Social interaction, noise and antibiotic-mediated switches in the intestinal microbiota

The intestinal microbiota plays important roles in digestion and resistance against entero-pathogens. As with other ecosystems, its species composition is resilient against small disturbances but strong perturbations such as antibiotics can affect the consortium dramatically. Antibiotic cessation does not necessarily restore pre-treatment conditions and disturbed microbiota are often susceptible to pathogen invasion. Here we propose a mathematical model to explain how antibiotic-mediated switches in the microbiota composition can result from simple social interactions between antibiotic-tolerant and antibiotic-sensitive bacterial groups. We build a two-species (e.g. two functional-groups) model and identify regions of domination by antibiotic-sensitive or antibiotic-tolerant bacteria, as well as a region of multistability where domination by either group is possible. Using a new framework that we derived from statistical physics, we calculate the duration of each microbiota composition state. This is shown to depend on the balance between random fluctuations in the bacterial densities and the strength of microbial interactions. The singular value decomposition of recent metagenomic data confirms our assumption of grouping microbes as antibiotic-tolerant or antibiotic-sensitive in response to a single antibiotic. Our methodology can be extended to multiple bacterial groups and thus it provides an ecological formalism to help interpret the present surge in microbiome data.Comment: 20 pages, 5 figures accepted for publication in Plos Comp Bio. Supplementary video and information availabl

Modeling of corium spreading under water layer - validation on the large mass prototypic PLINIUS-2 platform

International audienceCorium coolability after a postulated severe accident involving core meltdown and RPV failure is an important issue. This article deals with spreading and cooling of a corium in a water layer.Currently, the THEMA code, developed at CEA with EDF sponsorship , deals only with the spreading of corium on dry surface with a radiative-convective exchange coefficient. The spreading is then mainly controlled by inertial and viscous forces.In the presence of a water layer in the reactor pit, corium spreading is principally controlled by the yield stress in crust at the flow front. This required the development of a dedicated model. First, the corium crust formation modeling (upper and flow front) was needed. Thanks to this model, the crust thickness evolution with time can be described. An analogy with FinketGriffiths [1] model of volcanic lava was made.Parametric studies show that for a given flow rate, higher yield stresses gives higher height and smaller radius, and with the same stress, the larger the flow rate is, the smaller the height is and the larger the radius is.The aim is to improve the THEMA code taking into account the corium spreading in a water layer. For this, the following modifications should be done-The forces related to the yield stress in crust at the flow front have to be implemented in the momentum balance equation of THEMA; -The power associated with the tensile strength of the crust has to be added in the energy balance equation of THEMA.Finally, to validate the model, some experiments with large mass of prototypic corium are proposed. Indeed, as crust yield stress values and conductivity of corium crust are essential but very poorly known, first, dedicated experiments will be considered to measure these thermophysical data used in the model. Then, the validation of this new version of THEMA will require experiments in more representative conditions which could be carried out in the future PLINIUS-2 experimental platform in Cadarache

Needs for large mass prototypic corium experiments the PLINIUS-2 platform

International audienceCorium is the molten material formed after meltdown of a nuclear reactor core during a severe accident. In order to improve the understanding and modelling of corium behavior, experiments are needed both for LWRs and GenIV fast reactors. Experiments using low temperature simulant materials, thanks to lower costs and constraints, allow the testing of a larger number of configurations and the determination of correlations. But some crucial corium phenomena cannot be reproduced at low temperatures such as the importance of radiation heat transfer or the presence of a large (up to 1000 K) liquidus-solidus interval. Consequently, some experiments are performed with high temperature simulant materials alumina thermite as well as refractory oxides. However, it is not feasible to simulate all the aspects of corium phenomenology, especially its high temperature physico-chemistry. Therefore, even though the use of depleted uranium implies a series of protective and regulatory measures, the need for prototypic corium experimented is supported through several examples Another important aspect of experiment design deals with scaling. Small or medium scale corium experiments are easier to operate and only a few large scale (>100 kg) facilities have been built. Several effects are only visible with significant masses, as for instance, the formation of a corium cake during FCI or all the phenomena controlled by crust strength, such as underwater spreading or corium jet ablation. CEA is currently designing a new large prototypic corium platform PLINIUS-2 for both LWR and SFR corium experimental research. Its main characteristics will be presented