Effects of Spatial Structure and Reduced Growth Rates on Evolution in Bacterial Populations

Abstract

The evolutionary forces that create and maintain the awesome diversity observed in microbial communities and populations are not well understood. For the most part, previous studies on microbial evolution have been done using model species that are grown in well-mixed homogenous environments in which cells experience continuous or episodic periods of exponential growth. The relevance of these experimental systems to the evolution of naturally occurring populations is limited because bacterial populations in most environments reside in spatially structured heterogeneous habitats in which cell growth is slowed by nutrient limitation and cells often experience prolonged periods of stasis. Here we review and discuss how spatial structure and slow growth influence the evolution of microbial populations. We focus our discussion on microbial populations contained within biofilms, which are complex assemblages of microbial cells enclosed in self-made extracellular matrices. Biofilm-bound populations have spatial structure and contain subpopulations that are growing slowly. Studies have shown that spatial structure limits competition to a local scale thereby protracting selective sweeps. Additionally, reduced growth rates directly impact the rate at which selection can alter genotype frequencies in populations. Combined, these characteristics lead to the emergence and maintenance of genetic diversity within biofilms. We contend that the findings of studies on evolutionary processes in bacterial biofilms can readily be extrapolated to other spatially structured microbial habitats, such as soils and sediments, in which nutrient limitation causes slow growth.