biology, and
eComputational and Life Sciences Strategic Initiative, Emory University, Atlanta, GA 30322; and
cDepartment of Mathematics, Georgia Institute of Technology, Atlanta, GA 30332
Edited* by Robert May, University of Oxford, Oxford, United Kingdom, and approved January 12, 2011 (received for review September 9, 2010)
Abstract
Why is motility so common in bacteria? An obvious answer to this ecological and evolutionary question is that in almost all habitats, bacteria need to go someplace and particularly in the direction of food. Although the machinery required for motility and chemotaxis (acquiring and processing the information needed to direct movement toward nutrients) are functionally coupled in contemporary bacteria, they are coded for by different sets of genes. Moreover, information that resources are more abundant elsewhere in a habitat would be of no value to a bacterium unless it already had the means to get there. Thus, motility must have evolved before chemotaxis, and bacteria with flagella and other machinery for propulsion in random directions must have an advantage over bacteria relegated to moving at the whim of external forces alone. However, what are the selection pressures responsible for the evolution and maintenance of undirected motility in bacteria? Here we use a combination of mathematical modeling and experiments with Escherichia coli to generate and test a parsimonious and ecologically general hypothesis for the existence of undirected motility in bacteria: it enables bacteria to move away from each other and thereby obtain greater individual shares of resources in physically structured environments. The results of our experiments not only support this hypothesis, but are quantitatively and qualitatively consistent with the predictions of our model.
experimental evolution, population dynamics, partial differential equations
Footnotes
1To whom correspondence should be addressed. E-mail: blevin@emory.edu.
Author contributions: Y.W., H.W., and B.R.L. designed research; Y.W., X.W., J.L., and B.R.L. performed research; Y.W., X.W., J.L., H.W., and B.R.L. analyzed data; and Y.W., I.N., A.H.S., H.W., and B.R.L. wrote the paper.
The authors declare no conflict of interest.
↵*This Direct Submission article had a prearranged editor.
This article contains supporting information online at
Freely available online through the PNAS open access option.
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