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

Phenotypic heterogeneity is one of the hallmarks of the biofilm lifestyle, where even isogenic populations give rise to spatially organized and phenotypically distinct subpopulations. One such pattern is generated by the ability of several biofilm-forming bacteria to switch between a flagellated and a matrix producing state. Here, using Bacillus subtilis as a model system, we investigate the role of this switch during biofilm development on a solid-air interface. By comparing the matrix-flagella spatio-temporal patterns in wild-type biofilms with mixtures of flagella- and matrix-null mutants biofilms, we find that pattern formation does not require a phenotypic switch that enables individual cells to respond to the local environment, but can be explained by a completely stochastic switch coupled to a phenotype-dependent fitness landscape that selects phenotypes at the population level. Integration of experiments and physical models shows that the coexistence between flagellated and matrix-producing cells provides the population with enhanced resilience to environmental changes, by enabling cells to manipulate and harness the local morphological and transport properties within the biofilm. Our results not only reveal a new evolutionary advantage of phenotypic plasticity in biofilms, but also illustrate how the biology and ecology of these populations are intrinsically tied to their physical properties.