Publisher Summary This chapter considers the chemiosmotic interpretation of the synthesis of ATP, which takes place in the mitochondria and chloroplasts of eukaryotic micro-organisms The evidence that is available indicates that the ionic pumps of the plasma membranes of these organisms are not involved in ATP synthesis Thus the plasma membrane of yeast lacks a typical electron-transport chain, resembling in this respect the plasma membrane of anaerobic organisms such as Streptococcus faecalis rather than the plasma membrane of facultative aerobes like Escherichia coli, which carry out oxidative phosphorylation It discusses that a further comparison concerns the extensively studied transport systems of mammalian cells, which include the sodium pump based on a Na/K-ATPase A wide range of such cells involve Na + ions in secondary active transport of various carbohydrates and amino acids Similar coupled mechanisms are found in certain bacteria Their occasional occurrences in eukaryotic microorganisms are also considered in this chapter

Mechanisms of solute transport in selected eukaryotic micro-organisms.

Publisher Summary This chapter reviews the temperature profiles of yeasts. When an Arrhenius plot is prepared of the specific growth rates of yeast for temperatures that range from suboptimal to supermaximal values and an Arrhenius plot of the specific rates of thermal death is superimposed on the Arrhenius plot of growth, temperature profile of yeast is obtained. Temperature profiles of the specific rates of growth and thermal death may be enriched with curves displaying the temperature dependence of other significant parameters, such as growth yield, specific rates of production of extracellular enzymes and excretion of metabolic products. Temperature profiles of growth and death provide at least three types of information: (1) the position of the cardinal temperatures of the strain within the range of biological temperatures, (2) the dependence of the specific rates of growth and death on the temperature—that is, their activation parameters, (3) the presence or the absence of linkage between growth and thermal death in the superoptimal temperature range.

Temperature Profiles of Yeasts

The role of membrane integrity and the membrane ATPase in the mechanism of thermotolerance in Saccharomyces cerevisiae was investigated. The resistance to lethal heat of a mutant strain with reduced expression of the membrane ATPase was significantly less than that of the wild-type parent. However, prior exposure to sub-lethal temperatures resulted in the induction of similar levels of thermotolerance in the mutant compared to the parent strain, suggesting that the mechanism of sub-lethal heat-induced thermotolerance is independent of ATPase activity. Supporting this, exposure to sub-lethal heat stress did not result in increased levels of glucose-induced acid efflux at lethal temperatures and there was little correlation between levels of acid efflux and levels of heat resistance. ATPase activity in crude membrane preparations from sub-lethally heat-stressed cells was similar to that in preparations from unstressed cells. Study of net acid flux during heating revealed that pre-stressed cells were able to protect the proton gradient for longer. This may confer an ‘advantage’ to these cells that results in increased thermotolerance. This was supported by the observation that prior exposure to sub-lethal heat resulted in a transient protection against the large increase in membrane permeability that occurs at lethal temperatures. However, no protection against the large drop in intracellular pH was detected. Sub-lethal heat-induced protection of membrane integrity also occurred to the same extent in the reduced-expression membrane ATPase mutant, further implying that the mechanism of induced thermotolerance is independent of ATPase activity. To conclude, although the membrane ATPase is essential for basal heat resistance, thermotolerance induced by prior exposure to stress is largely conferred by a mechanism that is independent of the enzyme.

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Activity of the plasma membrane H+-ATPase is a key physiological determinant of thermotolerance in Saccharomyces cerevisiae

The influence of the kinetics of application of physical stresses on microbial viability is considered. For the main physical parameters used in food preservation, such as temperature and osmotic pressure, the optimal kinetics of their application can be determined in order to keep alive or to destroy the microbial population in a food product. Indeed, for both parameters, high rates of stress application as well as very low rates were found to drastically decrease the microbial population level. This important effect of the kinetics of thermal or drying stresses could be used to reduce the classical levels of heat or drying treatments used in food processing.

The importance of the kinetics of application of physical stresses on the viability of microorganisms: significance for minimal food processing

Effect of the kinetics of temperature variation on saccharomyces cerevisiae viability and permeability

SUMMARY: Anaerobically grown yeast, suspended in glucose solution, leaked cell contents non-selectively. The rate of leakage increased with temperature but reached a maximum rate close to the maximum temperature of growth (T
max) of the yeast. The total leakage induced by glucose was much greater above T
max than below it, because above T
max the yeast lost the ability to take up released material. The rate of uptake of 14C-labelled amino acids was also substantially slowed above T
max-Yeast heat-stressed in the presence of glucose lost the ability to establish and maintain a concentration gradient of sorbose, and simultaneously ATPase activity could be measured, all characteristics of yeast with a ruptured cytoplasmic membrane. These effects were uniquely caused by utilizable sugars, were essentially independent of sugar concentration and could be partially inhibited by Ca2+ or inhibitors that prevent sugar utilization. Yeast heated above T
max in water suspension was essentially undamaged as determined by the tests used, but if glucose was subsequently added below T
max an effect of heat damage could be demonstrated. We conclude that one factor that determines the T
max of a yeast is the temperature sensitivity of the cytoplasmic membrane in the presence of the utilizable sugar of the growth medium.

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A. N. Hagler

Papers

Effect of Glucose on Thermal Injury of Yeast that may Define the Maximum Temperature of Growth