Mechanically driven growth of quasi-two-dimensional microbial colonies.

We study colonies of nonmotile, rod-shaped bacteria growing on solid substrates. In our model, bacteria interact purely mechanically, by pushing each other away as they grow, and consume a diffusing nutrient. We show that mechanical interactions control the velocity and shape of the advancing front, which leads to features that cannot be captured by established Fisher-Kolmogorov models. In particular, we find that the velocity depends on the elastic modulus of bacteria or their stickiness to the surface. Interestingly, we predict that the radius of an incompressible, strictly two-dimensional colony cannot grow linearly in time, unless it develops branches. Importantly, mechanical interactions can also account for the nonequilibrium transition between circular and branching colonies, often observed in the lab.

Mechano-chemostats to study the effects of compressive stress on yeast.

Cells need to act upon the elastic extracellular matrix and against steric constraints when proliferating in a confined environment, leading to the build-up, at the population level, of a compressive, growth-induced, mechanical stress. Compressive mechanical stresses are ubiquitous to any cell population growing in a spatially-constrained environment, such as microbes or most solid tumors. They remain understudied, in particular in microbial populations, due to the lack of tools available to researchers. Here, we present various mechano-chemostats: microfluidic devices developed to study microbes under pressure. A mechano-chemostat permits researchers to control the intensity of growth-induced pressure through the control of cell confinement, while keeping cells in a defined chemical environment. These versatile devices enable the interrogation of physiological parameters influenced by mechanical compression at the single cell level and set a standard for the study of growth-induced compressive stress.

Dynamic structure factor of a stiff polymer in a glassy solution.

We provide a comprehensive overview of the current theoretical understanding of the dynamic structure factor of stiff polymers in semidilute solution based on the wormlike chain (WLC) model. We extend previous work by computing exact numerical coefficients and an expression for the dynamic mean square displacement (MSD) of a free polymer and compare various common approximations for the hydrodynamic interactions, which need to be treated accurately if one wants to extract quantitative estimates for model parameters from experimental data. A recent controversy about the initial slope of the dynamic structure factor is thereby resolved. To account for the interactions of the polymer with a surrounding (sticky) polymer solution, we analyze an extension of the WLC model, the glassy wormlike chain (GWLC), which predicts near power law and logarithmic long-time tails in the dynamic structure factor.

Stretching dynamics of semiflexible polymers.

We analyze the nonequilibrium dynamics of single inextensible semiflexible biopolymers as stretching forces are applied at the ends. Based on different (contradicting) heuristic arguments, various scaling laws have been proposed for the propagation speed of the backbone tension which is induced in response to stretching. Here, we employ a newly developed unified theory to systematically substantiate, restrict, and extend these approaches. Introducing the practically relevant scenario of a chain equilibrated under some prestretching force f (pre) that is suddenly exposed to a different external force f (ext) at the ends, we give a concise physical explanation of the underlying relaxation processes by means of an intuitive blob picture. We discuss the corresponding intermediate asymptotics, derive results for experimentally relevant observables, and support our conclusions by numerical solutions of the coarse-grained equations of motion for the tension.