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Influence of harvesting and concentration conditions on the physiological state and cryotolerance of lactobacillus delbrueckii subsp. bulgaricus cfl1

Fernanda Streit
Abstract : This study aimed at quantifying the effect of harvesting and concentration steps on Lactobacillus delbrueckii subsp. bulgaricus CFL1 cryotolerance. It intended to analyse the physiological mechanisms that explained the different levels of cryotolerance. Another goal of this work consisted to submit the cells to a moderate stress, thus allowing them to adapt themselves to better resist the stress during the freezing step. During the first part of this work, the effect of an acidification, at the end of the fermentation, was studied on cellular cryotolerance. An experimental design made it possible to identify the optimal acidification condition (pH 5.25 for 30 min) that permitted adaptation of the cells. This condition provided highly resistant cells to freezing and frozen storage at -20 °C. Two physiological mechanisms were found to explain this adaptation. The first one corresponded to the increase of the membrane fatty acid C18:1 content, and then to membrane fluidity. From proteome analysis, the second one was characterized by a reduction in protein metabolism, an increase in energetic and nucleotide metabolism and a higher synthesis of stress proteins. In the second part of this study, cells were concentrated according to different centrifugation conditions (speed, duration and temperature). These conditions did not affect the resistance of Lb. bulgaricus CFL1 to the concentration process itself. However, they had a small but significant effect on their cryotolerance. As the behaviour of cells resistance during freezing and frozen storage was opposite, the effect of centrifugation conditions on their activity after long time storage was not significant. The last part of this work made it possible to analyse the effects of microfiltration conditions on Lb. bulgaricus CFL1 resistance to concentration, freezing and frozen storage and on its physiological responses. Results show that cells were less resistant to microfiltration than centrifugation. In contrast, their resistance to freezing was significantly improved (in a range of 28% to 88%), according to the microfiltration conditions applied, as compared to the centrifuged cells. The higher cryotolerance was obtained with a cross flow velocity of 2.01 m.s-1 and a transmembrane pressure of 0.15 MPa. This improved resistance was related to an increase of C16:0, C18:1 and cycC19:0 membrane fatty acid concentrations and a reduction of the C14:0 content. Results obtained from proteome analysis displayed that cells exhibiting high activity and resistance were characterized by an increase in their energetic metabolism and, in contrast, by a reduction in their general and protein metabolisms. Finally, during this thesis, two interesting approaches were performed with the aim to adapt Lb. bulgaricus CFL1 for a better cryotolerance. Some general physiological responses, responsible for the cellular adaptation, were identified at membrane and cytosolic levels.
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Submitted on : Thursday, March 19, 2009 - 8:00:00 AM
Last modification on : Thursday, March 19, 2009 - 8:00:00 AM
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Fernanda Streit. Influence of harvesting and concentration conditions on the physiological state and cryotolerance of lactobacillus delbrueckii subsp. bulgaricus cfl1. Life Sciences [q-bio]. AgroParisTech, 2008. English. ⟨NNT : 2008AGPT0040⟩. ⟨pastel-00004299⟩

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