Liquid channels within B. subtilis biofilms allow the escape of trapped clones and population rescue*

Bacterial communities undergoing spatial expansions, typically exhibit a loss of genetic diversity due to gene surfing, a phenomenon by which only cells close to the expanding edge have a chance of contributing to the next generation. As a result, mutants that emerge behind the front remain trapped in the bulk of the population, even if they carry a significant growth advantage. While these findings are applicable to smooth, uniformly dense colonies, where cell spatial re-arrangement is limited, it is unclear whether they hold in bacterial communities with complex three-dimensional morphologies, such as wrinkly biofilms.

We employ an experimental procedure that enables a rapid switch of the chemical environment with minimal physical manipulation of the community to investigate the fate of trapped clones carrying a selective advantage in wrinkly B. subtilis biofilms. We find that these clones are able to use the channels underneath the wrinkles to traverse long distances and take over the expansion front. Moreover, we find that active motility is required to accomplish this task. Collectively, our findings reveal an intriguing novel role of wrinkles in enabling potentially advantageous mutants to propel themselves towards the leading edge of the expansion and rescue the population upon environmental change. Our results also point at motility, which has traditionally received little attention in communities on hard substrates, as a key mechanism for population rescue in biofilms with complex morphologies