Yeast resistance to high levels of osmotic pressure: Influence of kinetics

International audienceApplication of slow and progressive changes in osmotic pressure to cells of S. cerevisiae allowed the preservation of viability up to very high levels of osmotic pressure (125 MPa). Viability was examined in relation to water exit from the cell in terms of cell volume changes which were measured continuously using an on-line image analysis system.It has been shown that the high survival level of cells subjected to such progressive changes in osmotic pressure are due to physical factors alone and mainly to the water flow rate during the period just after the osmotic shift

Development of a sensor allowing the evaluation of the osmotic pressure of liquid media

International audienceThe energy level of water in liquid media is a powerful parameter which can be measured through the water activity of the solution. A sensor for on-line measurement of this parameter has been developed recently, but measurement of this parameter remains difficult for high water activity values. In this paper a sensor based on the measurement of the capillary ascension induced by a difference in osmotic pressure between two systems is developed theoretically, and the results of the application of a sensor of this type to osmotic flux measurement are presented

Yeast viability related to water potential variation: influence of the transient phase

International audienceThe decrease rate of the water potential was found to have a great effect on yeasts submitted to hypertonic shifts. The application of slow and linear decreases of the water potential of the medium to cells of Saccharomyces cerevisiae demonstrated that the cells could survive (90 to 100% of viability) very low levels of water potential (=–101 MPa). Resistance of the cells was examined through viability measurements and cell volume changes. The kinetics of cell volume variation were measured continuously using an on-line image analysis system coupled to a microscope. No biological response of the cells occurred, due to the lack of a usable carbon-energy source in the medium. The viability level was found to be a function of the water exit flow rate from the cells. The denaturation of the membrane structure was assumed to be involved in such phenomena

Etude de la cinétique et de l'amplitude de stress thermique froid sur différents modèles cellulaires (influence de la cristallisation sur la viabilité cellulaire)

L'objectif de ce travail est d'analyser la mortalité cellulaire au cours de la congélation en intégrant la vitesse de refroidissement. Après le développement de techniques de refroidissement sur une gamme de vitesses s'étendant de 5 à 30000C/min., la viabilité cellulaire a été déterminée, puis le volume des cellules a été mesuré par analyse d'image et microscopie électronique après cryosubstitution. Enfin les milieux de congélation ont été caractérisés par analyse enthalpique différentielle. Ce travail a été mené essentiellement sur S. cerevisiae et a été confirmé en utilisant d'autres modèles cellulaires : levure (C. utilis) , bactéries (E. Coli, L. plantarum) et cellules humaines (cellules leucémiques, globules rouges). Il a été montré que la cristallisation de l'eau est à l'origine de la mort des cellules : (i) Pour des vitesses de refroidissement allant jusqu'à 1800C/min, la cristallisation est exclusivement extracellulaire, ce qui entraîne une déshydratation des cellules. La forte viabilité cellulaire mesurée dans cette gamme de vitesse de refroidissement correspond à une vitesse de déshydratation optimale. (ii) ) Pour des vitesses de refroidissement supérieures à 1800C/min, une cristallisation se produit dans la cellule. Celle-ci est léthale si elle coïncide avec des mouvements d'eau transcellulaire. Pour des vitesses de refroidissement ultrarapides qui induisent de très faibles variations de volume cellulaire par la cristallisation et/ou vitrification quasi-instantanée des cellules, la viabilité cellulaire est préservée. Dans ce cas, la vitesse de réchauffement doit être optimisée afin d'éviter la recristallisation intracellulaire et préserver la survie cellulaire.DIJON-BU Sciences Economie (212312102) / SudocSudocFranceF

Viability of lactic bacteria submitted to dehydration: Importance of the dehydration kinetics \textbar Servie des bactéries lactiques soumises à la déshydratation : Importance de la cinétique de déshydratation

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Medium hydration effects on the growth of Trichoderma viride

International audienceThe hydration of gelled substrates was examined in relation to growth of the filamentous fungus Trichoderma viride. An experimental plan was designed to discriminate between the effects of thermodynamic (aw) and kinetic parameters (water content) on fungal growth. At a set thermodynamic level of water (aw), the decrease in water content caused by adding silica gel to the medium proved limiting to fungal growth. This limitation was attributed to diffusion as shown by experimental determination of rotational and translational diffusivities of glucose in such media

Cell Size and Water Permeability as Determining Factors for Cell Viability after Freezing at Different Cooling Rates

International audienceThis work studied the viabilities of five types of cells (two yeast cells, Saccharomyces cerevisiae CBS 1171 and Candida utilis; two bacterial strains, Escherichia coli and Lactobacillus plantarum; and one human leukemia K562 cell) as a function of cooling rate during freezing. The range of investigated cooling rates extended from 5 to 30,000°C/min. Cell viability was classified into three ranges: (i) high viability for low cooling rates (5 to 180°C/min), which allow cell water outflow to occur completely and do not allow any intracellular crystallization; (ii) low viability for rapid cooling rates (180 to 5,000°C/min), which allow the heat flow to prevail over water outflow (in this case, cell water crystallization would occur as water was flowing out of the cell); (iii) high viability for very high cooling rates (>5,000°C/min), which allow the heat flow to be very rapid and induce intracellular crystallization and/or vitrification before any water outflow from the cell. Finally, an assumption relating cell death to the cell water crystallization as water is flowing out of the cell is made. In addition, this general cell behavior is different for each type of cell and seems to be moderated by the cell size, the water permeability properties, and the presence of a cell wall

Involvement of two specific causes of cell mortality in freeze‐thaw cycles with freezing to ‐196°C

International audienceThe purpose of this study was to examine cell viability after freezing. Two distinct ranges of temperature were identified as corresponding to stages at which yeast cell mortality occurred during freezing to −196°C. The upper temperature range was related to the temperature of crystallization of the medium, which was dependent on the solute concentration; in this range mortality was prevented by high solute concentrations, and the proportion of the medium in the vitreous state was greater than the proportion in the crystallized state. The lower temperature range was related to recrystallization that occurred during thawing. Mortality in this temperature range was increased by a high cooling rate and/or high solute concentration in the freezing medium and a low temperature (less than −70°C). However, a high rate of thawing prevented yeast mortality in this lower temperature range. Overall, it was found that cell viability could be conserved better under freezing conditions by increasing the osmotic pressure of the medium and by using an increased warming rate

Effects of the kinetics of water potential variation on bacteria viability

International audienceThe effect of the kinetics of water potential variation (psi) on the viability of bacteria subjected to hyperosmotic stresses in water-glycerol solution was studied. The three bacteria used were Lactobacillus plantarum L-73, Leuconostoc mesenteroides LM057 and Escherichia coli TG1. These strains were submitted to a final water potential of -107.2 MPa, -170.9 MPa and/or -244.7 MPa. In any case the kinetics of water potential variation was found to have a great effect on the cell viability. The application of slow water potential decreases could maintain an important cell viability (about 80-100%) with regard to the corresponding viability observed after a sudden step change for the same final water potential (15-57%). For each strain tested, an optimal dehydration kinetics was determined which depended on the final water potential. The existence of this optimum could be explained thanks to the opposition of two actions affecting cell viability: a positive action relative to the slowness of the water potential variation and a negative action relative to the residence time of cells in a critical range of water potential