Effect of local hydrodynamic conditions on the behaviour of Lactococcus lactis NCDO2118 cultivated in a Couette bioreactor : a numerical and experimental study

Cette thèse présente les résultats de travaux visant à étudier les interactions entre les conditions hydrodynamiques et le comportement bactérien en bioréacteur. En s'intéressant plus particulièrement aux phénomènes agissant aux petites échelles, nous avons cherché à identifier, caractériser et quantifier les couplages hydro-bio à l'échelle de la cellule. Ici, la souche Lactococcus lactis a été choisie comme modèle microbiologique, alors qu’un réacteur de type Couette a été préféré afin d’engendrer des contraintes hydrodynamiques connues et définies. Il a été démontré que dans des conditions spécifiques d’écoulement (Modulated Wavy Vortex Flow), les bactéries lactiques s’agrègent au sein d’une matrice riche en polysaccharides. Le lien entre ce phénotype atypique et les contraintes locales liées à l’écoulement a été étudié { l’aide de simulation numérique directe de l’écoulement combiné { un suivi de particule. Cette approche permet d’établir les profils temporels des contraintes subies et de comparer la nature des forces disruptives subies au mécanisme d’agrégation séparation des cellules bactériennes au sein de leur matrice. Ce travail de recherche donne ainsi d’autres exemples d’interactions cellule – environement en bioréacteurs, mettant en exergue des effets mécaniquesThe aim of this work is to reach a better understanding of environmental effects on bacterial behaviour in bioreactors. Particular attention has been paid to hydrodynamically-induced stresses at the cell scale, with a view to characterizing and quantifying these local interactions. As a “model experiment”, Lactococcus lactis NCDO2118 has been cultivated in a CouetteBioreactor, a device generating a known and defined flow field. Under specific flow regime (Modulated Wavy Vortex Flow), the cells end up being entrapped in a polysaccharidic matrix. The phenotype of the cells has been demonstrated to be strongly affected by the flow conditions. The stress signal encountered by the cells has been characterized, through umerical simulation (Direct Numerical Simulation) and lagrangian particle tracking, and linked to the phenotypic expression. These studies provide further examples of bacterial response to local hydrodynamic condition

A Unique Phenotypic Modification of Lactococcus lactis Cultivated in a Couette Bioreactor

International audienceBatch cultures of Lactococcus lactis NCDO 2118 and IL 1403 were performed in a Couette bioreactor operated in the modulated wavy vortex flow and the turbulent regimes. This study provides an overall analysis taking into account both mechanical stress and mixing in a Couette bioreactor. A unique phenotypic aspect has been proved to occur only in the modulated wavy vortex flow regime for the two studied strains, namely that the cells become entrapped in a filamentous form. No change in the metabolic behavior of the cells has been observed. The polymeric matrix has been microscopically observed through FISH and fluorescent lectin binding, showing cells entrapped in a glycoconjugate matrix. All hypotheses regarding insufficient mixing as a cause of this phenotype have been discarded, leading to the conclusion that this particular phenotypic feature is essentially due a combined effect of mechanical stress and flow structure. Particle size measurement during the fermentation course indicates that formation of filamentous form results from a continuous aggregation started in the early stages of the cultivation. According to our results a minimum shear is required to induce the ability for cells to aggregate. Then, it appears that both flow structure and mechanical stress (shear) are responsible for the appearance of such a filamentous form. As far as the authors know, this is the first experimental evidence of a bio polymerization induced by the flow structure. Biotechnol. Bioeng. 2011;108: 559-571. (C) 2010 Wiley Periodicals, Inc