Abstract
Biofilms are social entities where bacteria live in tightly packed agglomerations, surrounded by self-secreted exopolymers. Since production of exopolymers is costly and potentially exploitable by non-producers, mechanisms that prevent invasion of non-producing mutants are hypothesized. Here we study long-term dynamics and evolution in Bacillus subtilis biofilm populations consisting of wild-type (WT) matrix producers and mutant non-producers. We show that non-producers initially fail to incorporate into biofilms formed by the WT cells, resulting in 100-fold lower final frequency compared to the WT. However, this is modulated in a long-term scenario, as non-producers evolve the ability to better incorporate into biofilms, thereby slightly decreasing the productivity of the whole population. Detailed molecular analysis reveals that the unexpected shift in the initially stable biofilm is coupled with newly evolved phage-mediated interference competition. Our work therefore demonstrates how collective behaviour can be disrupted as a result of rapid adaptation through mobile genetic elements.
Highlights
Biofilms are social entities where bacteria live in tightly packed agglomerations, surrounded by self-secreted exopolymers
Competition assays with the WT ancestor revealed that the transformants had comparable incorporation probabilities to the eNMPs (Fig. 7). These results indicated that producers and nonproducers showed very similar general adaptation patterns involving major changes in mobile genetic elements, some of these changes were specific to the evolved non-producers, resulting in their improved incorporation into pellicles, most likely through an advantage in interference competition
The evolution of improved invasion of biofilms by non-producers was previously observed by Zhang et al.[15]. They excluded the possibility of general adaptation being responsible for the changed social dynamics in biofilms since the evolved producer did not increase its performance towards the ancestor producer
Summary
Biofilms are social entities where bacteria live in tightly packed agglomerations, surrounded by self-secreted exopolymers. We show that non-producers initially fail to incorporate into biofilms formed by the WT cells, resulting in 100-fold lower final frequency compared to the WT This is modulated in a long-term scenario, as non-producers evolve the ability to better incorporate into biofilms, thereby slightly decreasing the productivity of the whole population. Data from various bacterial models suggest that defectors can leave a fingerprint on the evolution of social strains and promote the evolution of novel cheating-suppression mechanisms[16] These can be linked to lowering the cost of cooperation by the wild-type (WT) cells[17]. Long-term scenarios in socially heterogeneous populations of microbes are still very difficult to predict In this manuscript, we study the long-term social dynamics of co-cultures comprising matrix producer and non-producer strains using the widespread soil bacterium Bacillus subtilis. We further demonstrate how unexpected adaptive events involving mobile genetic elements can shift the social dynamics in the population and reduce biofilm formation
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