Substrate stiffness impacts early biofilm formation by modulating twitching motility.

Surface-associated lifestyles dominate in the bacterial world. Large multicellular assemblies, called biofilms, are essential to the survival of bacteria in harsh environments and are closely linked to antibiotic resistance in pathogenic strains. Biofilms stem from the surface colonization of a wide variety of substrates encountered by bacteria, from living tissues to inert materials.

Symmetry-breaking, motion and bistability of active drops through polarization-surface coupling.

Cell crawling crucially depends on the collective dynamics of the acto-myosin cytoskeleton. However, it remains an open question to what extent cell polarization and persistent motion depend on continuous regulatory mechanisms and autonomous physical mechanisms. Experiments on cell fragments and theoretical considerations for active polar liquids have highlighted that physical mechanisms induce motility through splay and bend configurations in a nematic director field.

Self-repair protects microtubules from destruction by molecular motors.

Microtubule instability stems from the low energy of tubulin dimer interactions, which sets the growing polymer close to its disassembly conditions. Molecular motors use ATP hydrolysis to produce mechanical work and move on microtubules. This raises the possibility that the mechanical work produced by walking motors can break dimer interactions and trigger microtubule disassembly.

Thin-film modeling of resting and moving active droplets.

We propose a generic model for thin films and shallow drops of a polar active liquid that have a free surface and are in contact with a solid substrate. The model couples evolution equations for the film height and the local polarization in the form of a gradient dynamics supplemented with active stresses and fluxes. A wetting energy for a partially wetting liquid is incorporated allowing for motion of the liquid-solid-gas contact line.

CLASP Mediates Microtubule Repair by Restricting Lattice Damage and Regulating Tubulin Incorporation.

Microtubules play a key role in cell division, motility, and intracellular trafficking. Microtubule lattices are generally regarded as stable structures that undergo turnover through dynamic instability of their ends [1]. However, recent evidence suggests that microtubules also exchange tubulin dimers at the sites of lattice defects, which can be induced by mechanical stress, severing enzymes, or occur spontaneously during polymerization [2-6].

Lattice defects induce microtubule self-renewal.

Microtubules are dynamic polymers, which grow and shrink by addition and removal of tubulin dimers at their extremities. Within the microtubule shaft, dimers adopt a densely packed and highly ordered crystal-like lattice structure, which is generally not considered to be dynamic. Here we report that thermal forces are sufficient to remodel the microtubule shaft, despite its apparent stability.

Modelling of surfactant-driven front instabilities in spreading bacterial colonies.

The spreading of bacterial colonies at solid-air interfaces is determined by the physico-chemical properties of the involved interfaces. The production of surfactant molecules by bacteria is a widespread strategy that allows the colony to efficiently expand over the substrate. On the one hand, surfactant molecules lower the surface tension of the colony, effectively increasing the wettability of the substrate, which facilitates spreading.

Equilibrium Contact Angle and Adsorption Layer Properties with Surfactants.

The three-phase contact line of a droplet on a smooth surface can be characterized by the Young equation. It relates the interfacial energies to the macroscopic contact angle θ. On the mesoscale, wettability is modeled by a film-height-dependent wetting energy f( h).

Continuous versus Arrested Spreading of Biofilms at Solid-Gas Interfaces: The Role of Surface Forces.

We introduce and analyze a model for osmotically spreading bacterial colonies at solid-air interfaces that includes wetting phenomena, i.e., surface forces.

Self-repair promotes microtubule rescue.

The dynamic instability of microtubules is characterized by slow growth phases stochastically interrupted by rapid depolymerizations called catastrophes. Rescue events can arrest the depolymerization and restore microtubule elongation. However, the origin of these rescue events remains unexplained.

A variational approach to the growth dynamics of pre-stressed actin filament networks.

In order to model the growth dynamics of elastic bodies with residual stresses a thermodynamically consistent approach is needed such that the cross-coupling between growth and mechanics can be correctly described. In the present work we apply a variational principle to the formulation of the interfacial growth dynamics of dendritic actin filament networks growing from biomimetic beads, an experimentally well studied system, where the buildup of residual stresses governs the network growth. We first introduce the material model for the network via a strain energy density for an isotropic weakly nonlinear elastic material and then derive consistently from this model the dynamic equations for the interfaces, i.

Microtubules self-repair in response to mechanical stress.

Microtubules--which define the shape of axons, cilia and flagella, and provide tracks for intracellular transport--can be highly bent by intracellular forces, and microtubule structure and stiffness are thought to be affected by physical constraints. Yet how microtubules tolerate the vast forces exerted on them remains unknown. Here, by using a microfluidic device, we show that microtubule stiffness decreases incrementally with each cycle of bending and release.

Traveling waves and global oscillations triggered by attractive molecular interactions in an excitable system.

During pattern formation in spatially extended systems, different mechanisms with different characteristic length scales, e.g., reaction-diffusion processes or molecular interactions, can be active.

Spontaneous polarization in an interfacial growth model for actin filament networks with a rigorous mechanochemical coupling.

Many processes in eukaryotic cells, including cell motility, rely on the growth of branched actin networks from surfaces. Despite its central role the mechanochemical coupling mechanisms that guide the growth process are poorly understood, and a general continuum description combining growth and mechanics is lacking. We develop a theory that bridges the gap between mesoscale and continuum limit and propose a general framework providing the evolution law of actin networks growing under stress.

Nonlinear elasticity of cross-linked networks.

Cross-linked semiflexible polymer networks are omnipresent in living cells. Typical examples are actin networks in the cytoplasm of eukaryotic cells, which play an essential role in cell motility, and the spectrin network, a key element in maintaining the integrity of erythrocytes in the blood circulatory system. We introduce a simple mechanical network model at the length scale of the typical mesh size and derive a continuous constitutive law relating the stress to deformation.

Complex wave patterns in an effective reaction-diffusion model for chemical reactions in microemulsions.

An effective medium theory is employed to derive a simple qualitative model of a pattern forming chemical reaction in a microemulsion. This spatially heterogeneous system is composed of water nanodroplets randomly distributed in oil. While some steps of the reaction are performed only inside the droplets, the transport through the extended medium occurs by diffusion of intermediate chemical reactants as well as by collisions of the droplets.

Self-ratcheting Stokes drops driven by oblique vibrations.

We develop and analyze a minimal hydrodynamic model to understand why a drop climbs a smooth homogeneous incline that is harmonically vibrated at an angle different from the substrate normal [P. Brunet, J. Eggers, and R.

Ratchet-driven fluid transport in bounded two-layer films of immiscible liquids.

We present a two-layer thin film model allowing us to study the behavior of a general class of 'wettability ratchets' that can be employed to transport a continuous phase. Brownian ratchets, in contrast, are normally used to transport particles or molecules within a continuous carrier fluid without transporting the fluid itself. The wettability ratchet is based on a switchable, spatially asymmetric, periodic interaction of the free surface of the film and the walls.

Nonlinear study of symmetry breaking in actin gels: implications for cellular motility.

Force generation by actin polymerization is an important step in cellular motility and can induce the motion of organelles or bacteria, which move inside their host cells by trailing an actin tail behind. Biomimetic experiments on beads and droplets have identified the biochemical ingredients to induce this motion, which requires a spontaneous symmetry breaking in the absence of external fields. We find that the symmetry breaking can be captured on the basis of elasticity theory and linear flux-force relationships.

Alternative mechanisms of structuring biomembranes: self-assembly versus self-organization.

We study two mechanisms for the formation of protein patterns near membranes of living cells by mathematical modelling. Self-assembly of protein domains by electrostatic lipid-protein interactions is contrasted with self-organization due to a nonequilibrium biochemical reaction cycle of proteins near the membrane. While both processes lead eventually to quite similar patterns, their evolution occurs on very different length and time scales.

Travelling lipid domains in a dynamic model for protein-induced pattern formation in biomembranes.

Cell membranes are composed of a mixture of lipids. Many biological processes require the formation of spatial domains in the lipid distribution of the plasma membrane. We have developed a mathematical model that describes the dynamic spatial distribution of acidic lipids in response to the presence of GMC proteins and regulating enzymes.

Dynamical model for chemically driven running droplets.

We propose coupled evolution equations for the thickness of a liquid film and the density of an adsorbate layer on a partially wetting solid substrate. Therein, running droplets are studied assuming a chemical reaction underneath the droplets that induces a wettability gradient on the substrate and provides the driving force for droplet motion. Two different regimes for moving droplets--reaction-limited and saturated regime--are described.

Transbilayer movement of phospholipids at the main phase transition of lipid membranes: implications for rapid flip-flop in biological membranes.

The transbilayer movement of fluorescent phospholipid analogs in liposomes was studied at the lipid phase transition of phospholipid membranes. Two NBD-labeled analogs were used, one bearing the fluorescent moiety at a short fatty acid chain in the sn-2 position (C(6)-NBD-PC) and one headgroup-labeled analog having two long fatty acyl chains (N-NBD-PE). The transbilayer redistribution of the analogs was assessed by a dithionite-based assay.

Rapid transbilayer movement of the fluorescent sterol dehydroergosterol in lipid membranes.

This study establishes a new assay for measuring the transbilayer movement of dehydroergosterol (DHE) in lipid membranes. The assay is based on the rapid extraction of DHE by methyl-beta-cyclodextrin (M-CD) from liposomes. The concentration of DHE in the liposomal membrane was measured by using fluorescence resonance energy transfer (FRET) from DHE to dansyl-phosphatidylethanolamine, which is not extracted from liposomes by M-CD.