Trophic relationships between Saccharomyces cerevisiae and Lactobacillus plantarum and their metabolism of glucose and citrate

Glucose and citrate are two major carbon sources in fruits or fruit juices such as orange juice. Their metabolism and the microorganisms involved in their degradation were studied by inoculating with an aliquot of fermented orange juice a synthetic model medium containing glucose and citrate. At pH 3.6, their degradation led, first, to the formation of ethanol due to the activity of yeasts fermenting glucose and, eventually, to the formation of acetate resulting from the activity of lactobacilli. The yeast population always outcompeted the lactobacilli even when the fermented orange juice used as inoculum was mixed with fermented beet leaves containing a wider variety of lactic acid bacteria. The evolution of the medium remained similar between pH 3.3 and 5.0. At pH 3.0 or below, the fermentation of citrate was totally inhibited. Saccharomyces cerevisiae and Lactobacillus plantarum were identified as the only dominant microorganisms. The evolution of the model medium with the complex microbial community was successfully reconstituted with a defined coculture of S. cerevisiae and L. plantarum. The study of the fermentation of the defined model medium with a reconstituted microbial community allows us to better understand the behavior not only of fermented orange juice but also of many other fruit fermentations utilized for the production of alcoholic beverages

Dynamics and structure of decaying shallow dipolar vortices

The current work reports on a numerical and experimental study of the evolution of decaying dipolar vortices in a shallow fluid layer. The dynamics and the structure of such vortices are investigated as a function of both their Reynolds number Re and the aspect ratio of vertical and horizontal length scales δ. By quantifying the strength of the secondary motions (vertical motions and nonzero horizontal divergence) with respect to the swirling motions of the primary vortex cores, it was found that the three-dimensionality of a shallow (δ << 1) dipolar vortex only depends on a single parameter: δ²Re. Depending on the value of this parameter, three flow regimes are observed for shallow dipolar vortices: (1) a quasi-two-dimensional regime where the structure of the dipolar vortex remains almost unchanged throughout its lifetime, (2) a transitional regime where the structure presents some three-dimensional characteristics but remains coherent, and (3) a three-dimensional regime where the structure of the dipolar vortex acquires a complicated three-dimensional shape with a persistent spanwise vortex at its front

Quelques données récentes sur la biologie des écosystèmes méthanogènes

La production biologique de méthane est actuellement décrite par schéma en trois étapes. Deux d'entre elles restent mal connues : la méthanogenèse à partir de l'acétate et surtout l'acétogenèse avec production obligée d'hydrogène. Pourtant elle sont souvent les étapes limitant la vitesse globale de la méthanisation. Après une revue des acquis récents, des espèces impliquées et de leurs interactions au sein écosystèmes méthanogènes, nous discutons l'application de ces concepts à la biologie des fosses septiques

Biomass Retention in Advanced Anaerobic Reactors

Recognition of the advantages of anaerobic wastewater treatment induced the development of high rate processes, i.e. reactors designed to allow an efficient treatment of even diluted streams. The performance of these advanced reactors is mainly dependent on the retention within the reactor of high bacterial concentrations. The prevailing mechanism is either the formation of bacterial aggregates with good settling characteristics, the development of methanogenic biolayers at the surface of inert carriers or both. During the past decade information on the biology of methanogenic ecosystems became available at an increasing rate. From a practical point of view it can be stated that the biological conversion of organic compounds to methane is reasonably well understood. However the current knowledge on the aggregation and adhesion of methanogenic consortia remains very limited. In most cases reactor start-up procedures are rather long and appear to be more empirical than rational. This paper is a brief presentation on the current knowledge of methanogenic aggregates and biofilms. The fundamental aspects of bacterial adhesion and the modelling of anaerobic biofilms growth are presented elsewhere.</jats:p

Ecophysiologie des boues activées induites dans les stations d'épuration industrielles ou mixtes par les effluents à forte charge glucidique

Des boues activées traitant des effluents riches en glucides (ceux des industries agro-alimentaires en particulier) sont prédisposées aux accidents de décantation de type foisonnement des boues ou « bulking filamenteux ». Après un rappel bibliographique de l'écologie et de la physiologie des germes présents dans les boues activées adaptées à des effluents riches en glucides, nous présentons les mécanismes probables d'implantation du bulking filamenteux pour ce type particulier de boues activées. Les travaux réalisés dans notre laboratoire montrent que la prolifération préférentielle des espèces filamenteuses est à relier au métabolisme des réserves endocellulaires synthétisées par deux types de microorganismes lorsque l'oxygène ou l'azote sont des facteurs limitants (a-glucanes chez les espèces non filamenteuses, P.H.B. chez les espèces filamenteuses). Dans la dernière partie de notre exposé, nous discutons les modifications qu'il convient d'apporter au procédé par boues activées classique pour traiter avec succès ce type d'effluent (supplémentation de l'eau résiduaire, compartimentage du bassin d'alimentation, utilisation du procédé « contact - stabilisation » à forte charge)