Destabilization of a flow focused suspension of magnetotactic bacteria

Active matter is a new class of material, intrinsically out-of equilibrium with intriguing properties. So far, the recent upsurge of studies has mostly focused on the spontaneous behavior of these systems --in the absence of external constraints or driving--. Yet, many real life systems evolve under constraints, being both submitted to flow and various taxis. In the present work, we demonstrate a new experimental system which opens up the way for quantitative investigations, and discriminating examinations, of the challenging theoretical description of such systems. We explore the behavior of magnetotactic bacteria as a particularly rich and versatile class of driven matter, which behavior can be studied under contrasting and contradicting stimuli. In particular we demonstrate that the competing driving of an orienting magnetic field and hydrodynamic flow lead not only to jetting, but also unveils a new pearling instability. This illustrates new structuring capabilities of driven active matter

Les bactéries magnétotactiques en tant que fluide actif dirigé : du comportement individuel vers des effets collectifs

.We report the work we lead on magneto tactic bacteria, from the point of view of active matter. The ability of this bacterium to swim at 100µm/s directed by the magnetic field makes it a good candidate to study driven active matter. Indeed, in this configuration, the self-propelled system is not dragged by an external force, and its directed motion comes from its biased orientation. We choose the strain MC-1 for our study, for the robustness of its individual behavior and its swimming speed. We studied the individual behavior, confirming previous results where the bacteria passively aligns on the magnetic field being disoriented solely by the magnetic field, but also succeeded in triggering activity in their reorientation, suspending it in different chemical environments, or directing them against a solid interface, where this bacteria could tumble. This tumbling behavior, very common amongst non-magnetic bacteria, was not reported for Mangetotactic bacteria. These new results leaded us to develop a model of Run and Tumble under a magnetic field. We studied their behavior when densely concentrated in a micro-channel, in jammed configuration, using standard microfluidics tools. We observed their motion in hour glass shaped micro-channels, without any flow, and characterized the chronology of the jamming process. We investigated the interaction of their swim with a shear, in a counter flow experiments, where MC-1 would be directed against a Poiseuille flow. Due to equilibrium between the magnetic torque and the hydrodynamic shear, bacteria would focus instantly in the middle of the channel. We studied this phenomenon theoretically, and checked our model with the experiments. We discovered a instability of a new kind in the same configuration, for high magnetic fields. Indeed, beyond a threshold the focused suspension would become wavy to end up in segregated droplets of bacteria. We characterized experimentally this phenomenon which reminds us of Rayleigh-Plateau and Kelvin-Helmholtz instabilities, varying the flow rate, the Magnetic field and the density of the suspension. Recirculation in the droplets is observed and explained. We interpret these convection droplets as the source of the instability of the focused suspensio

Self-transport of swimming bacteria is impaired by porous microstructure

International audienceMotility is a fundamental survival strategy of bacteria to navigate porous environments, where they mediate essential biogeochemical processes in quiescent wetlands and sediments. However, a comprehensive understanding of the mechanisms regulating self-transport in the confined interstices of porous media is lacking, and determining the interactions between cells and surfaces of the solid matrix becomes paramount. Here, we precisely track the movement of bacteria (Magnetococcus marinus) through a series of microfluidic porous media with broadly varying geometries and show how successive scattering events from solid surfaces decorrelate cell motion. Ordered versus disordered media impact the cells' motility over short ranges, but their large-scale transport properties are regulated by the cutoff of their persistent motility. An effective mean free path is established as the key geometrical parameter controlling cell transport, and we implement a theoretical model that universally predicts the effective cell diffusion for the diverse geometries studied here. These results aid in our understanding of the physical ecology of swimming cells, and their role in environmental and health hazards in stagnant porous media. Bacteria often reside within complex microenvironments through which they have to navigate efficiently. This paper presents an experimental study of bacterial motility and dispersion within ordered and disordered arrays of obstacles as a proxy to a realistic porous medium and established a bacterial mean free path as the determining factor for bacterial navigation

Morphological and physiological impacts of salinity on colonial strains of the cyanobacteria Microcystis aeruginosa

Abstract In the context of global change and enhanced toxic cyanobacterial blooms, cyanobacterial transfer to estuaries is likely to increase in frequency and intensity and impact animal and human health. Therefore, it is important to evaluate the potential of their survival in estuaries. In particular, we tested if the colonial form generally observed in natural blooms enhanced the resistance to salinity shock compared to the unicellular form generally observed in isolated strains. We tested the impact of salinity on two colonial strains of Microcystis aeruginosa, producing different amounts of mucilage by combining classical batch methods with a novel microplate approach. We demonstrate that the collective organization of these pluricellular colonies improves their ability to cope with osmotic shock when compared to unicellular strains. The effect of a sudden high salinity increase (S ≥ 20) over 5 to 6 days had several impacts on the morphology of M. aeruginosa colonies. For both strains, we observed a gradual increase in colony size and a gradual decrease in intercellular spacing. For one strain, we also observed a decrease in cell diameter with an increase in mucilage extent. The pluricellular colonies formed by both strains could withstand higher salinities than unicellular strains studied previously. In particular, the strain producing more mucilage displayed a sustained autofluorescence even at S = 20, a limit that is higher than the most robust unicellular strain. These results imply survival and possible M. aeruginosa proliferation in mesohaline estuaries

Anomalous percolation flow transition of yield stress fluids in porous media

Yield stress fluid (YSF) flows through porous materials are fundamental to biological, industrial, and geophysical processes, from blood and mucus transport to enhanced oil recovery. Despite their widely recognized importance across scales, the emergent transport properties of YSFs in porous environments remain poorly understood due to the nonlinear interplay between complex fluid rheology and pore microstructure. Here, we combine microfluidic experiments and nonlinear network theory to uncover an anomalous, hierarchical yielding process in the fluidization transition of a generic YSF flowing through a random medium. Percolation of a single fluidized filament gives way to pathways that branch and merge to form a complex flow network within the saturated porous medium. The evolution of the fluidized network with the flowing fraction of YSF results in a highly nonlinear flow conductivity and reveals a novel dispersion mechanism, resulting from the rerouting of fluid streamlines. The identified flow percolation phenomenon has broad implications for YSF transport in natural and engineered systems, and provides a tractable archetype for a diverse class of breakdown phenomena. © 2019 American Physical Society.National Science Foundation (U.S.) (Award: CBET-1510768)James S. McDonnell Foundatio

Bacterial scattering in microfluidic crystal flows reveals giant active Taylor–Aris dispersion

The natural habitats of planktonic and swimming microorganisms, from algae in the oceans to bacteria living in soil or intestines, are characterized by highly heterogeneous fluid flows. The complex interplay of flow-field topology, self-propulsion, and porous microstructure is essential to a wide range of biophysical and ecological processes, including marine oxygen production, remineralization of organic matter, and biofilm formation. Although much progress has been made in the understanding of microbial hydrodynamics and surface interactions over the last decade, the dispersion of active suspensions in complex flow environments still poses unsolved fundamental questions that preclude predictive models for microbial transport and spreading under realistic conditions. Here, we combine experiments and simulations to identify the key physical mechanisms and scaling laws governing the dispersal of swimming bacteria in idealized porous media flows. By tracing the scattering dynamics of swimming bacteria in microfluidic crystal lattices, we show that hydrodynamic gradients hinder transverse bacterial dispersion, thereby enhancing stream-wise dispersion ∼100-fold beyond canonical Taylor–Aris dispersion of passive Brownian particles. Our analysis further reveals that hydrodynamic cell reorientation and Lagrangian flow structure induce filamentous density patterns that depend upon the incident angle of the flow and disorder of the medium, in striking analogy to classical light-scattering experiments.National Science Foundation (Award CBET- 1510768

Genomic study of a novel magnetotactic Alphaproteobacteria uncovers the multiple ancestry of magnetotaxis

La publication est en ligne ici : https://onlinelibrary.wiley.com/doi/abs/10.1111/1462-2920.14364International audienc

The abolition of the localisation of assets requirement for EU insurers and the emerging 'asset allocation gap' An empirical analysis

SIGLEAvailable from British Library Document Supply Centre-DSC:7755.03445(97/7) / BLDSC - British Library Document Supply CentreGBUnited Kingdo