Emergence of non-motile cells in collectively-expanding populations of Proteus mirabilis

Collective motility is a cooperative behavior characterized by the coordinated migration of individuals, leading to emergent dynamics. While such self-organization can benefit a population through efficient expansion, natural microbial communities often display mixed motility. The evolutionary implications of this mixed motility are not yet well understood, particularly regarding the interactions between nonmotile and cooperating motile cells. To explore this dynamic, we investigate the swarming bacterium Proteus mirabilis, known for its robust collective migration across hard surfaces. Intriguingly, despite engineering P. mirabilis strains to prioritize swarming expansion over cell growth, we consistently observe the emergence of a nonmotile subpopulation in swarming colonies across diverse environmental conditions. Our research aims to elucidate the origin and enrichment of nonmotile cells within swarming colonies, examining factors such as mutation rates, selective pressures, and cell-to-cell interactions. By dissecting how collective motility influences the establishment of nonmotile cells, we seek to shed light on the adaptive strategies employed by microbial populations and the potential evolutionary consequences of collective motility.