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Dynamic motility selection drives population segregation in a bacterial swarm

Significance Ecological models usually take growth rate fitness as the essential driver of population dynamics. However, as a widespread natural phenomenon, population expansion in space (or range expansion) is often governed by both motility and growth. Microbial communities offer unique systems to study how an individual’s motility contributes to space competition among heterogeneous microbial populations during range expansion. Here we show that motility heterogeneity can promote the spatial segregation of subpopulations in structured microbial communities via a dynamic motility selection mechanism. Our findings are relevant to microbial stress response and microbial ecology. The results may also provide new insight to range expansion in other biological systems, such as tumor invasion and collective stress tolerance of cancer cells in densely packed environments. Population expansion in space, or range expansion, is widespread in nature and in clinical settings. Space competition among heterogeneous subpopulations during range expansion is essential to population ecology, and it may involve the interplay of multiple factors, primarily growth and motility of individuals. Structured microbial communities provide model systems to study space competition during range expansion. Here we use bacterial swarms to investigate how single-cell motility contributes to space competition among heterogeneous bacterial populations during range expansion. Our results revealed that motility heterogeneity can promote the spatial segregation of subpopulations via a dynamic motility selection process. The dynamic motility selection is enabled by speed-dependent persistence time bias of single-cell motion, which presumably arises from physical interaction between cells in a densely packed swarm. We further showed that the dynamic motility selection may contribute to collective drug tolerance of swarming colonies by segregating subpopulations with transient drug tolerance to the colony edge. Our results illustrate that motility heterogeneity, or “motility fitness,” can play a greater role than growth rate fitness in determining the short-term spatial structure of expanding populations.