The ATP pool in relation to the production of glycerol and heat during growth of the halotolerant yeast Debaryomyces hansenii

Debaryomyces hansenii Strains Isolated From Danish Cheese Brines Act as Biocontrol Agents to Inhibit Germination and Growth of Contaminating Molds

The antagonistic activities of native Debaryomyces hansenii strains isolated from Danish cheese brines were evaluated against contaminating molds in the dairy industry. Determination of chromosome polymorphism by use of pulsed-field gel electrophoresis (PFGE) revealed a huge genetic heterogeneity among the D. hansenii strains, which was reflected in intra-species variation at the phenotypic level. 11 D. hansenii strains were tested for their ability to inhibit germination and growth of contaminating molds, frequently occurring at Danish dairies, i.e., Cladosporium inversicolor, Cladosporium sinuosum, Fusarium avenaceum, Mucor racemosus, and Penicillium roqueforti. Especially the germination of C. inversicolor and P. roqueforti was significantly inhibited by cell-free supernatants of all D. hansenii strains. The underlying factors behind the inhibitory effects of the D. hansenii cell-free supernatants were investigated. Based on dynamic headspace sampling followed by gas chromatography-mass spectrometry (DHS-GC-MS), 71 volatile compounds (VOCs) produced by the D. hansenii strains were identified, including 6 acids, 22 alcohols, 15 aldehydes, 3 benzene derivatives, 8 esters, 3 heterocyclic compounds, 12 ketones, and 2 phenols. Among the 71 identified VOCs, inhibition of germination of C. inversicolor correlated strongly with three VOCs, i.e., 3-methylbutanoic acid, 2-pentanone as well as acetic acid. For P. roqueforti, two VOCs correlated with inhibition of germination, i.e., acetone and 2-phenylethanol, of which the latter also correlated strongly with inhibition of mycelium growth. Low half-maximal inhibitory concentrations (IC50) were especially observed for 3-methylbutanoic acid, i.e., 6.32-9.53 × 10-5 and 2.00-2.67 × 10-4 mol/L for C. inversicolor and P. roqueforti, respectively. For 2-phenylethanol, a well-known quorum sensing molecule, the IC50 was 1.99-7.49 × 10-3 and 1.73-3.45 × 10-3 mol/L for C. inversicolor and P. roqueforti, respectively. For acetic acid, the IC50 was 1.35-2.47 × 10-3 and 1.19-2.80 × 10-3 mol/L for C. inversicolor and P. roqueforti, respectively. Finally, relative weak inhibition was observed for 2-pentanone and acetone. The current study shows that native strains of D. hansenii isolated from Danish brines have antagonistic effects against specific contaminating molds and points to the development of D. hansenii strains as bioprotective cultures, targeting cheese brines and cheese surfaces.

Characterization of glucose transport mutants of Saccharomyces cerevisiae during a nutritional upshift reveals a correlation between metabolite levels and glycolytic flux

Saccharomyces cerevisiae shows a marked preference for glucose and fructose, revealed by the repression of genes whose products are involved in processing other carbon sources. This response seems to be driven by sugar phosphorylation in the first steps of glycolysis rather than by the external sugar concentration. To gain a further insight into the role of the internal sugar signalling mechanisms, were measured the levels of upper intracellular glycolytic metabolites and adenine nucleotides in three mutant strains, HXT1, HXT7 and TM6*, with progressively reduced uptake capacities in comparison with the wild type. Reducing the rate of sugar consumption caused an accumulation of hexose phosphates upstream of the phosphofructokinase (PFK) and a reduction of fructose-1,6-bisphosphate levels. Mathematical modelling showed that these effects may be explained by changes in the kinetics of PFK and phosphoglucose isomerase. Moreover, the model indicated a modified sensitivity of the pyruvate dehydrogenase and the trichloroacetic acid cycle enzymes towards the NAD/NADH in the TM6* strain. The activation of the SNF1 sugar signalling pathway, previously observed in the TM6* strain, does not correlate with a reduction of the ATP : AMP ratio as reported in mammals. The mechanisms that may control the glycolytic rate at reduced sugar transport rates are discussed.

Determination of adenine and pyridine nucleotides in glucose-limited chemostat cultures of Penicillium simplicissimum by one-step ethanol extraction and ion-pairing liquid chromatography

Under specific conditions Penicillium simplicissimum excretes large amounts of organic acids, mainly citrate. As the energetic status of the hyphae might play a role in that respect, we developed a method for the determination of adenine (adenosine triphosphate, adenosine diphosphate, and adenosine monophosphate) and pyridine (nicotinamide adenine dinucleotide and reduced nicotinamide adenine dinucleotide (NADH)) nucleotides in hyphae of P. simplicissimum. An optimum separation of the five compounds in less than 15 min was possible on a C-8 column, utilizing 50 mM aqueous triethylamine-buffer (pH 6.5) and acetonitrile as mobile phase; detection was performed at 254 nm. With the exception of NADH, which could not be determined accurately due to stability problems, the method was sensitive (LOD < or = 0.7 ng on-column), repeatable (sigma(rel) < or = 4.4%), accurate (recovery rates between 97.9 and 104.9%), and precise (intraday variation < or = 9.4%, interday variation < or = 6.2 %). For an optimum extraction of the nucleotides the chemostat samples were directly placed into hot (90 degrees C) 50% ethanol, and shaken for 10 min, followed by evaporation of the solvent and a solid phase extraction cleanup of the redissolved aqueous samples. With this method the nucleotide concentrations in hyphae from a glucose-limited chemostat culture and the respective energy charge were determined. Additionally, the effect of the time lag between sampling and extraction and the effect of a glucose pulse on nucleotide concentrations were determined.

The effect of lactic acid on anaerobic carbon or nitrogen limited chemostat cultures of Saccharomyces cerevisiae

Weak organic acids are well-known metabolic effectors in yeast and other micro-organisms. High concentrations of lactic acid due to infection of lactic acid bacteria often occurs in combination with growth under nutrient-limiting conditions in industrial yeast fermentations. The effects of lactic acid on growth and product formation of Saccharomyces cerevisiae were studied, with cells growing under carbon- or nitrogen-limiting conditions in anaerobic chemostat cultures (D=0.1 h(-1)) at pH values 3.25 and 5. It was shown that lactic acid in industrially relevant concentrations had a rather limited effect on the metabolism of S. cerevisiae. However, there was an effect on the energetic status of the cells, i.e. lactic acid addition provoked a reduction in the adenosine triphosphate (ATP) content of the cells. The decrease in ATP was not accompanied by a significant increase in the adenosine monophosphate levels.

Rapamycin pre-treatment preserves viability, ATP level and catabolic capacity during carbon starvation of Saccharomyces cerevisiae

Saccharomyces cerevisiae growing exponentially in anaerobic batch cultures that are suddenly exposed to carbon starvation will rapidly lose almost all ATP. This will cause an energy deficiency and adaptation to starvation conditions is prohibited. As a result, viability and fermentative capacity will be drastically reduced during prolonged starvation. However, if the cells are incubated in the presence of rapamycin (which will inactivate the TOR pathway) before carbon starvation ATP levels, viability and fermentative capacity will be preserved to a much larger extent compared to untreated cells. The beneficial effect of rapamycin cannot be explained by induction of a stationary phase phenotype. In fact, under these anaerobic well-controlled growth conditions, rapamycin-treated cells were still metabolically active and continued to grow, albeit not exponentially and with a reduced protein content. It is hypothesized that the loss of ATP during carbon starvation occurs because protein synthesis does not make an immediate arrest at the onset of starvation. Since there are no external or internal energy sources, this will rapidly deplete the cells of ATP. Rapamycin-treated cells, on the other hand, have already downregulated the protein-synthesizing machinery and are thus better suited to cope with a sudden carbon starvation condition. This hypothesis is strengthened by the fact that treating the cells with the protein synthesis inhibitor cycloheximide also improves the carbon starvation tolerance, although not to the same extent as rapamycin. The even better effect of rapamycin is explained by accumulation of storage carbohydrates, which is not observed for cycloheximide-treated cells.

Starvation response of Saccharomyces cerevisiae grown in anaerobic nitrogen- or carbon-limited chemostat cultures

Anaerobic starvation conditions are frequent in industrial fermentation and can affect the performance of the cells. In this study, the anaerobic carbon or nitrogen starvation response of Saccharomyces cerevisiae was investigated for cells grown in anaerobic carbon or nitrogen-limited chemostat cultures at a dilution rate of 0.1 h(-1) at pH 3.25 or 5. Lactic or benzoic acid was present in the growth medium at different concentrations, resulting in 16 different growth conditions. At steady state, cells were harvested and then starved for either carbon or nitrogen for 24 h under anaerobic conditions. We measured fermentative capacity, glucose uptake capacity, intracellular ATP content, and reserve carbohydrates and found that the carbon, but not the nitrogen, starvation response was dependent upon the previous growth conditions. All cells subjected to nitrogen starvation retained a large portion of their initial fermentative capacity, independently of previous growth conditions. However, nitrogen-limited cells that were starved for carbon lost almost all their fermentative capacity, while carbon-limited cells managed to preserve a larger portion of their fermentative capacity during carbon starvation. There was a positive correlation between the amount of glycogen before carbon starvation and the fermentative capacity and ATP content of the cells after carbon starvation. Fermentative capacity and glucose uptake capacity were not correlated under any of the conditions tested. Thus, the successful adaptation to sudden carbon starvation requires energy and, under anaerobic conditions, fermentable endogenous resources. In an industrial setting, carbon starvation in anaerobic fermentations should be avoided to maintain a productive yeast population.

Inactivation of Escherichia coli, Listeria innocua and Saccharomyces cerevisiae in relation to membrane permeabilization and subsequent leakage of intracellular compounds due to pulsed electric field processing

Membrane permeabilization, caused by pulsed electric field (PEF) processing of microbial cells, was investigated by measurement of propidium iodide (PI) uptake with flow cytometry. Inactivation of Escherichia coli, Listeria innocua and Saccharomyces cerevisiae was determined by viable counts, and leakage of intracellular compounds, such as ATP and UV-absorbing substances, was measured in the extracellular environment. Electrical field strength and pulse duration influenced membrane permeabilization of all three tested organisms of which S. cerevisiae was the most PEF sensitive, followed by E. coli and L. innocua. It was shown by viable counts, PI uptake and leakage of intracellular compounds that L. innocua was the most resistant. Increased inactivation corresponded to greater numbers of permeabilized cells, which were reflected by increased PI uptake and larger amounts of intracellular compounds leaking from cells. For E. coli and L. innocua, a linear relationship was observed between the number of inactivated cells (determined as CFU) and cells with permeated membranes (determined by PI uptake), with higher number of inactivated cells than permeated cells. Increased leakage of intracellular compounds with increasing treatment severity provided further evidence that cells were permeabilized. For S. cerevisiae, there was higher PI uptake after PEF treatments, although very little or no inactivation was observed. Results suggest that E. coli and L. innocua cells, which took up PI, lost their ability to multiply, whereas cells of S. cerevisiae, which also took up PI, were not necessarily lethally permeabilized.

Carbon starvation can induce energy deprivation and loss of fermentative capacity in Saccharomyces cerevisiae

Seven different strains of Saccharomyces cerevisiae were tested for the ability to maintain their fermentative capacity during 24 h of carbon or nitrogen starvation. Starvation was imposed by transferring cells, exponentially growing in anaerobic batch cultures, to a defined growth medium lacking either a carbon or a nitrogen source. After 24 h of starvation, fermentative capacity was determined by addition of glucose and measurement of the resulting ethanol production rate. The results showed that 24 h of nitrogen starvation reduced the fermentative capacity by 70 to 95%, depending on the strain. Carbon starvation, on the other hand, provoked an almost complete loss of fermentative capacity in all of the strains tested. The absence of ethanol production following carbon starvation occurred even though the cells possessed a substantial glucose transport capacity. In fact, similar uptake capacities were recorded irrespective of whether the cells had been subjected to carbon or nitrogen starvation. Instead, the loss of fermentative capacity observed in carbon-starved cells was almost surely a result of energy deprivation. Carbon starvation drastically reduced the ATP content of the cells to values well below 0.1 micro mol/g, while nitrogen-starved cells still contained approximately 6 micro mol/g after 24 h of treatment. Addition of a small amount of glucose (0.1 g/liter at a cell density of 1.0 g/liter) at the initiation of starvation or use of stationary-phase instead of log-phase cells enabled the cells to preserve their fermentative capacity also during carbon starvation. The prerequisites for successful adaptation to starvation conditions are probably gradual nutrient depletion and access to energy during the adaptation period.

The catabolic capacity of Saccharomyces cerevisiae is preserved to a higher extent during carbon compared to nitrogen starvation

A comparison of catabolic capacity was made between S. cerevisiae cells subjected to 24 h carbon or nitrogen starvation. The cells were shifted to starvation conditions at the onset of respiratory growth on ethanol in aerobic batch cultures, using glucose as the carbon and energy source. The results showed that the catabolic capacity was preserved to a much larger extent during carbon compared to nitrogen starvation. Nitrogen starvation experiments were made in the presence of ethanol (not glucose) to exclude the effect of glucose transport inactivation (Busturia and Lagunas, 1986). Hence, the difference in catabolic capacity could not be attributed to differences in glucose transport capacity during these conditions. In order to understand the reason for this difference in starvation response, measurement of protein composition, adenine nucleotides, inorganic phosphate, polyphosphate and storage carbohydrates were performed. No clear correlation between any of these variables and catabolic capacity after starvation could be obtained. However, there was a positive correlation between total catabolic activity and intracellular ATP concentration when glucose was added to starved cells. The possible mechanism for this correlation, as well as what determines the ATP level, is discussed.

The yeast glycerol 3-phosphatases Gpp1p and Gpp2p are required for glycerol biosynthesis and differentially involved in the cellular responses to osmotic, anaerobic, and oxidative stress

We have characterized the strongly homologous GPP1/RHR2 and GPP2/HOR2 genes, encoding isoforms of glycerol 3-phosphatase. Mutants lacking both GPP1 and GPP2 are devoid of glycerol 3-phosphatase activity and produce only a small amount of glycerol, confirming the essential role for this enzyme in glycerol biosynthesis. Overproduction of Gpp1p and Gpp2p did not significantly enhance glycerol production, indicating that glycerol phosphatase is not rate-limiting for glycerol production. Previous studies have shown that expression of both GPP1 and GPP2 is induced under hyperosmotic stress and that induction partially depends on the HOG (high osmolarity glycerol) pathway. We here show that expression of GPP1 is strongly decreased in strains having low protein kinase A activity, although it is still responsive to osmotic stress. The gpp1Delta/gpp2Delta double mutant is hypersensitive to high osmolarity, whereas the single mutants remain unaffected, indicating GPP1 and GPP2 substitute well for each other. Transfer to anaerobic conditions does not affect expression of GPP2, whereas GPP1 is transiently induced, and mutants lacking GPP1 show poor anaerobic growth. All gpp mutants show increased levels of glycerol 3-phosphate, which is especially pronounced when gpp1Delta and gpp1Delta/gpp2Delta mutants are transferred to anaerobic conditions. The addition of acetaldehyde, a strong oxidizer of NADH, leads to decreased glycerol 3-phosphate levels and restored anaerobic growth of the gpp1Delta/gpp2Delta mutant, indicating that the anaerobic accumulation of NADH causes glycerol 3-phosphate to reach growth-inhibiting levels. We also found the gpp1Delta/gpp2Delta mutant is hypersensitive to the superoxide anion generator, paraquat. Consistent with a role for glycerol 3-phosphatase in protection against oxidative stress, expression of GPP2 is induced in the presence of paraquat. This induction was only marginally affected by the general stress-response transcriptional factors Msn2p/4p or protein kinase A activity. We conclude that glycerol metabolism plays multiple roles in yeast adaptation to altered growth conditions, explaining the complex regulation of glycerol biosynthesis genes.

The importance of ATP as a regulator of glycolytic flux in Saccharomyces cerevisiae

The control of glycolytic flux in the yeast Saccharomyces cerevisiae was studied by using permeabilized cells. Cells were harvested from chemostat cultures and, after removal of the cell wall, nystatin was used to permeabilize the spheroplasts. By this method it is possible to study the performance and regulation of a complete and functional metabolic pathway and not only a single enzymatic step. The results showed that ATP has a strong negative effect on glycolytic activity affecting several of the glycolytic enzymes. However, the main targets for ATP inhibition was phosphofructokinase and pyruvate kinase. Phospofructokinase was inhibited by ATP concentrations starting at about 1-2 mM, while pyruvate kinase required ATP levels above 2.5 mM before any inhibition was visible. These ATP concentrations were in the same range as measured for nitrogen- and glucose-limited cells cultivated in chemostat cultures. Other potential candidates as enzymes susceptible to ATP inhibition included hexokinase and enolase. The ATP:ADP ratio, as well as trehalose-6-phosphate levels, did not seem to influence the glycolytic activity.

Glycolytic flux is conditionally correlated with ATP concentration in Saccharomyces cerevisiae: a chemostat study under carbon- or nitrogen-limiting conditions

Anaerobic and aerobic chemostat cultures of Saccharomyces cerevisiae were performed at a constant dilution rate of 0.10 h(-1). The glucose concentration was kept constant, whereas the nitrogen concentration was gradually decreasing; i.e., the conditions were changed from glucose and energy limitation to nitrogen limitation and energy excess. This experimental setup enabled the glycolytic rate to be separated from the growth rate. There was an extensive uncoupling between anabolic energy requirements and catabolic energy production when the energy source was present in excess both aerobically and anaerobically. To increase the catabolic activity even further, experiments were carried out in the presence of 5 mM acetic acid or benzoic acid. However, there was almost no effect with acetate addition, whereas both respiratory (aerobically) and fermentative activities were elevated in the presence of benzoic acid. There was a strong negative correlation between glycolytic flux and intracellular ATP content; i.e., the higher the ATP content, the lower the rate of glycolysis. No correlation could be found with the other nucleotides tested (ADP, GTP, and UTP) or with the ATP/ADP ratio. Furthermore, a higher rate of glycolysis was not accompanied by an increasing level of glycolytic enzymes. On the contrary, the glycolytic enzymes decreased with increasing flux. The most pronounced reduction was obtained for HXK2 and ENO1. There was also a correlation between the extent of carbohydrate accumulation and glycolytic flux. A high accumulation was obtained at low glycolytic rates under glucose limitation, whereas nitrogen limitation during conditions of excess carbon and energy resulted in more or less complete depletion of intracellular storage carbohydrates irrespective of anaerobic or aerobic conditions. However, there was one difference in that glycogen dominated anaerobically whereas under aerobic conditions, trehalose was the major carbohydrate accumulated. Possible mechanisms which may explain the strong correlation between glycolytic flux, storage carbohydrate accumulation, and ATP concentrations are discussed.

Purification and characterization of glycerol-3-phosphate dehydrogenase of Saccharomyces cerevisiae

The NAD-dependent glycerol-3-phosphate dehydrogenase (glycerol-3-phosphate:NAD+ oxidoreductase; EC 1.1.1.8; G3P DHG) was purified 178-fold to homogeneity from Saccharomyces cerevisiae strain H44-3D by affinity- and ion-exchange chromatography. SDS-PAGE indicated that the enzyme had a molecular mass of approximately 42,000 (+/- 1,000) whereas a molecular mass of 68,000 was observed using gel filtration, implying that the enzyme may exist as a dimer. The pH optimum for the reduction of dihydroxyacetone phosphate (DHAP) was 7.6 and the enzyme had a pI of 7.4. NADPH will not substitute for NADH as coenzyme in the reduction of DHAP. The oxidation of glycerol-3-phosphate (G3P) occurs at 3% of the rate of DHAP reduction at pH 7.0. Apparent Km values obtained were 0.023 and 0.54 mM for NADH and DHAP, respectively. NAD, fructose-1,6-bisphosphate (FBP), ATP and ADP inhibited G3P DHG activity. Ki values obtained for NAD with NADH as variable substrate and FBP with DHAP as variable substrate were 0.93 and 4.8 mM, respectively.

Osmotic significance of glycerol accumulation in exponentially growing yeasts

Natural-abundance 13C-nuclear magnetic resonance spectroscopy has shown glycerol to be the major osmotically significant low-molecular-weight solute in exponentially growing, salt-stressed cells of the yeasts Saccharomyces cerevisiae, Zygosaccharomyces rouxii, and Debaromyces hansenii. Measurement of the intracellular nonosmotic volume (i.e., the fraction of the cell that is osmotically unresponsive) by using the Boyle-van't Hoff relationship (for nonturgid cells, the osmotic volume is directly proportional to the reciprocal of the external osmotic pressure) showed that the nonosmotic volume represented up to 53% of the total cell volume; the highest values were recorded in media with maximum added NaCl. Determinations of intracellular glycerol levels with respect to cell osmotic volumes showed that increases in intracellular glycerol may counterbalance up to 95% of the external osmotic pressure due to added NaCl. The lack of other organic osmotica in 13C-nuclear magnetic resonance spectra indicates that inorganic ions may constitute the remaining component of intracellular osmotic pressure.

Glycerol production in relation to the ATP pool and heat production rate of the yeasts Debaryomyces hansenii and Saccharomyces cerevisiae during salt stress

Changes in glycerol production and two parameters related to energy metabolism i.e. the heat production rate and the ATP pool, were assayed during growth of Saccharomyces cerevisiae and Debaryomyces hansenii in 4 mM and 1.35 M NaCl media. For both of the yeasts, the specific ATP pool changed during the growth cycle and reached maximum values around 10 nmol per mg dry weight in both types of media. The levels of glycerol were markedly enhanced by high salinity. In the presence of 1.35 M NaCl, D. hansenii retained most of its glycerol produced intracellularly, while S. cerevisiae extruded most of the glycerol to the environment. The intracellular glycerol level of S. cerevisiae equalled or exceeded that of D. hansenii, however, with values never lower than 3 mumol per mg dry weight at all phases of growth. When D. hansenii was grown at this high salinity the intracellular level of glycerol was found to correlate with the specific heat production rate. No such correlation was found for S. cerevisiae. We concluded that during salt stress, D. hansenii possesses the capacity to regulate the metabolism of glycerol to optimize growth, while S. cerevisiae may not be able to regulate when exposed to different demands on the glycerol metabolism.

Effect of NaCl on kinetics of D-glucosamine uptake in yeasts differing in halotolerance

The initial rate of D-glucosamine uptake by the non-halotolerant yeast Saccharomyces cerevisiae was approximately halved as the apparent half saturation constant (Km) and the apparent maximum velocity (Vmax) changed from 6.6 mM to 16.4 mM and from 22 mumol.g-1.min-1 to 16 mumol.g-1.min-1, respectively, when the salinity in the medium was increased from zero M to 0.68 M NaCl. Corresponding changes in a high affinity transport system in the halotolerant yeast Debaryomyces hansenii were from 1.1 mM to 4.6 mM and from 3.1 mumol.g-1.min-1 to 4.5 mumol.g-1.min-1, implying a practically unchanged transport capacity. In 2.7 M NaCl, Km and Vmax in this system were 24.5 mM and 1.1 mumol.g-1.min-1, respectively, representing a marked decrease in transport capability. Nevertheless, the degree of affinity in this extreme salinity must still be regarded as noteworthy. In addition to the high affinity transport system in D. hansenii, a low affinity system, presumably without relevance in D-glucosamine transport, was observed.