Random motility regulation drives the fragmentation of microbial ecosystems

The self-organization of microbial ecosystems involves a large variety of mechanisms, ranging from biochemical signaling to population dynamics. Among these, the role of motility regulation has been little studied, despite the importance of active migration processes. Here we show how weak, random motility regulation generically induces the fragmentation of bacterial ecosystems comprising a large number of coexisting strains. To do so, we simulate microscopic models of run-and-tumble bacteria whose self-propulsion speeds are regulated by the local density of each strain. Our simulations reveal that, as the heterogeneity of the interaction network increases, the ecosystem undergoes a phase transition leading to the emergence of distinct communities. To account for these results and assess their robustness, we use random-matrix theory to analyze the hydrodynamic description of the bacterial ecosystem, obtaining a quantitative agreement with our microscopic simulations. Our results are shown to hold for a variety of motility-regulation mechanisms and should be relevant to the study of community formation by motile organisms.