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.

The diffusion law of DMPC and DPPC in Supported Lipid Bilayers (SLB), on different substrates, has been investigated in details by Fluorescence Recovery After Patterned Photobleaching (FRAPP). Over micrometer length scales, we demonstrate the validity of a purely Brownian diffusive law both in the gel and the fluid phases of the lipids. Measuring the diffusion coefficient as a function of temperature, we characterize the gel-to-liquid phase transition of DMPC and DPPC. It is shown that, depending on the type of substrate and the method used for bilayer preparation, completely different behaviours can be observed. On glass substrates, using the Langmuir-Blodgett deposition technique, both leaflets of the bilayer have the same dynamics. On mica, the dynamics of the proximal leaflet is slower than the dynamics of the distal leaflet, although the transition temperature is the same for both layers. Preparing bilayers from vesicle fusion in same conditions leads to more random behaviours and shifted transition temperatures.

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 recognizes a geometric cue in 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. SpoVM 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 SpoVM 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.Proteins often localize to particular positions within bacteria, sometimes in a dynamic manner. A striking but mysterious example of subcellular localization occurs during spore formation in Bacillus subtilis when SpoVM (VM) localizes to a particular patch of membrane (1). How VM discriminates between different membrane surfaces in the same cell is unknown.During sporulation, the cell divides asymmetrically to create mother cell and forespore compartments. Next, the mother cell engulfs the forespore, enveloping it with inner and outer membranes (Fig. 1A). Following engulfment a protein coat is deposited around the outer forespore membrane (2). Coat assembly depends on VM, a 26-residue peptide that is produced in the mother cell (3). VM is an amphipathic α-helix (4) that inserts into the membrane with its long axis parallel to the membrane and its hydrophobic face buried in the lipid bilayer (5). During engulfment, VM localizes to the membrane that tracks around the forespore, eventually decorating the entire surface of the forespore, as visualized using a fusion to the Green Fluorescent Protein (VM-GFP; Fig 1C) (1). Proline 9 (P9; Fig. 1B) is critical for this localization (1), as substitution of P9 with alanine (VM P9A -GFP) resulted in localization to both the cytoplasmic and the outer forespore membranes (Fig. 1D).Following engulfment, the outer forespore membrane becomes topologically isolated from the cytoplasmic membrane. We wondered if VM would adhere to the outer forespore membrane after isolation. We engineered cells to produce VM-GFP in response to an inducer and triggered synthesis of the fusion protein after engulfment. To monitor topological isolation, we stained the membranes with a membrane permeable dye, which stains all membranes, and a membrane impermeable dye, which can only access the engulfment membrane before membrane fusion (6). VM-GFP localized almost exclusively to the outer forespore membrane even when the forespore was topologically isolated (Fig. 1G-I). As a control, VM P9A -GFP synthesized after * Publisher's Disclaimer: This manuscript has been accepted for publication in Science. This version has not undergone final editing.Please refer to the complete version of record at http://www....

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.

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.