  Bacterial biofilms, surface-attached communities of cells, are in some
respects similar to colloidal solids; both are densely packed with non-zero
yield stresses. However, unlike non-living materials, bacteria reproduce and
die, breaking mechanical equilibrium and inducing collective dynamic responses.
We report experiments and theory investigating the motion of immotile Vibrio
cholerae, which can kill each other and reproduce in biofilms. We vary
viscosity by using bacterial variants that secrete different amounts of
extracellular matrix polymers, but are otherwise identical. Unlike
thermally-driven diffusion, in which diffusivity decreases with increased
viscosity, we find that cellular motion mediated by death and reproduction is
independent of viscosity over timescales relevant to bacterial reproduction. To
understand this surprising result, we use two separate modeling approaches.
First we perform explicitly mechanical simulations of one-dimensional chains of
Voigt-Kelvin elements that can die and reproduce. Next, we perform an
independent statistical approach, modeling Brownian motion with the classic
Langevin equation under an effective temperature that depends on cellular
division rate. The diffusion of cells in both approaches agrees quite well,
supporting a kinetic interpretation for the effective temperature used here and
developed in previous work. As the viscoelastic behavior of biofilms is
believed to play a large role in their anomalous biological properties, such as
antibiotic resistance, the independence of cellular diffusive motion ---
important for biofilm growth and remodeling --- on viscoelastic properties
likely holds ecological, medical, and industrial relevance.
