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We introduce and analyze a model for osmotically spreading bacterial colonies at solid-air interfaces that includes wetting phenomena, i.e., surface forces. The model is based on a hydrodynamic description for liquid suspensions which is supplemented by bioactive processes. We show that surface forces determine whether a biofilm can expand laterally over a substrate and provide experimental evidence for the existence of a transition between continuous and arrested spreading for Bacillus subtilis biofilms. In the case of arrested spreading, the lateral expansion of the biofilm is confined, albeit the colony is biologically active. However, a small reduction in the surface tension of the biofilm is sufficient to induce spreading. The incorporation of surface forces into our hydrodynamic model allows us to capture this transition in biofilm spreading behavior.

Bacteria inhabit a wide variety of environments in which fluid flow is present, including healthcare and food processing settings and the vasculature of animals and plants. The motility of bacteria on surfaces in the presence of flow has not been well characterized. Here we focus on Pseudomonas aeruginosa, an opportunistic human pathogen that thrives in flow conditions such as in catheters and respiratory tracts. We investigate the effects of flow on P. aeruginosa cells and describe a mechanism in which surface shear stress orients surface-attached P. aeruginosa cells along the flow direction, causing cells to migrate against the flow direction while pivoting in a zig-zag motion. This upstream movement is due to the retraction of type IV pili by the ATPase motors PilT and PilU and results from the effects of flow on the polar localization of type IV pili. This directed upstream motility could be beneficial in environments where flow is present, allowing bacteria to colonize environments that cannot be reached by other surface-attached bacteria.

In most environments, such as natural aquatic systems, bacteria are found predominantly in self-organized sessile communities known as biofilms. In the presence of a significant flow, mature multispecies biofilms often develop into long filamentous structures called streamers, which can greatly influence ecosystem processes by increasing transient storage and cycling of nutrients. However, the interplay between hydrodynamic stresses and streamer formation is still unclear. Here, we show that suspended thread-like biofilms steadily develop in zigzag microchannels with different radii of curvature. Numerical simulations of a low-Reynolds-number flow around these corners indicate the presence of a secondary vortical motion whose intensity is related to the bending angle of the turn. We demonstrate that the formation of streamers is directly proportional to the intensity of the secondary flow around the corners. In addition, we show that a model of an elastic filament in a two-dimensional corner flow is able to explain how the streamers can cross fluid streamlines and connect corners located at the opposite sides of the channel.

Bacterial biofilms have an enormous impact on medicine, industry and ecology. These microbial communities are generally considered to adhere to surfaces or interfaces. Nevertheless, suspended filamentous biofilms, or streamers, are frequently observed in natural ecosystems where they play crucial roles by enhancing transport of nutrients and retention of suspended particles. Recent studies in streamside flumes and laboratory flow cells have hypothesized a link with a turbulent flow environment. However, the coupling between the hydrodynamics and complex biofilm structures remains poorly understood. Here, we report the formation of biofilm streamers suspended in the middle plane of curved microchannels under conditions of laminar flow. Experiments with different mutant strains allow us to identify a link between the accumulation of extracellular matrix and the development of these structures. Numerical simulations of the flow in curved channels highlight the presence of a secondary vortical motion in the proximity of the corners, which suggests an underlying hydrodynamic mechanism responsible for the formation of the streamers. Our findings should be relevant to the design of all liquid-carrying systems where biofilms are potentially present and provide new insights on the origins of microbial streamers in natural and industrial environments.

Proteins in bacteria often deploy to particular places within the cell, but the cues for localization are frequently mysterious. We found that the peripheral membrane protein SpoVM (VM) recognizes a geometric cue when localizing to a particular membrane during sporulation in Bacillus subtilis . Sporulation involves an inner cell maturing into a spore and an outer cell nurturing the developing spore. VM is produced in the outer cell, where it embeds in the membrane that surrounds the inner cell but not in the cytoplasmic membrane of the outer cell. We found that VM localized by discriminating between the positive curvature of the membrane surrounding the inner cell and the negative curvature of the cytoplasmic membrane. Membrane curvature could be a general cue for protein localization in bacteria.

We report results concerning the destabilisation of supported phospholipid bilayers in a well-defined geometry. When heating up supported phospholipid membranes deposited on highly hydrophilic glass slides from room temperature (i.e. with lipids in the gel phase), unbinding was observed around the main gel to fluid transition temperature of the lipids. It lead to the formation of relatively monodisperse vesicles, of which most remained tethered to the supported bilayer. We interpret these observations in terms of a sharp decrease of the bending rigidity modulus $\kappa$ in the transition region, combined with a weak initial adhesion energy. On the basis of scaling arguments, we show that our experimental findings are consistent with this hypothesis., Comment: 11 pages, 3 figures

Bacteria are the smallest and most abundant form of life. They have traditionally been considered as primarily planktonic organisms, swimming or floating in a liquid medium, and this view has shaped many of the approaches to microbial processes, including for example the design of most antibiotics. However, over the last few decades it has become clear that many bacteria often adopt a sessile, surface-associated lifestyle, forming complex multicellular communities called biofilms. Bacterial biofilms are found in a vast range of environments and have major consequences on human health and industrial processes, from biofouling of surfaces to the spread of diseases. Although the study of biofilms has been biologists' territory for a long time, a multitude of phenomena in the formation and development of biofilms hinges on physical processes. We are pleased to present a collection of research papers that discuss some of the latest developments in many of the areas to which physicists can contribute a deeper understanding of biofilms, both experimentally and theoretically. The topics covered range from the influence of physical environmental parameters on cell attachment and subsequent biofilm growth, to the use of local probes and imaging techniques to investigate biofilm structure, to the development of biofilms in complex environments and the modeling of colony morphogenesis. The results presented contribute to addressing some of the major challenges in microbiology today, including the prevention of surface contamination, the optimization of biofilm disruption methods and the effectiveness of antibiotic treatments., New Journal of Physics, 17, ISSN:1367-2630

Understanding interactions between membranes requires measurements on well-controlled systems close to natural conditions, in which fluctuations play an important role. We have determined, by grazing incidence X-ray scattering, the interaction potential between two lipid bilayers, one adsorbed on a solid surface and the other floating close by. We find that interactions in this highly hydrated model system are two orders of magnitude softer than in previously reported work on multilayer stacks. This is attributed to the weak electrostatic repulsion due to the small fraction of ionized lipids in supported bilayers with a lower number of defects. Our data are consistent with the Poisson-Boltzmann theory, in the regime where repulsion is dominated by the entropy of counter ions. We also have unique access to very weak entropic repulsion potentials, which allowed us to discriminate between the various models proposed in the literature. We further demonstrate that the interaction potential between supported bilayers can be tuned at will by applying osmotic pressure, providing a way to manipulate these model membranes, thus considerably enlarging the range of biological or physical problems that can be addressed., Comment: 14 pages, 8 figures

Twitching motility is a mechanism in which bacteria move about solid surfaces by repeatedly extending and retracting long extracellular filamentous structures known as pili. We investigated the effects of flow on P. aeruginosa cells and found that surface shear stress causes surface-attached P. aeruginosa cells to migrate against the flow direction while pivoting in a zig-zag motion. Because this directed motility depends on polar type IV pili and results from the effects of flow on the polar attachment of bacterial cells, we describe it as twitching flow-taxis. Finally, we examined the function of two molecular motors responsible for this motion and characterized their individual functions. Our results suggest a model in which there are two distinct modes of twitching motility.

Microstructured surfaces are common in many materials applications such as microcontact printing,[1,2] biomimetic arrays, [3]controlled-wetting surfaces,[4] superhydrophobic surfaces, [5] and self-cleaning surfaces[6]among others. The majority of strategies for surface fabrication utilize some form of photolithography to achieve patterning. Photolithographic patterning is essentially two-dimensional;[1–4]it does not allow control over geometric parameters in the third dimension such as the surface profile and curvature (the topography) of fabricated features. To overcome the inherent limitations of photolithography, the fabrication of topographically patterned substrates for applications of supported lipid bilayers[7, 8]requires a combination of microfabrication techniques: photolithography followed by anisotropic plasma dry etching and wet oxide etching,[7] or chemical vapor deposition followed by photolithography and chemical etching.[8] These pioneering methods, while successful in producing topographically patterned surfaces capable of imposing gradients of curvature on supported bilayers, are technically complex and require costly clean room or microfabrication facilities. An alternative method is thus desirable, particularly since there is intense interest in the role of curvature in the thermodynamics and dynamics of lipid bilayers[7–11]and membrane proteins[12–14]in the wider fields of biology and physics.

We present here a neutron reflectivity study of the influence of an alternative electric field on a supported phospholipid double bilayer. We report for the first time a reproducible increase of the fluctuation amplitude leading to the complete unbinding of the floating bilayer. Results are in good agreement with a semi-quantitative interpretation in terms of negative electrostatic surface tension., Comment: 12 pages, 7 figures, 1 table accepted for publication in European Physical Journal E Replaced with with correct bibliography

International audience; We demonstrate an electrohydrodynamic ͑EHD technique for separating giant unilamellar vesicles by size in polydisperse suspensions. An oscillatory electric field ͑ϳ30 Hz generates EHD flow around each vesicle close to an electrode. Nearby vesicles are entrained in the flow and the vesicles move toward one another. Upon aggregation, smaller vesicles are pulled underneath the larger vesicles, which ultimately lifts them off of the electrode. A brief spike in the electric field then serves to irreversibly adhere the bottom layer of smaller vesicles to the electrode, and the large vesicles are subsequently removed by flow. We demonstrate that a single application of this technique can remove more than 90% of the smallest vesicles (diameter < 20 µm) from a suspension of electroformed giant lipid vesicles.

Resume. Les molecules amphiphiles s’auto-associent en solutions pour former des structures complexes. Les membranes sont un exemple de ce type de structures. Ce sont des objets bidimensionnels fluctuants qui presentent des proprietes physiques remarquables, mettant en jeu une gamme d’echelles spatiales tres large : echelle moleculaire nanometrique pour l’epaisseur de la membrane, de la centaine de nanometre a la dizaine de microns pour la taille des vesicules, et jusqu’au centimetre pour lasurface des membranes supportees. Ceci explique la grande variete de techniques experimentales utilisees pour leur etude : microscopie optique classique (contraste de phase...) et interferentielle (RICM), microscopie de fluorescence, microscopie a force atomique (AFM), diffusion de rayonnement. Cette derniere technique permet en particulier d’obtenir des informations uniques sur la structure moleculaire des membranes, mais aussi sur leurs fluctuations a des echelles submicroniques. Dans ce cours nous nous focaliserons sur ce dernier point, essentiellement au travers d’exemples d’etudes des fluctuations par diffusion de neutrons et de rayons X.

Although ubiquitous, the processes by which bacteria colonize surfaces remain poorly understood. Here we report results for the influence of the wall shear stress on the early-stage adhesion of Pseudomonas aeruginosa PA14 on glass and polydimethylsiloxane surfaces. We use image analysis to measure the residence time of each adhering bacterium under flow. Our main finding is that, on either surface, the characteristic residence time of bacteria increases approximately linearly as the shear stress increases (∼0–3.5 Pa). To investigate this phenomenon, we used mutant strains defective in surface organelles (type I pili, type IV pili, or the flagellum) or extracellular matrix production. Our results show that, although these bacterial surface features influence the frequency of adhesion events and the early-stage detachment probability, none of them is responsible for the trend in the shear-enhanced adhesion time. These observations bring what we believe are new insights into the mechanism of bacterial attachment in shear flows, and suggest a role for other intrinsic features of the cell surface, or a dynamic cell response to shear stress.

Supported phospholipid bilayers are interesting model systems for biologists and present fascinating physical properties. The authors present an extensive experimental study of the dynamic properties of supported bilayers. The structure and the equilibrium properties of single and double supported bilayers were investigated with neutron reflectivity. The submicronic fluctuation spectrum of a nearly free “floating” bilayer was determined using off-specular x-ray scattering: the surface tension of the bilayer, its bending modulus, and the intermembrane potential could be determined. Using fluorescence microscopy, the authors showed that this well-controlled single bilayer can form vesicles. Destabilization occurred either at the main gel-fluid transition of the lipids and could be interpreted in terms of a decrease in the bending rigidity or under an ac low-frequency electric field applied in the fluid phase. In the latter case, the authors also studied the effect of the electric field at the molecular length scale by neutron reflectivity. In both cases, destabilization leads to the formation of relatively monodisperse vesicles. This could give further understanding on the vesicle formation mechanism and on the parameters that determine the vesicle size.