Réponses de Streptococcus salivarius K12 à l'environnement et à la dynamique de la bouche simulés en bioréacteur

Abstract : This thesis aims to better understand the effect of oral environmental conditions on the behaviorof the probiotic bacteria Streptococcus salivarius K12. Growth and maintenance of S. salivarius K12 have been characterized in a complemented artificial saliva (CAS), designed for this study. In this medium, S. salivarius demonstrated highspecific growth rate and low lag time, but it did not produce active bacteriocins. However, the survival of S. salivarius K12 during stationary phase was affected during fermentation in CAS medium. This was mainly explained by a reduced synthesis of proteins involved in energy and glycogen metabolisms. Thus, despite an increased sensitivity in stationary phase, the "complemented artificial saliva" allowed the growth and maintenance of S. salivarius K12. The effects of several environmental oral factors on S. salivarius K12 were determined in complemented artificial saliva. Adding sucrose decreased cellular viability. Enzymes added to their physiological concentration also affected the bacteria. Lysozyme increased S. salivarius mortality by acting on cellular wall. Peroxidase enhanced viability, by reducing the redox potential. The key role of redox potential on S. salivarius K12 was confirmed by the negative impact of the injection of air containing 5% CO2, which increased redox potential. The amylase demonstrated both positive (biomass increase) and negative roles (reduced growth rate). Consequently, studies involving oral bacteria must integrate these environmental factors that affected the bacterial physiological state. Continuous cultures, taking into account the variations in salivary flow and the periodical supply of nutrients, and combining all environmental conditions previously identified, allowed simulating oral dynamic conditions. From our results, a good adaptation of S. salivarius K12 took place in these culture conditions. Cells were able maintaining a constant level of cultivability despite nutritional starvation and wash out. Some molecular mechanisms explaining this bacterial adaptation have been characterized: activation of alternative carbon sources pathways, energy storage, and increase of natural genetic competence. Finally, this work made it possible identifying some mechanisms used by Streptococcussalivarius K12 to adapt itself to the oral environment, through the establishment of in vitro methods for studying the behavior of oral bacteria.
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Perrine Roger. Réponses de Streptococcus salivarius K12 à l'environnement et à la dynamique de la bouche simulés en bioréacteur. Biochimie, Biologie Moléculaire. AgroParisTech, 2011. Français. ⟨NNT : 2011AGPT0071⟩. ⟨pastel-01061385⟩

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