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Clostridioides difficileis the major cause of nosocomial diarrhea, which are associated with gut microbiome dysbiosis. Biofilms ofC. difficilehave been progressively linked to the pathogenesis of this bacterium and the recurrences of its infections. Though the number of conditions in whichC. difficilebiofilms are being produced is increasing, little is known about how and when biofilms are formed in the gut. Here we report that succinate, a metabolite abundantly produced by the dysbiotic gut microbiota, inducesin vitrobiofilm formation ofC. difficilestrains. We characterized the morphology and spatial composition of succinate-induced biofilms, and compared to non-induced or deoxycholate-induced biofilms, biofilms induced by succinate are significantly thicker, structurally more complex, and poorer in proteins and exopolysaccharides (EPS). We then applied transcriptomics and genetics to characterize the early stages of succinate-induced biofilm formation and we showed that succinate-induced biofilm results from major metabolic shifts and cell-wall composition changes. Similar to deoxycholate-induced biofilms, biofilms induced by succinate depend on the presence of a rapidly metabolized sugar. Finally, although succinate can be consumed by the bacteria, we found that the extracellular succinate is in fact responsible for the induction of biofilm formation through complex regulation involving global metabolic regulators and the osmotic stress response. In the context of human gut dysbiosis, succinate can limit bacterial infections through the control of innate immune responses. Collectively, our results suggest that succinate is an intestinal signal which can drive the biofilm formation and persistence ofC. difficilein the gut and increase the risk of relapse.

Thin elastic two-dimensionnal systems under compressive stresses may relieve part of their stretching energy by developing out of plane undulations. We investigate experimentally and theoretically the indentation of an elastic disk supported by a circular ring and show that compressive stresses are relieved via two different routes : either developing \textit{buckles} which are spread over the system or developing a \textit{d-cone} where deformation is concentrated in a subregion of the system. We characterize the indentation threshold for wrinkles or d-cone existence as a function of aspect ratio.

Enterococcus cecorumis an emerging pathogen responsible for osteomyelitis, spondylitis, and femoral head necrosis causing animal suffering, mortality, and requiring antimicrobial use in poultry. Paradoxically,E. cecorumis a common inhabitant of the intestinal microbiota of adult chickens. Despite evidence suggesting the existence of clones with pathogenic potential, the genetic and phenotypic relatedness of disease-associated isolates remains little investigated. Here, we sequenced and analyzed the genomes and characterized the phenotypes of more than 100 isolates, the majority of which were collected over the last ten years in 16 French broiler farms. Comparative genomics, genome-wide association study, and measured susceptibility to serum, biofilm forming capacity, and adhesion to chicken type II collagen were used to identify features associated with clinical isolates. We found that none of the tested phenotypes could discriminate origin of the isolates or phylogenetic group. Instead, we found that most clinical isolates are grouped phylogenetically and our analyses selected six genes that discriminate 94% of isolates associated with disease from those that are not. Analysis of the resistome and the mobilome revealed that multidrug-resistant clones ofE. cecorumcluster in few clades and that integrative conjugative elements and genomic islands are the main carriers of antimicrobial resistance. This comprehensive genomic analysis shows that disease-associated clones ofE. cecorumbelong mainly to one phylogenetic clade.IMPORTANCEEnterococcus cecorumis an important pathogen in poultry worldwide. It causes a number of locomotor disorders and septicemia, particularly in fast-growing broilers. Animal suffering, antimicrobial use, and associated economic losses require a better understanding of disease-associatedE. cecorumisolates. To address this need, we performed whole genome sequencing and analysis of a large collection of isolates responsible for outbreaks in France. By providing the first dataset on the genetic diversity and resistome ofE. cecorumstrains circulating in France, we pinpoint an epidemic lineage probably also circulating elsewhere and which should be targeted preferentially by preventive strategies in order to reduce the burden ofE. cecorum-related diseases.

Significance Biofilms are communities of surface-attached bacteria exhibiting biofilm-specific properties. Although anaerobic biofilms impact health, industry, and environment, they are mostly studied in aerobic bacterial species. Here, we studied biofilm formation in Bacteroides thetaiotaomicron , an anaerobic gut symbiont degrading diet sugars and contributing to gut maturation. Although B. thetaiotaomicron adhesion contributes to intestinal colonization, little is known about the determinants of its biofilm capacities. We identified that bile is a physiologically relevant gut signal inducing biofilm formation in B. thetaiotaomicron and other gut Bacteroidales. Moreover, we showed that, in contrast to the known scaffolding role of extracellular DNA, bile-dependent biofilm requires a DNase degrading matrix DNA, thus revealing a previously unrecognized factor contributing to the adhesion capacity of major gut symbionts.

Bacillus subtilis is a widely used bacterial model to decipher biofilm formation, genetic determinants and their regulation. For several years, studies were conducted on colonies or pellicles formed at the interface with air, but more recent works showed that non-domesticated strains were able to form thick and structured biofilms on submerged surfaces. Taking advantage of time-lapse confocal laser scanning microscopy, we monitored bacterial colonization on the surface and observed an unexpected biphasic submerged biofilm development. Cells adhering to the surface firstly form elongated chains before being suddenly fragmented and released as free motile cells in the medium. This switching coincided with an oxygen depletion in the well which preceded the formation of the pellicle at the liquid-air interface. Residual bacteria still associated with the solid surface at the bottom of the well started to express matrix genes under anaerobic metabolism to build the typical biofilm protruding structures.

Bacteroides thetaiotaomicron is a gut symbiont that inhabits the mucus layer and adheres to and metabolizes food particles, contributing to gut physiology and maturation. Whereas adhesion and biofilm formation could be key features for B. thetaiotaomicron stress resistance and gut colonization, little is known about the determinants of B. thetaiotaomicron biofilm formation. We previously showed that the B. thetaiotaomicron reference strain VPI-5482 is a poor in vitro biofilm former. Here we demonstrated that bile, a gut-relevant environmental cue, triggers the formation of biofilm in many B. thetaiotaomicron isolates and common gut Bacteroidales species. We identified the genetic determinants of this bile-dependent biofilm formation and showed that it involves the production of the DNase BT3563, degrading extracellular DNA, in biofilms formed in the presence of bile. Our study therefore identifies a physiologically relevant condition inducing B. thetaiotaomicron biofilm and shows that, in contrast to the biofilm-promoting role played by bacterial eDNA scaffold in Firmicutes and Proteobacteria models, degradation of eDNA by BT3563 DNAse and its widespread homologs is required to achieve B. thetaiotaomicron bile-dependent biofilm formation.

Surface-associated multicellular assemblage is an important bacterial trait to withstand harsh environmental conditions. Bacillus subtilis is one of the most studied Gram-positive bacteria, serving as a model for the study of genetic pathways involved in the different steps of 3D biofilm formation. B. subtilis biofilm studies have mainly focused on pellicle formation at the air-liquid interface or complex macrocolonies formed on nutritive agar. However, only few studies focus on the genetic features of B. subtilis submerged biofilm formation and their link with other multicellular models at the air interface. NDmed, an undomesticated B. subtilis strain isolated from a hospital, has demonstrated the ability to produce highly structured immersed biofilms when compared to strains classically used for studying B. subtilis biofilms. In this contribution, we have conducted a multi-culturing comparison (between macrocolony, swarming, pellicle, and submerged biofilm) of B. subtilis multicellular communities using the NDmed strain and mutated derivatives for genes shown to be required for motility and biofilm formation in pellicle and macrocolony models. For the 15 mutated NDmed strains studied, all showed an altered phenotype for at least one of the different culture laboratory assays. Mutation of genes involved in matrix production (i.e., tasA, epsA-O, cap, ypqP) caused a negative impact on all biofilm phenotypes but favored swarming motility on semi-solid surfaces. Mutation of bslA, a gene coding for an amphiphilic protein, affected the stability of the pellicle at the air-liquid interface with no impact on the submerged biofilm model. Moreover, mutation of lytF, an autolysin gene required for cell separation, had a greater effect on the submerged biofilm model than that formed at aerial level, opposite to the observation for lytABC mutant. In addition, B. subtilis NDmed with sinR mutation formed wrinkled macrocolony, less than that formed by the wild type, but was unable to form neither thick pellicle nor structured submerged biofilm. The results are discussed in terms of the relevancy to determine whether genes involved in colony and pellicle formation also govern submerged biofilm formation, by regarding the specificities in each model.

We study the deflection by gravity of a circular elastic disk deposited on a rigid support. The axisymmetric deflection induces compressive orthoradial stresses which leads to a wrinkling instability above a critical threshold of the dimensionless gravity force. We study this instability by a combination of experiments, numerical simulations and analytical tools, with a particular focus on the role of geometry. We show that aspect ratio is a crucial parameter that controls both the threshold of instability and the most unstable mode. The influence of this parameter on the threshold can be caught by introducing a new nondimensionalization of the transverse load.

International audience; The behavior of Listeria monocytogenes communities in the food chain is closely associated with their spatial organization. Whether as biofilms on industrial surfaces or as microcolonies in food matrices, the resulting physiological diversification combined with the presence of extracellular polymeric substances (EPS) triggers emergent community functions involved in the pathogen survival and persistence (e.g., tolerance to dehydration, biocides, or preservatives). In this contribution, we present a noninvasive confocal laser microscopy (CLM) protocol allowing exploration of the spatial organization of L. monocytogenes communities on various inert or nutritive materials relevant for the food industry.

The behavior of Listeria monocytogenes communities in the food chain is closely associated with their spatial organization. Whether as biofilms on industrial surfaces or as microcolonies in food matrices, the resulting physiological diversification combined with the presence of extracellular polymeric substances (EPS) triggers emergent community functions involved in the pathogen survival and persistence (e.g., tolerance to dehydration, biocides, or preservatives). In this contribution, we present a noninvasive confocal laser microscopy (CLM) protocol allowing exploration of the spatial organization of L. monocytogenes communities on various inert or nutritive materials relevant for the food industry.

Over the last 20 years, Lactobacillus species inhabiting the gastrointestinal tract (GIT) have received much attention, and their health-promoting properties are now well-described. Probiotic effects cannot be generalized, and their uses cover a wide range of applications. It is thus important to proceed to an accurate selection and evaluation of probiotic candidates. We evaluate the probiotic potential of six strains of Lactobacillus in different in vitro models representing critical factors of either survival, efficacy, or both. We characterized the strains for their ability to (i) modulate intestinal permeability using transepithelial electrical resistance (TEER), (ii) form biofilms and resist stressful conditions, and (iii) produce beneficial host and/or bacteria metabolites. Our data reveal the specificity of Lactobacillus strains to modulate intestinal permeability depending on the cell type. The six isolates were able to form spatially organized biofilms, and we provide evidence that the biofilm form is beneficial in a strongly acidic environment. Finally, we demonstrated the ability of the strains to produce &gamma, aminobutyric acid (GABA) that is involved in the gut-brain axis and beneficial enzymes that promote the bacterial tolerance to bile salts. Overall, our study highlights the specific properties of Lactobacillus strains and their possible applications as biofilms.

Clostridium difficile is a major cause of nosocomial infections. Bacterial persistence in the gut is responsible for infection relapse; sporulation and other unidentified mechanisms contribute to this process. Intestinal bile salts cholate and deoxycholate stimulate spore germination, while deoxycholate kills vegetative cells. Here, we report that sub-lethal concentrations of deoxycholate stimulate biofilm formation, which protects C. difficile from antimicrobial compounds. The biofilm matrix is composed of extracellular DNA and proteinaceous factors that promote biofilm stability. Transcriptomic analysis indicates that deoxycholate induces metabolic pathways and cell envelope reorganization, and represses toxin and spore production. In support of the transcriptomic analysis, we show that global metabolic regulators and an uncharacterized lipoprotein contribute to deoxycholate-induced biofilm formation. Finally, Clostridium scindens enhances biofilm formation of C. difficile by converting cholate into deoxycholate. Together, our results suggest that deoxycholate is an intestinal signal that induces C. difficile persistence and may increase the risk of relapse.

The functional properties of biofilms are intimately related to their spatial architecture. Structural data are therefore of prime importance to dissect the complex social and survival strategies of biofilms and ultimately to improve their control. Confocal laser microscopy (CLM) is the most widespread microscopic tool to decipher biofilm structure, enabling noninvasive 3D investigation of their dynamics down to single cell scale. The emergence of fully automated high content screening (HCS) systems, associated with large-scale image analysis, radically amplifies the flow of available biofilm structural data. In this contribution, we present an HCS-CLM protocol used to analyze biofilm 4D structural dynamics at high throughput. Meta-analysis of the quantitative variates extracted from HCS-CLM will contribute to a better biological understanding of biofilm traits.

IF 3.682; International audience; Few studies have extensively investigated probiotic functions associated with biofilms. Here, we show that strains of Lactobacillus plantarum and Lactobacillus fermentum are able to grow as biofilm on abiotic surfaces, but the biomass density differs between strains. We performed microtiter plate biofilm assays under growth conditions mimicking to the gastrointestinal environment. Osmolarity and low concentrations of bile significantly enhanced Lactobacillus spatial organization. Two L. plantarum strains were able to form biofilms under high concentrations of bile and mucus. We used the agar well-diffusion method to show that supernatants from all Lactobacillus except the NA4 isolate produced food pathogen inhibitory molecules in biofilm. Moreover, TNF-alpha production by LPS-activated human monocytoid cells was suppressed by supernatants from Lactobacillus cultivated as biofilms but not by planktonic culture supernatants. However, only L. fermentum NA4 showed anti-inflammatory effects in zebrafish embryos fed with probiotic bacteria, as assessed by cytokine transcript level (TNF-alpha, IL-1 beta and IL-10). We conclude that the biofilm mode of life is associated with beneficial probiotic properties of lactobacilli, in a strain dependent manner. Those results suggest that characterization of isolate phenotype in the biofilm state could be additional valuable information for the selection of probiotic strains.

International audience; Bacillus subtilis QST713 is extensively used as a biological control agent in agricultural fields including in the button mushroom culture, Agaricus bisporus. This last use exploits its inhibitory activity against microbial pathogens such as Trichoderma aggressivum f. europaeum, the main button mushroom green mould competitor. Here, we report the complete genome sequence of this bacterium with a genome size of 4 233 757 bp, 4263 predicted genes and an average GC content of 45.9%. Based on phylogenomic analyses, strain QST713 is finally designated as Bacillus velezensis. Genomic analyses revealed two clusters encoding potential new antimicrobials with NRPS and TransATPKS synthetase. B. velezensis QST713 genome also harbours several genes previously described as being involved in surface colonization and biofilm formation. This strain shows a strong ability to form in vitro spatially organized biofilm and to antagonize T. aggressivum. The availability of this genome sequence could bring new elements to understand the interactions with micro or/and macroorganisms in crops.

Enterococcus faecalis, an organism generally not pathogenic for healthy humans, has the potential to cause disease in susceptible hosts. While it seems to be equipped to interact with and circumvent host immune defense, most of the molecular and cellular mechanisms underlying the enterococcal infectious process remain elusive. Here, we investigated the role of the Enterococcal Leucine Rich protein A (ElrA), an internalin-like protein of E. faecalis also known as a virulence factor. ElrA was previously shown to prevent adhesion to macrophages. We show that ElrA does not inhibit the basic phagocytic process, but is able to prevent sensing and migration of macrophages toward E. faecalis. Presence or absence of FHL2, a eukaryotic partner of ElrA, does not affect the ElrA-dependent mechanism preventing macrophage migration. However, we highlight a partial contribution of FHL2 in ElrA-mediated virulence in vivo. Our results indicate that ElrA plays at least a dual role of which anti-phagocytic activity may contribute to dissemination of extracellular E. faecalis during infection.

International audience; This work investigated the effects of diamond-like carbon (DLC) coatings on the architecture and biocide reactivity of dual-species biofilms mimicking food processing contaminants. Biofilms were grown using industrial isolates of Escherichia coli and Pantoea agglomerans on bare stainless steel (SST) and on two DLC surface coatings (a-C:H:Si:O designated by SICON® and a-C:H:Si designated by SICAN) in order to evaluate their antifouling activities. Quantification and spatial organization in single- and dual-species biofilms were examined by confocal laser scanning microscopy (CLSM) using a strain specific labelling procedure. Those assays revealed that the E. coli isolate exhibited a higher adhesion to the modified surfaces and a decreased susceptibility to disinfectant in presence of P. agglomerans than alone in axenic culture. While SICON® reduced the short-term growth of E. coli in axenic conditions, both DLC surfaces increased the E. coli colonization in presence of P. agglomerans. However, both modified surfaces triggered a significantly higher log reduction of E. coli cells within mixed-species biofilms, thus the use of SICON® and SICAN surfaces may be a good approach to facilitate the disinfection process in critical areas of food processing plants. This study presents a new illustration of the importance of interspecies interactions in surface-associated community functions, and of the need to evaluate the effectiveness of hygienic strategies with relevant multi-species consortia.

We study the continuous-time limit of a class of Markov chains coming from the evolution of classical open systems undergoing repeated interactions. This repeated interaction model has been initially developed for dissipative quantum systems in Attal and Pautrat (2006) and was recently set up for the first time in Deschamps (2012) for classical dynamics. It was particularly shown in the latter that this scheme furnishes a new kind of Markovian evolutions based on Hamilton’s equations of motion. The system is also proved to evolve in the continuous-time limit with a stochastic differential equation. We here extend the convergence of the evolution of the system to more general dynamics, that is, to more general Hamiltonians and probability measures in the definition of the model. We also present a natural way to directly renormalize the initial Hamiltonian in order to obtain the relevant process in a study of the continuous-time limit. Then, even if Hamilton’s equations have no explicit solution in general, we obtain some bounds on the dynamics allowing us to prove the convergence in law of the Markov chain on the system to the solution of a stochastic differential equation, via the infinitesimal generators.

Integrative conjugative elements (ICEs) are chromosomal elements that are widely distributed in bacterial genomes, hence contributing to genome plasticity, adaptation, and evolution of bacteria. Conjugation requires a contact between both the donor and the recipient cells and thus likely depends on the composition of the cell surface envelope. In this work, we investigated the impact of different cell surface molecules, including cell surface proteins, wall teichoic acids, lipoteichoic acids, and exopolysaccharides, on the transfer and acquisition of ICESt3 from Streptococcus thermophilus. The transfer of ICESt3 from wild-type (WT) donor cells to mutated recipient cells increased 5- to 400-fold when recipient cells were affected in lipoproteins, teichoic acids, or exopolysaccharides compared to when the recipient cells were WT. These mutants displayed an increased biofilm-forming ability compared to the WT, suggesting better cell interactions that could contribute to the increase of ICESt3 acquisition. Microscopic observations of S. thermophilus cell surface mutants showed different phenotypes (aggregation in particular) that can also have an impact on conjugation. In contrast, the same mutations did not have the same impact when the donor cells, instead of recipient cells, were mutated. In that case, the transfer frequency of ICESt3 decreased compared to that with the WT. The same observation was made when both donor and recipient cells were mutated. The dominant effect of mutations in donor cells suggests that modifications of the cell envelope could impair the establishment or activity of the conjugation machinery required for DNA transport.

International audience; Soft microcapsules moving in a cylindrical capillary deform from quasi-spherical shapes to elongated shapes with an inversion of curvature at the rear. We investigated the perturbation of the flow by particle tracking velocimetry around deformed microcapsules in confined flow. These experiments are completed by numerical simulations. Microcapsules are made of a thin membrane of polymerized human albumin and their shear elastic moduli are previously characterized in a cross flow chamber. Firstly, the velocity of the microcapsule can be calculated by theoretical predictions for rigid spheres, even for large deformations as 'parachute- like' shapes, if a relevant definition of the ratio of confinement is chosen. Secondly, at the rear and the front of the microcapsule, the existence of multiple recirculation regions is governed by the local curvature of the membrane. The amplitudes of these perturbations increase with the microcapsule deformation, whereas their axial extents are comparable to the radius of the capillary whatever the confinement and the capillary number. We conclude that whereas the motion of microcapsules in confined flow has quantitative similitudes with rigid spheres in terms of velocity and axial extent of the perturbation, their presence induces variations in the flow field that are related to the local deformation of the membrane as in droplets.

Bacteria of the genus Thiomonas are found ubiquitously in arsenic contaminated waters such as acid mine drainage (AMD), where they contribute to the precipitation and the natural bioremediation of arsenic. In these environments, these bacteria have developed a large range of resistance strategies among which the capacity to form particular biofilm structures. The biofilm formation is one of the most ubiquitous adaptive response observed in prokaryotes to various stresses, such as those induced in the presence of toxic compounds. This study focused on the process of biofilm formation in three Thiomonas strains (CB1, CB2 and CB3) isolated from the same AMD. The results obtained here show that these bacteria are all capable of forming biofilms, but the architecture and the kinetics of formation of these biofilms differ depending on whether arsenite is present in the environment and from one strain to another. Indeed, two strains favoured biofilm formation, whereas one favoured motility in the presence of arsenite. To identify the underlying mechanisms, the patterns of expression of some genes possibly involved in the process of biofilm formation were investigated in Thiomonas sp. CB2 in the presence and absence of arsenite, using a transcriptomic approach (RNA-seq). The findings obtained here shed interesting light on how the formation of biofilms, and the motility processes contribute to the adaptation of Thiomonas strains to extreme environments.

Summary The predominant form of life for microorganisms in their natural habitats is the biofilm mode of growth. The adherence and colonization of probiotic bacteria are considered as essential factors for their immunoregulatory function in the host. Here, we show that Lactobacillus casei ATCC334 adheres to and colonizes the gut of zebrafish larvae. The abundance of pro-inflammatory cytokines and the recruitment of macrophages were low when inflammation was induced in probiotic-fed animals, suggesting that these bacteria have anti-inflammatory properties. We treated human macrophage-differentiated monocytic THP-1 cells with supernatants of L. casei ATCC334 grown in either biofilm or planktonic cultures. TNF-α production was suppressed and the NF-κB pathway was inhibited only in the presence of supernatants from biofilms. We identified GroEL as the biofilm supernatant compound responsible, at least partially, for this anti-inflammatory effect. Gradual immunodepletion of GroEL demonstrated that the abundance of GroEL and TNF-α were inversely correlated. We confirmed that biofilm development in other Lactobacillus species affects the immune response. The biofilms supernatants of these species also contained large amounts of GroEL. Thus, our results demonstrate that the biofilm enhances the immunomodulatory effects of Lactobacillus sp. and that secreted GroEL is involved in this beneficial effect.

Listeria monocytogenes has a dichotomous lifestyle, existing as an ubiquitous saprophytic species and as an opportunistic intracellular pathogen. Besides its capacity to grow in a wide range of environmental and stressful conditions, L. monocytogenes has the ability to adhere to and colonize surfaces. Morphotype variation to elongated cells forming rough colonies has been reported for different clinical and environmental isolates, including biofilms. This cell differentiation is mainly attributed to the reduced secretion of two SecA2-dependent cell-wall hydrolases, CwhA and MurA. SecA2 is a non-essential SecA paralogue forming an alternative translocase with the primary Sec translocon. Following investigation at temperatures relevant to its ecological niches, i.e. infection (37 degrees C) and environmental (20 degrees C) conditions, inactivation of this SecA2-only protein export pathway led, despite reduced adhesion, to the formation of filamentous biofilm with aerial structures. Compared to the wild type strain, inactivation of the SecA2 pathway promoted extensive cell aggregation and sedimentation. At ambient temperature, this effect was combined with the abrogation of cell motility resulting in elongated sedimented cells, which got knotted and entangled together in the course of filamentous-biofilm development. Such a cell differentiation provides a decisive advantage for listerial surface colonization under environmental condition. As further discussed, this morphotypic conversion has strong implication on listerial physiology and is also of potential significance for asymptomatic human/animal carriage.

International audience

International audience; A microfluidic technique is used to characterize the mechanical behavior of capsules that are produced in a two-step process: first, an emulsification step to form droplets, followed by a cross-linking step to encapsulate the droplets within a thin membrane composed of cross-linked proteins. The objective is to study the influence of the capsule size and protein concentration on the membrane mechanical properties. The microcapsules are fabricated by cross-linking of human serum albumin (HSA) with concentrations from 15 to 35 % (w/v). A wide range of capsule radii (∼40–450 μm) is obtained by varying the stirring speed in the emulsification step. For each stirring speed, a low threshold value in protein concentration is found, below which no coherent capsules could be produced. The smaller the stirring speed, the lower the concentration can be. Increasing the concentration from the threshold value and considering capsules of a given size, we show that the surface shear modulus of the membrane increases with the concentration following a sigmoidal curve. The increase in mechanical resistance reveals a higher degree of cross-linking in the membrane. Varying the stirring speed, we find that the surface shear modulus strongly increases with the capsule radius: its increase is two orders of magnitude larger than the increase in size for the capsules under consideration. It demonstrates that the cross-linking reaction is a function of the emulsion size distribution and that capsules produced in batch through emulsification processes inherently have a distribution in mechanical resistance.

We study a particular class of complex-valued random variables and their associated random walks: complex obtuse random variables. They generalize, to the complex case, the real-valued obtuse random variables introduced in Equations de structure pour des martingales vectorielles. (Seminaire de Probabilites, XXVIII, p. 256278. Lecture Notes in Math., vol. 1583. Springer, Berlin (1994)) in order to understand the structure of normal martingales in \(\mathbb {R}^N\). The extension to the complex case is motivated by Quantum Statistical Mechanics, in particular for characterizing those quantum baths acting as classical noises. The extension of obtuse random variables to the complex case is far from obvious and makes use of very interesting algebraical structures. We show that complex obtuse random variables are characterized by a 3-tensor which admits certain symmetries; we show that these symmetries are the exact 3-tensor analogue of the normal character for 2-tensors (i.e. matrices), that is, a necessary and sufficient condition for being diagonalizable in some orthonormal basis. We discuss the passage to the continuous-time limit for these random walks and show that they converge in distribution to normal martingales in \(\mathbb {C}^N\). We show that the 3-tensor associated to these normal martingales encodes their behavior, in particular the diagonalization directions of the 3-tensor indicate the directions of the space where the martingales behaves like a diffusion and those where it behaves like a Poisson process. We finally prove the convergence, in the continuous-time limit, of the corresponding multiplication operators on the canonical Fock space, with an explicit expression in terms of the associated 3-tensor again.

This papers addresses the connection between two classical models of phase transition phenomena describing different stages of the growth of clusters. The Becker-D��ring model (BD) describes discrete-sized clusters through an infinite set of ordinary differential equations. The Lifshitz-Slyozov equation (LS) is a transport partial differential equation on the continuous half-line $x\in (0,+\infty)$. We introduce a scaling parameter $\varepsilon>0$, which accounts for the grid size of the state space in the BD model, and recover the LS model in the limit $\varepsilon\to 0$. The connection has been already proven in the context of outgoing characteristic at the boundary $x=0$ for the LS model, when small clusters tend to shrink. The main novelty of this work resides in a new estimate on the growth of small clusters, which behave at a fast time scale. Through a rigorous quasi steady state approximation, we derive boundary conditions for the incoming characteristic case, when small clusters tend to grow., 29 pages

International audience; We investigated experimentally the deformation of soft microcapsules and the dynamics of their membrane in simple shear flows. Firstly, the tank-treading motion, i.e. the rotation of the membrane, was visualized and quantified by tracking particles included in the membrane by a new protocol. The period of membrane rotation increased quadratically with the extension of the large axis. The tracking of the distance between two close micro-particles showed membrane contraction at the tips and stretching on the sides, a specific property of soft particles such as capsules. Present experimental results are discussed in regard to previous numerical simulations. This analysis showed that the variation of the tank-treading period with the Taylor parameter (deformation) cannot be explained by purely elastic membrane models. It suggests a strong effect of membrane viscosity whose order of magnitude is determined. Secondly, two distinct shapes of sheared microcapsules were observed. For moderate deformations, the shape was a steady ellipsoid in the shear plane. For larger deformations, the capsule became asymmetric and presented a S-like shape. When the viscous shear stress increased by three orders of magnitude, the small axis decreased by 70 % whereas the large axis increased by 100% before any break-up. The inclination angle decreased from 40 • to 8 • , almost aligned with the flow direction as expected by theory/numerics on capsules and experiments/theory/numerics on drops and vesicles. Whatever the microcapsule size and the concentration of proteins, the characteristic lengths of the shape, the Taylor parameter and the inclination angle satisfy master curves versus the longest axis or the normalized shear stress or the capillary number in agreement with theory for non negligible membrane viscosity in the regime of moderate deformations. Finally, we observed that very small deviation from sphericity gave rise to swinging motion, i.e. shape oscillations, in the small deformation regime. In conclusion, this study of tank-treading motion supports the role of membrane viscosity on the dynamics of microcapsules in shear flow by both independent methods which compare experimental data with numerical results in the regime of large deformations and with the theory in the regime of moderate deformations.

International audience

Salmonella is recognized as one of the most significant enteric foodborne bacterial pathogens. In recent years, the resistance of pathogens to biocides and other environmental stresses, especially when they are embedded in biofilm structures, has led to the search for and development of novel antimicrobial strategies capable of displaying both high efficiency and safety. In this direction, the aims of the present work were to evaluate the antimicrobial activity of hydrosol of the Mediterranean spice Thymbra capitata against both planktonic and biofilm cells of Salmonella enterica serovar Typhimurium and to compare its action with that of benzalkonium chloride (BC), a commonly used industrial biocide. In order to achieve this, the disinfectant activity following 6-min treatments was comparatively evaluated for both disinfectants by calculating the concentrations needed to achieve the same log reductions against both types of cells. Their bactericidal effect against biofilm cells was also comparatively determined by in situ and real-time visualization of cell inactivation through the use of time-lapse confocal laser scanning microscopy (CLSM). Interestingly, results revealed that hydrosol was almost equally effective against biofilms and planktonic cells, whereas a 200-times-higher concentration of BC was needed to achieve the same effect against biofilm compared to planktonic cells. Similarly, time-lapse CLSM revealed the significant advantage of the hydrosol to easily penetrate within the biofilm structure and quickly kill the cells, despite the three-dimensional (3D) structure of Salmonella biofilm. IMPORTANCE The results of this paper highlight the significant antimicrobial action of a natural compound, hydrosol of Thymbra capitata , against both planktonic and biofilm cells of a common foodborne pathogen. Hydrosol has numerous advantages as a disinfectant of food-contact surfaces. It is an aqueous solution which can easily be rinsed out from surfaces, it does not have the strong smell of the essential oil (EO) and it is a byproduct of the EO distillation procedure without any industrial application until now. Consequently, hydrosol obviously could be of great value to combat biofilms and thus to improve product safety not only for the food industries but probably also for many other industries which experience biofilm-related problems.

Travelling through slime - bacterial movements in the EPS matrix

Resistance of micro-organisms to disinfection is frequently associated with the presence of biofilms on surfaces. To partially overcome biofilms resistance to biocides alternative strategies are being developped. So a wealth of biological antibiofilm agents are under development in the research laboratories but there are only few products on the market and none of them proved to be universal antibiofilm molecules. However, most of these antibiofilm agents may benefit from the "swimmers" approach that would act synergistically by allowing a better infiltration and/or an in situ delivery in the targets biofilms. The goal of the project, Greenswimmers 1, is to identify the "best" bacteria for this purpose of infiltration and biocide delivery. In this technical report we explain how we evaluated the efficiency of swimmers with respect to infiltration of Stapyloccocus Aureus biofilms.This evaluation relies on the kinematics properties of swimmers and on the kinematics of the porous network created by the swimmers, in interaction with the biofilm. Practically, we addressed the tracking of the swimmers with the successful multiple particle tracking algorithm, U-Track, we improved in order to exploit the information from two channels of the biofilm video under the microscope. We also combined swimmers detection and poresegmentation for a more robust estimation of pores. A first step in pore segmentation consists in partitioning the images from the movies with superpixel decomposition. Eventually, more than three hundreds movies were analyzed for eight species and several strains of each of them. Parameters such as speed, persistence, pore area, e-DNA proportion were estimated from the data.

An approach to quantitatively study vesicle dynamics as well as biologically-related micro-objects in a fluid flow, which is based on the combination of a dynamical trap and a control parameter, the ratio of the vorticity to the strain rate, is suggested. The flow is continuously varied between rotational, shearing, and elongational in a microfluidic 4-roll mill device, the dynamical trap, that allows scanning of the entire phase diagram of motions, i.e., tank-treading (TT), tumbling (TU), and trembling (TR), using a single vesicle even at λ = η in /η out = 1, where η in and η out are the viscosities of the inner and outer fluids. This cannot be achieved in pure shear flow, where the transition between TT and either TU or TR is attained only at λ>1. As a result, it is found that the vesicle dynamical states in a general are presented by the phase diagram in a space of only 2 dimensionless control parameters. The findings are in semiquantitative accord with the recent theory made for a quasi-spherical vesicle, although vesicles with large deviations from spherical shape were studied experimentally. The physics of TR is also uncovered.

We investigate unitary operators acting on a tensor product space, with the property that the quantum channels they generate, via the Stinespring dilation theorem, are of a particular type, independently of the state of the ancilla system in the Stinespring relation. The types of quantum channels we consider are those of interest in quantum information theory: unitary conjugations, constant channels, unital channels, mixed unitary channels, PPT channels, and entanglement breaking channels. For some of the classes of bipartite unitary operators corresponding to the above types of channels, we provide explicit characterizations, necessary and/or sufficient conditions for membership, and we compute the dimension of the corresponding algebraic variety. Inclusions between these classes are considered, and we show that for small dimensions, many of these sets are identical., v2: references [14,15] added, as well as various comments and clarifications