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Marc Hennes

Publications and source records attributed to Marc Hennes.

6 recordsLinked to original sources
arXiv2022

Non-equilibrium dynamics of bacterial colonies -- growth, active fluctuations, segregation, adhesion, and invasion

Colonies of bacteria endowed with a pili-based self-propulsion machinery are ideal models for investigating the structure and dynamics of active many-particle systems. We study Neisseria gonorrhoeae colonies with a molecular-dynamics-based approach. A generic, adaptable simulation method for particle systems with fluctuating bond-like interactions is devised. The simulations are employed to investigate growth of bacterial colonies and the dependence of the colony structure on cell-cell interactions. In colonies, pilus retraction enhances local ordering. For colonies consisting of different types of cells, the simulations show a segregation depending on the pili-mediated interactions among different cells. These results agree with experimental observations. Next, we quantify the power-spectral density of colony-shape fluctuations in silico. Simulations predict a strong violation of the equilibrium fluctuation-response relation. Furthermore, we show that active force generation enables colonies to spread on surfaces and to invade narrow channels. The methodology can serve as a foundation for future studies of active many-particle systems at boundaries with complex shape.

cond-mat.soft↗
arXiv2019

Molecular motors govern liquid-like ordering and fusion dynamics of bacterial colonies

Bacteria can adjust the structure of colonies and biofilms to enhance their survival rate under external stress. Here, we explore the link between bacterial interaction forces and colony structure. We show that the activity of extracellular pilus motors enhances local ordering and accelerates fusion dynamics of bacterial colonies. The radial distribution function of mature colonies shows local fluid-like order. The degree and dynamics of ordering are dependent on motor activity. At a larger scale, the fusion dynamics of two colonies shows liquid-like behavior whereby motor activity strongly affects tension and viscosity.

cond-mat.soft↗
arXiv2017

Extraction du solvant d'un hydrogel par des gouttes de bactéries B. subtilis

We observe that small drops of a Bacillus subtilis suspension deposited on agar strongly increase in volume while similar bacteria-void drops do not. By measuring the bacterial concentration within the drop at different heights, we show that the biomass increase due to the constant bacterial cell-division is too small to explain the drop bloating. Rather, the increased volume is caused by the presence of surfactin - a surfactant produced by the bacteria - which induces a water flow out of the environment by an osmotic capillary effect. The required concentration is very low (< 1 mM), four orders of magnitude smaller than the concentration of, for example, glucose to produce a similar effect. The ability of B. subtilis to extract water from its environment probably contributes to collective migration modes like mass swarming. It also gives rise to a new displacement mode independent of cellular motility: By combining the osmotic and wetting effects of the surfactant, B. subtilis can actively induce the sliding of the bacterial colony on substrates tilted by angles as small as 0.1 degrees.

cond-mat.soft↗
arXiv2017

Active depinning of bacterial droplets: the collective surfing of Bacillus subtilis

How systems are endowed with migration capacity is a fascinating question with implications ranging from the design of novel active systems to the control of microbial populations. Bacteria, which can be found in a variety of environments, have developed among the richest set of locomotion mechanisms both at the microscopic and collective levels. Here, we uncover experimentally a new mode of collective bacterial motility in humid environment through the depinning of bacterial droplets. While capillary forces are notoriously enormous at the bacterial scale, even capable of pinning water droplets of millimetric size on inclined surfaces, we show that bacteria are able to harness a variety of mechanisms to unpin contact lines, hence inducing a collective slipping of the colony across the surface. Contrary to flagella-dependent migration modes like swarming we show that this much faster `colony surfing' still occurs in mutant strains of \textit{Bacillus subtilis} lacking flagella. The active unpinning seen in our experiments relies on a variety of microscopic mechanisms which could each play an important role in the migration of microorganisms in humid environment.

physics.bio-ph↗
arXiv2014

Self-induced polar order of active Brownian particles in a harmonic trap

Hydrodynamically interacting active particles in an external harmonic potential form a self-assembled fluid pump at large enough Péclet numbers. Here, we give a quantitative criterion for the formation of the pump and show that particle orientations align in the self-induced flow field in surprising analogy to ferromagnetic order where the active Péclet number plays the role of inverse temperature. The particle orientations follow a Boltzmann distribution $Φ(\mathbf{p}) \sim \exp(A p_z)$ where the ordering mean field $A$ scales with active Péclet number and polar order parameter. The mean flow field in which the particles' swimming directions align corresponds to a regularized stokeslet with strength proportional to swimming speed. Analytic mean-field results are compared with results from Brownian dynamics simulations with hydrodynamic interactions included and are found to capture the self-induced alignment very well.

cond-mat.soft↗