Turning bacteria suspensions into a "superfluid"

The rheological response under simple shear of an active suspension of Escherichia coli is determined in a large range of shear rates and concentrations. The effective viscosity and the time scales characterizing the bacterial organization under shear are obtained. In the dilute regime, we bring evidences for a low shear Newtonian plateau characterized by a shear viscosity decreasing with concentration. In the semi-dilute regime, for particularly active bacteria, the suspension display a "super-fluid" like transition where the viscous resistance to shear vanishes, thus showing that macroscopically, the activity of pusher swimmers organized by shear, is able to fully overcome the dissipative effects due to viscous loss

Effect of motility on the transport of bacteria populations through a porous medium

The role of activity on the hydrodynamic dispersion of bacteria in a model porous medium is studied by tracking thousands of bacteria in a microfluidic chip containing randomly placed pillars. We first evaluate the spreading dynamics of two populations of motile and nonmotile bacteria injected at different flow rates. In both cases, we observe that the mean and the variance of the distances covered by the bacteria vary linearly with time and flow velocity, a result qualitatively consistent with the standard geometric dispersion picture. However, quantitatively, the motile bacteria display a systematic retardation effect when compared to the nonmotile ones. Furthermore, the shape of the traveled distance distribution in the flow direction differs significantly for both the motile and the nonmotile strains, hence probing a markedly different exploration process. For the nonmotile bacteria, the distribution is Gaussian, whereas for the motile ones, the distribution displays a positive skewness and spreads exponentially downstream akin to a Γ distribution. The detailed microscopic study of the trajectories reveals two salient effects characterizing the exploration process of motile bacteria: (1) the emergence of an "active" retention effect due to an extended exploration of the pore surfaces and (2) an enhanced spreading at the forefront due to the transport of bacteria along "fast tracks" where they acquire a velocity larger than the local flow velocity. We finally discuss the practical applications of these effects on the large-scale macroscopic transfer and contamination processes caused by microbes in natural environments.Fil: Creppy, Adama. Centre National de la Recherche Scientifique; FranciaFil: Clément, Eric. Université Paris Diderot - Paris 7; FranciaFil: Douarche, Carine. Centre National de la Recherche Scientifique; FranciaFil: D'angelo, María Verónica. Universidad de Buenos Aires. Facultad de Ingeniería; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Auradou, Harold. Centre National de la Recherche Scientifique; Franci

Magnetotactic bacteria in a droplet self-assemble into a rotary motor

From intracellular protein trafficking to large scale motion of animal groups, the physical concepts driving the self-organization of living systems are still largely unraveled. Selforganization of active entities, leading to novel phases and emergent macroscopic properties, recently shed new lights on these complex dynamical processes. Here we show that, under the application of a constant magnetic field, motile magnetotactic bacteria confined in water-in-oil droplets self-assemble into a rotary motor exerting a torque on the external oil phase. A collective motion in the form of a large-scale vortex, reversable by inverting the field direction, builds-up in the droplet with a vorticity perpendicular to the magnetic field. We study this collective organization at different concentrations, magnetic fields and droplets radii and reveal the formation of two torque-generating areas close to the droplet interface. We characterize quantitatively the mechanical energy extractable from this new biological and self-assembled motor

3D spatial exploration by E. coli echoes motor temporal variability

Unraveling bacterial strategies for spatial exploration is crucial for understanding the complexity in the organization of life. Bacterial motility determines the spatio-temporal structure of microbial communities, controls infection spreading and the microbiota organization in guts or in soils. Most theoretical approaches for modeling bacterial transport rely on their run-and-tumble motion. For Escherichia coli, the run time distribution was reported to follow a Poisson process with a single characteristic time related to the rotational switching of the flagellar motors. However, direct measurements on flagellar motors show heavy-tailed distributions of rotation times stemming from the intrinsic noise in the chemotactic mechanism. Currently, there is no direct experimental evidence that the stochasticity in the chemotactic machinery affect the macroscopic motility of bacteria. In stark contrast with the accepted vision of run-and-tumble, here we report a large behavioral variability of wild-type \emph{E. coli}, revealed in their three-dimensional trajectories. At short observation times, a large distribution of run times is measured on a population and attributed to the slow fluctuations of a signaling protein triggering the flagellar motor reversal. Over long times, individual bacteria undergo significant changes in motility. We demonstrate that such a large distribution of run times introduces measurement biases in most practical situations. Our results reconcile the notorious conundrum between run time observations and motor switching statistics. We finally propose that statistical modeling of transport properties currently undertaken in the emerging framework of active matter studies, should be reconsidered under the scope of this large variability of motility features.Comment: 12 pages, 7 figures, Supplementary information include

Étude de l'adsorption de l'ADN simple brin et double brin aux interfaces

Les interactions acides nucléiques - interfaces jouent un rôle important à la fois dans de nombreux phénomènes biologiques et dans les biotechnologies. Ce travail de thèse est consacré l'étude de l'adsorption de l'ADN sur deux types d'interfaces: (i) des surfaces solides de silicium fonctionnalisées et (ii) l'interface eau/air. L'étude de ces systèmes simples a été entreprise afin de mieux comprendre les interactions existant in vivo et de faire progresser les biotechnologies impliquant des surfaces. (i) Nous avons élaboré un protocole de fonctionnalisation chimique du silicium permettant d'obtenir des surfaces atomiquement planes, sans oxyde, et qui possèdent une fonctionnalité chimique choisie. La caractérisation des molécules greffées (nature chimique, densité, homogénéité) a été faite par spectroscopie infrarouge et par microscopie AFM. La technique de greffage moléculaire développée permet d'obtenir des monocouches denses de molécules organiques. Nous avons alors étudié par différentes techniques les interactions moléculaires entre les acides nucléiques simple brin et double brin et différents types de surfaces: a) des surfaces greffées de composés aromatiques (phénol) par microscopie AFM et b) des surfaces chargées positivement (par greffage de groupement amines) par réflectivité de rayons X. (ii) Nous avons étudié l'adsorption de l'ADN simple brin à l'interface eau/air par une technique d'autoradiographie. Nous avons montré que cette adsorption est directement corrélée à une agrégation de l'ADN, soit en présence d'ions multivalents, soit à haute concentration en sels monovalents.LILLE1-BU (590092102) / SudocSudocFranceF

Bacillus subtilis Bacteria generate an Internal Mechanical Force within a Biofilm

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Bacterial suspensions under flow

Fluids laden with motile bacteria enter in the category of active matter, a new field currently developing at the convergence of biology, hydrodynamics and statistical physics. Such suspensions were shown recently to exhibit singular macroscopic transport properties. In this paper we review some recent results, either theoretical or experimental, on the active fluid rheology. We focus principally on bacteria suspensions and the objective is to provide the basis for understanding the emergence of the singular constitutive relations characterizing the macroscopic transport properties of such an active fluid under flow