Effects of weak acid preservatives on the growth and thermal death of the yeast Pichia membranifaciens in a commercial apple juice.

Pichia membranifaciens exhibited a dissociative temperature profile (the temperature range of thermal death was distinct from the temperature range of growth) when incubation took place either in a commercial apple juice (AJ) or in a synthetic mineral medium with glucose and vitamins (MGV). In AJ the maximum temperature for growth (Tmax) was 38.6 degrees C, which decreased to 36 degrees C in the presence of either 1 mM sorbic or 1 mM benzoic acid. The minimum temperatures of thermal death (Tmind) were, respectively, 40 and 38 degrees C with either of the acids. The yeast could grow with up to 2 mM sorbic or 3 mM benzoic acid, at 25 degrees C, which is close to the optimum temperature for growth (Top). At temperatures slightly above Tmind, sorbic acid was an actual enhancer of death rather than benzoic, the latter conferring some protection. However, these effects were reversed at higher temperatures (above 43 degrees C), at which benzoic acid was the most operative, in contrast to sorbic which was highly protective of the yeast against thermal death. The addition of acetaldehyde to sulphur-dioxide-containing juice reduced the lag phase and increased the overall specific growth rates. Sporulated or stationary vegetative cultures were more heat-resistant than exponential cultures, particularly at temperatures above 45 degrees C.

Effects of salts on Debaryomyces hansenii and Saccharomyces cerevisiae under stress conditions.

The effect of Na+ and K+ on growth and thermal death of Debaryomyces hansenii and Saccharomyces cerevisiae were compared under stress conditions as those commonly found in food environments. At the supraoptimal temperature of 34 degrees C both cations at concentrations of 0.5 M stimulated growth of D. hansenii, while K+ had no effect and Na+ inhibited growth of S. cerevisiae. At 8 degrees C, close to the minimum temperature for growth in both species, both cations inhibited both yeasts, this effect being more pronounced with Na+ in S. cerevisiae. At extreme pH values (7.8 and 3.5) both cations at concentrations of 0.25 M stimulated D. hansenii while Na+ inhibited S. cerevisiae. K+ inhibited this yeast at pH 3.5. Thermal inactivation rates, measured at 38 degrees C in D. hansenii and at 48 degrees C in S. cerevisiae, decreased in the presence of both cations. This protective effect could be observed in a wider range of concentrations in D. hansenii. These results call the attention to the fact that not all yeasts have the same behaviour on what concerns synergy or antagonism of salt together with other stress factors and should be taken into consideration in the establishment of food preservation procedures.

Depression by miconazole of heat-induced respiratory-deficiency in Saccharomyces cerevisiae.

Miconazole, at 0.2 microM, decreased, by two orders of magnitude, the specific mutation rate of Saccharomyces cerevisiae to respiratory-deficient mutants (petites), which had been induced at either 37 degrees C or 39 degrees C. Identical concentrations of ketoconazole did not change the mutation rate. The results fit in the mechanisms of action which have been proposed for imidazole antimycotics, and constitute further support for the hypothesis that the targets of thermal death, in petite-positive associately-profiled yeasts, lie in the mitochondria.

A comparison of miconazole, ketoconazole and fluconazole in their effects on temperature-dependent growth and thermal death in Candida albicans.

A strain of Candida albicans isolated from human sputum exhibited an associative temperature profile, with the initial maximum temperature = 42 degrees C, the final maximum temperature = 38 degrees C, and the minimum temperature of thermal death = 33 degrees C, showed a decrease in its cardinal temperatures and a reduction in the specific rates of growth and thermal death throughout the novel temperature ranges in the presence of either 25 microM of miconazole, ketoconazole or fluconazole. In the concentration range 0-30 microM, each drug concertedly depressed the kinetic and energetic parameters of growth, with lesser variation on the specific glucose transfer rate. The overall effect of miconazole was the greatest (up to one order of magnitude), while that of fluconazole was the least.

Conditioning by mitochondria and energy source of thermal death parameters in yeast.

Two strains of Saccharomyces cerevisiae of opposite mating type (a and alpha), with different resistance to thermal death, and their respective mitochondrial respiratory-deficient mutants (petites ap and alpha p) were used to prepare the following diploid strains: a alpha, a alpha p and ap alpha. Specific thermal death rates were determined at supramaximum temperatures for growth, under conditions of mitochondrial derepression (glycerol medium). Comparison of the entropies of activation of thermal death showed that diploids a alpha and ap alpha behaved like the more resistant haploid alpha, and diploid a alpha p like haploid a. In glucose medium strains alpha and a alpha became even more thermoresistant. The results favour the concept that thermal death determinants are located in the mitochondrial genome, and that mitochondria repressed cells repair thermal injuries more efficiently.

Thermal death potentiation by amphotericin B in Cryptococcus neoformans and its dependence on pre-incubation temperature.

Thermal death of Cryptococcus neoformans in the presence of amphotericin B was strongly dependent upon the temperature of pre-incubation. The entropy coefficient, that is, the increase in entropy of activation of thermal death per square unit concentration of the drug in the medium, was 35 times higher after pre-incubation at 25 degrees C than at 39 degrees C. This means that C. neoformans cells grown at lower temperatures were much more sensitive to the temperature-dependent fungicidal effect of amphotericin B.

Comparative study of the temperature profiles of growth and death of the pathogenic yeast Cryptococcus neoformans and the non-pathogenic Cryptococcus albidus.

The temperature profiles of two species of Cryptococcus were compared. The pathogenic Cr. neoformans had a maximum temperature for growth of 39.8 degrees C and the non-pathogenic Cr. albidus, of 30.2 degrees C. The specific growth rates measured in the former were of an order of magnitude higher than in the latter, whereas the Arrhenius plots of the specific thermal death rates did not show a significant difference.

The effect of 5-fluorocytosine on the temperature profile of Candida albicans.

The temperature association of exponential thermal death with exponential growth, observed in a strain of Candida albicans, was disrupted by 5-fluorocytosine which, at a concentration of 2 micrograms ml-1, shifted the maximum temperature for growth from 38 degrees C to 33 degrees C but did not affect thermal death.

Effects of cycloheximide on the temperature profile of Sacharomyces cerevisiae.

Tmax, the maximum temperature for growth of Saccharomyces cerevisiae, decreased linearly with increasing concentrations of cycloheximide added to the medium, to about 20 degrees C at 2.5 microgram ml-1. In this concentration range thermal death was not enhanced. The Arrhenius plot of growth was shifted to lower temperatures as a function of the cycloheximide concentration and became dissociated from the Arrhenius plot of thermal death. It was concluded that the target site of cycloheximide, the cytoplasmic ribosome, is not identical with the physiological Tmax site of S. cerevisiae and that the binding of cycloheximide to its target sites is strongly enhanced by the temperature.

The temperature profile of the pathogenic yeast Candida albicans.

A strain of Candida albicans was found to have an associative temperature profile with respect to growth, thermal death and yield on glucose. The ARRHENIUS plot of sustained exponential growth displayed two branches in the supraoptimal temperature range with the optimum temperature for growth around 33 degrees C, the final maximum temperature for growth around 38 degrees C and the initial maximum temperature for growth around 42 degrees C. The yield on glucose was temperature dependent in the supraoptimal range and declined linearly to zero between the optimum and the initial maximum temperature for growth.

Effects of chloramphenicol on the thermal profile of Saccharomyces cerevisiae.

Chloramphenicol decreased the maximum temperature for growth of a petite mutant of Saccharomyces cerevisiae, shifted the ARRHENIUS plot of thermal death to lower temperatures and shortened correspondingly, the ARRHENIUS plot of growth, while an associative thermal profile was maintained. At saturating concentrations (about 5 mg per ml) of chloramphenicol in liquid mineral medium with vitamins and glucose the final maximum temperature for growth was depressed from about 40 degrees C to about 37 degrees C. The results suggested that chloramphenicol acted in the mutant on targets other than mitochondrial ribosomes and that these targets are identical or associated with the death and Tmax sites of the yeast.

Kinetics of petite mutation and thermal death in Saccharomyces cerevisiae growing at superoptimal temperatures.

Mass formation of petite mutants took place in a strain of Saccharomyces cerevisiae when grown at superoptimal temperatures. After an initial period of exponential growth, a second period followed during which exponential death and net exponential petite mutation concurred with exponential growth. The specific rates of the three exponential processes were of the same order of magnitude and varied with the temperature. Net exponential petite mutation did not occur during the deathless first period of growth at superoptimal temperatures nor at any time during growth at suboptimal temperatures. Mitochondria are discussed as possible targets of thermal death in mesophilic yeasts.

Yield and maintenance relations of yeast growth in the chemostat at superoptimal temperatures.

A model is proposed that accounts for the decreases in yield which occur in chemostat cultures of mesophilic yeasts at superoptimal growth temperatures. Two yield depressing effects were identified, one due to increased maintenance requirements by the viable fraction of the population, the other due to energy substrate dissipation by the nonviable function. The two effects are functions of the dilution rate, as is the fraction of nonviable cells. Experimental results were obtained on the yield, maintenance, and dissipation of energy substrate in a glucose-limited chemostat culture of a respiration-deficient mutant of Saccharomyces cerevisiae at 39 degrees C. The rates of glucose utilization for maintenance and for dissipation constituted, respectively, 33-28% and 15-9% of the total glucose utilization rate over the range of dilution rates tested (0.038-0.064 hr-1), while the yield varied over this range from 0.066-0.085 g of biomass (dry wt) per gram of glucose.

Dependence of the maximum temperature for growth of Saccharomyces cerevisiae on nutrient concentration.

Saccharomyces cerevisae was grown in a chemostat under glucose limitation at three superoptimal temperatures. In each steady state the specific growth rate was the sum of the dilution rate and the specific death rate, exponential death occurring with exponential growth. The specific death rate was a function of both the temperature and the concentration of the limiting nutrient. Each superoptimal temperature was characterized by a critical glucose concentration below which net growth was not possible. The critical glucose concentration increased with the temperature. Consequently the maximum temperature for growth was a function of the concentration of the limiting nutrient and approached the optimum temperature for growth with decreasing glucose concentrations.