Studies of natural and experimental biofilms provide a way to explore the importance of spatial structure on maintenance and generation of diversity in microbial ecosystems. Biofilms are complex communities of microorganisms attached to surfaces and encased in an intricate matrix of extracellular polymeric substances which serve to create complex environmental gradients. Relative to their planktonic counterparts, cells within biofilms can exhibit exceptional resistance to antibiotics and disinfectants. Capable of forming on many indwelling medical aids, such as catheters and artificial joints, biofilms pose a significant risk of infection in humans. In industrial settings they enhance pipe corrosion, decrease the efficiency of heat transfer in water-cooled systems, and adversely affect drinking water quality.
It is not well understood how phenotypic heterogeneity originates within biofilms and to what extent interactions among individual cells are influenced by that diversity. To address these topics, we have developed a simple conceptual model of biofilm growth where new cells are descended from ancestors positioned closer to the substratum. This conceptual model (termed the ‘Onion Model’) can be mathematically modeled and explicitly tested because the distribution of clonal variants within spatially structured biofilms should be consistent with their temporal phylogeny. Our empirical data and modeling studies have shown that genetic diversification in biofilms is rapid and extensive, and is consistent with a selectively neutral model of biofilm development.