Immotile Active Matter: Activity from Death and Reproduction

Abstract

Unlike equilibrium atomic solids, biofilms-soft solids composed of bacterial cells-do not experience significant thermal fluctuations at the constituent level. However, living cells stochastically reproduce and die, provoking a mechanical response. We investigate the mechanical consequences of cellular death and reproduction by measuring surface-height fluctuations of biofilms containing two mutually antagonistic strains of Vibrio cholerae that kill one another on contact via the type VI secretion system. While studies of active matter typically focus on activity via constituent mobility, here, activity is mediated by reproduction and death events in otherwise immobilized cells. Biofilm surface topography is measured in the nearly homeostatic limit via white light interferometry. Although biofilms are far from equilibrium systems, measured surface-height fluctuation spectra resemble the spectra of thermal permeable membranes but with an activity-mediated effective temperature, as predicted by Risler, Peilloux, and Prost [Phys. Rev. Lett. 115, 258104 (2015)PRLTAO0031-900710.1103/PhysRevLett.115.258104]. By comparing the activity of killer strains of V. cholerae with that of genetically modified strains that cannot kill each other and validating with individual-based simulations, we demonstrate that extracted effective temperatures increase with the amount of death and reproduction and that death and reproduction can fluidize biofilms. Together, these observations demonstrate the unique physical consequences of activity mediated by death and reproduction events.

Cellular organization in lab-evolved and extant multicellular species obeys a maximum entropy law

“…This was later confirmed by similar studies of shape fluctuations exhibited by large lipid vesicles ( Schneider et al, 1984 ). The generalization of these considerations to homeostatic tissues with cell division, rearrangements and apoptosis has also been considered ( Risler et al, 2015 ; Kalziqi et al, 2018 ). While such membrane systems may differ greatly in the specific values of their elastic modulus (and, indeed, of their microscopic membrane constituents), the viscosity of the surrounding fluid, and their physical size, the space-time correlation function of fluctuations about the equilibrium shape adopts a universal form in appropriately rescaled length and frequency variables.…”

Accumulation of dead cells from contact killing facilitates coexistence in bacterial biofilms

“…It is likely that physical effects observed in studies of lateral range expansions [67][68][69][70] also play a role in the upward growth of unconfined biofilms. Previous studies found that proliferating cells in crowded environments interact mechanically [71][72][73], pushing cells towards the expanding cell front. In analogy, during vertical biofilm growth, the few labelled, non-killer cells may be mechanically pushed off the agar surface by neighbouring cells that proliferate.…”

Spatial propagation of electrical signals in circular biofilms: A combined experimental and agent-based fire-diffuse-fire study

“…Biofilms are responsible for most clinical infections [8] and related processes, such as biofouling, are costly to industry [9]. Central to this mode of growth is the ability of bacteria to coordinate behavior and act as a multicellular organism [5][6][7][10][11][12]. Therefore, understanding biofilm regulation is important in trying to control or prevent biofilm formation.…”