Cellular reproduction and death induce diffusion of immotile bacteria in growing biofilms

Biofilms, surface-adhered communities of bacteria, are often studied in the framework of colloidal soft matter physics. Yet, in contrast to non-living colloids, cells are living matter which can reproduce and die. We study the statistical-mechanical consequences of these life events in Vibrio cholerae biofilms. While the bacterial cells are inherently immotile in biofilms, reproduction and death induce cellular rearrangement, providing an effective diffusive behavior of individual cells. We confirm such diffusive dynamics experimentally by developing a protocol to track individual bacteria during biofilm growth. We vary the rate of reproduction and death to explore the relation between these life and death events and the resulting cellular dynamics. We find that cellular rearrangement is strongly time-dependent, exhibiting stages of diffusive, super-diffusive, and sub-diffusive behavior. Further, cellular dynamics in biofilms emerges from an interplay of growth and death and reorganization dynamics in biofilms. The results demonstrate that life and death events are important aspects in dense cellular populations, and provide a rich source of non-equilibrium dynamics in living soft matter.