On growth and form of Bacillus subtilis biofilms

International audienceA general feature of mature biofilms is their highly heterogeneous architecture that partitions the microbial city into sectors with specific micro-environments. To understand how this heterogeneity arises, we have investigated the for- mation of a microbial community of the model organism Bacillus subtilis. We first show that the growth of macroscopic colonies is inhibited by the accumulation of ammoniacal by-products. By constraining biofilms to grow approximately as two-dimensional layers, we then find that the bacteria which differentiate to produce extracellular polymeric substances form tightly packed bacterial chains. In addition to the process of cellular chaining, the bio- mass stickiness also strongly hinders the reorganization of cells within the biofilm. Based on these observations, we then write a biomechanical model for the growth of the biofilm where the cell density is constant and the physical mechanism responsible for the spreading of the biomass is the pressure gener- ated by the division of the bacteria. Besides reproducing the velocity field of the biomass across the biofilm, the model predicts that, although bacteria divide everywhere in the biofilm, fluctuations in the growth rates of the bacteria lead to a coarsening of the growing bacterial layer. This process of kin- etic roughening ultimately leads to the formation of a rough biofilm surface exhibiting self-similar properties. Experimental measurements of the biofilm texture confirm these predictions

Liquid plugs flow in capillary tube : application to pulmonary congestion diseases

Les maladies pulmonaires obstructives touchent aujourd'hui plusieurs millions de personnes dans le monde. Ces maladies se manifestent par l'accumulation d'un liquide appelé mucus dans les poumons, pouvant aboutir, lorsqu’elle est trop importante, à la formation de ponts liquides entravant la circulation de l’air. Les voies pulmonaires peuvent néanmoins se rouvrir via la rupture de ces ponts liquides. Ces réouvertures peuvent résulter du cycle respiratoire, d’écoulements plus violents provoqués par le mécanisme de toux ou encore nécessiter des séances de kinésithérapie respiratoire pour les malades atteints de bronchites chroniques ou de mucoviscidose. Dans cette thèse nous nous sommes intéressés à la rupture de ponts liquides engendrée par un cycle de respiration ou via un forçage unidirectionnel d’intensité suffisante. En particulier nous avons caractérisé expérimentalement et théoriquement les pressions critiques nécessaires pour rouvrir des voies obstruées. Nous avons aussi étudié les derniers instants de vie d’un pont liquide et mis en évidence à la fois expérimentalement et numériquement différents régimes de rupture. A forte vitesse, cette rupture est obtenue via l’atomisation du liquide, c’est à dire la formation de gouttelettes qui pourraient correspondre aux éjectas lors de la toux. Enfin nous nous sommes intéressés à un problème plus éloigné des poumons mais néanmoins fondamental en microfluidique : la dynamique de ponts liquides sur des surfaces partiellement mouillantes. Nous avons montré qu’au-dessus d’un certain seuil en vitesse, le déplacement d’un simple doigt de liquide à débit constant aboutit à la formation d’un train de bulles et de ponts liquides calibrés.Obstructive pulmonary disease now affects several million people worldwide. These diseases are manifested by the accumulation of a liquid called mucus in the lungs which, when too large, can lead to the formation of liquid plugs which impede air circulation Pulmonary pathways can nevertheless reopen via the rupture of these liquid plugs. These reopenings may result from the respiratory cycle, more violent outflows caused by the cough mechanism or require respiratory physiotherapy sessions for patients with chronic bronchitis or cystic fibrosis. In this thesis we were interested in the rupture of liquid plugs generated by a breathing cycle or by a unidirectional forcing of sufficient intensity. In particular, we have experimentally and theoretically characterized the critical pressures necessary to reopen obstructed pathways. We also studied the last moments of life of a liquid plug and demonstrated both experimentally and numerically different regimes of rupture. At high velocity, this rupture is obtained via the atomization of the liquid, ie the formation of droplets that could correspond to the ejections during the cough. Finally, we were interested in a problem more distant from lungs but nevertheless fundamental in microfluidics: the dynamics of liquid plugs on partially wet surfaces. We have shown that above a certain velocity threshold, the displacement of a single liquid finger at constant flow results in the formation of a train of calibrated bubbles and liquid plugs