Acidification power: indicator of metabolic activity and autolytic changes in Saccharomyces cerevisiae

Unraveling the interaction of co-encapsulated Saccharomyces cerevisiae and Metarhizium brunneum in calcium alginate-based attract-and-kill beads

BACKGROUND

Attract-and-kill (AK) beads are biological, microbial insecticides developed as an alternative to synthetic soil insecticides. For wireworm control, beads are based on calcium alginate/starch co-encapsulating the carbon dioxide (CO2) producing yeast Saccharomyces cerevisiae H205 as the attract component, and the entomopathogenic fungus Metarhizium brunneum CB15-III as the kill component. However, the physicochemical processes inside beads during co-cultivation are still unclear. Here we reveal for the first time the spatiotemporal conditions of oxygen and pH inside AK beads measured with microelectrodes and describe the impact of S. cerevisiae on CO2 and conidia formation.

RESULTS

Measurements revealed a steep oxygen gradient already 2 days after co-encapsulation, with an internal hypoxic zone. Encapsulating either S. cerevisiae or M. brunneum already decreased the average pH from 5.5 to 4.7 and 4.6, respectively. However, on day 3, co-cultivation lead to temporal strong acidification of beads down to pH 3.6 which followed the maximum CO2 productivity and coincided with the maximum conidiation rate. Decreasing the yeast load decreased the total CO2 productivity to half, and the conidial production by 93%, while specific productivities normalized to 1% yeast load increased eight-fold and three-fold, respectively, with day 3 being an exception.

CONCLUSION

Our findings indicate a general beneficial interaction between M. brunneum and S. cerevisiae, but also suggest competition for resources. These findings will contribute to develop innovative co-formulations with maximum efficiency to save application rates and costs. © 2024 The Author(s). Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

The effect of growth rate on the production and vitality of non-Saccharomyces wine yeast in aerobic fed-batch culture

Non-Saccharomyces wine yeasts are of increasing importance due to their influence on the organoleptic properties of wine and thus the factors influencing the biomass production of these yeasts, as starter cultures, are of commercial value. Therefore, the effects of growth rates on the biomass yield (Y_x/s) and fermentation performance of non-Saccharomyces yeasts at bench and pilot scale were examined. The fermentative performance and (Y_x/s) were optimised, in aerobic fed-batch cultivations, to produce commercial wine seed cultures of Lachancea thermotolerans Y1240, Issatchenkia orientalis Y1161 and Metschnikowia pulcherrima Y1337. Saccharomyces cerevisiae (Lalvin EC1118) was used as a benchmark. A Crabtree positive response was shown by L. thermotolerans in a molasses-based industrial medium, at growth rates exceeding 0.21 h^-1 (µ_crit), resulting in a Y_x/s of 0.76 g/g at 0.21 h^-1 (46% of µ_max) in the aerobic bioreactor-grown fed-batch culture at bench scale. At pilot scale and 0.133 h^-1 (36% of µ_max), this yeast exhibited ethanol concentrations reaching 10.61 g/l, as a possible result of substrate gradients. Crabtree negative responses were observed for I. orientalis and M. pulcherrima resulting in Y_x/s of 0.83 g/g and 0.68 g/g, respectively, below 32% of µ_max. The Y_x/s of M. pulcherrima, I. orientalis and L. thermotolerans was maximised at growth rates between 0.10 and 0.12 h^-1 and the fermentative capacity of these yeasts was maximised at these lower growth rates.

Repairing Creep-Resistant and Kinetically Inert Hydrogels via Yeast Activity-Regulated Energy Dissipation

Energy dissipation, a ubiquitous process in biological systems, has been intensively studied and widely used to guide the transient assembly of natural or synthetic molecules, but very few examples of material transient healability controlled by this important process have been reported. Herein, we realize the healing of creep-resistant and kinetically inert polymer hydrogels that is driven by the respiration of baker's yeast (Saccharomyces cerevisiae) and spontaneous energy dissipation. The entire healing process can be simply controlled by a single variable: sucrose concentration. Due to the high activity and stability of yeast in the hydrogels, multiple local healing events become possible and healing of damaged hydrogels is also efficient after a long waiting time. All these results indicate that our yeast-containing polymer hydrogels are kinetically stable materials, which can be readily healable on demand.

Physiological responses to acid stress by Saccharomyces cerevisiae when applying high initial cell density

High initial cell density is used to increase volumetric productivity and shorten production time in lignocellulosic hydrolysate fermentation. Comparison of physiological parameters in high initial cell density cultivation of Saccharomyces cerevisiae in the presence of acetic, formic, levulinic and cinnamic acids demonstrated general and acid-specific responses of cells. All the acids studied impaired growth and inhibited glycolytic flux, and caused oxidative stress and accumulation of trehalose. However, trehalose may play a role other than protecting yeast cells from acid-induced oxidative stress. Unlike the other acids, cinnamic acid did not cause depletion of cellular ATP, but abolished the growth of yeast on ethanol. Compared with low initial cell density, increasing initial cell density reduced the lag phase and improved the bioconversion yield of cinnamic acid during acid adaptation. In addition, yeast cells were able to grow at elevated concentrations of acid, probable due to the increase in phenotypic cell-to-cell heterogeneity in large inoculum size. Furthermore, the specific growth rate and the specific rates of glucose consumption and metabolite production were significantly lower than at low initial cell density, which was a result of the accumulation of a large fraction of cells that persisted in a viable but non-proliferating state.

Two-component signal transduction system SaeRS positively regulates Staphylococcus epidermidis glucose metabolism

Staphylococcus epidermidis, which is a causative pathogen of nosocomial infection, expresses its virulent traits such as biofilm and autolysis regulated by two-component signal transduction system SaeRS. In this study, we performed a proteomic analysis of differences in expression between the S. epidermidis 1457 wild-type and saeRS mutant to identify candidates regulated by saeRS using two-dimensional gel electrophoresis (2-DE) combined with matrix-assisted laser desorption/lonization mass spectrometry (MALDI-TOF-MS). Of 55 identified proteins that significantly differed in expression between the two strains, 15 were upregulated and 40 were downregulated. The downregulated proteins included enzymes related to glycolysis and TCA cycle, suggesting that glucose is not properly utilized in S. epidermidis when saeRS was deleted. The study will be helpful for treatment of S. epidermidis infection from the viewpoint of metabolic modulation dependent on two-component signal transduction system SaeRS.

Peculiar H⁺ homeostasis of Saccharomyces cerevisiae during the late stages of wine fermentation

Intracellular pH (pH(in)) is a tightly regulated physiological parameter, which controls cell performance in all living systems. The purpose of this work was to evaluate if and how H(+) homeostasis is accomplished by an industrial wine strain of Saccharomyces cerevisiae while fermenting real must under the harsh winery conditions prevalent in the late stages of the fermentation process, in particular low pH and high ethanol concentrations and temperature. Cells grown at 15, 25, and 30°C were harvested in exponential and early and late stationary phases. Intracellular pH remained in the range of 6.0 to 6.4, decreasing significantly only by the end of glucose fermentation, in particular at lower temperatures (pH(in) 5.2 at 15°C), although the cells remained viable and metabolically active. The cell capability of extruding H(+) via H(+)-ATPase and of keeping H(+) out by means of an impermeable membrane were evaluated as potential mechanisms of H(+) homeostasis. At 30°C, H(+) efflux was higher in all stages. The most striking observation was that cells in late stationary phase became almost impermeable to H(+). Even when these cells were challenged with high ethanol concentrations (up to 20%) added in the assay, their permeability to H(+) remained very low, being almost undetectable at 15°C. Comparatively, ethanol significantly increased the H(+) permeability of cells in exponential phase. Understanding the molecular and physiological events underlying yeast H(+) homeostasis at late stages of fermentations may contribute to the development of more robust strains suitable to efficiently produce a high-quality wine.

Studies of the cell-wall properties of Saccharomyces cerevisiae during fermentation

The cell-wall properties of three strains of the yeast Sacharomyces cerevisiae have been experimentally studied at various times during fermentation. The cell walls have been characterized by electrophoretic mobility measurements, from which zeta potentials may be calculated. They have also been characterized by computerized pH titration, which gives direct information on the number and nature of groups in the yeast cell wall. The data have been quantitatively analyzed in three ways. First, a simplified analysis of the electrokinetic data of a type used by previous workers has been applied. Second, such a simplified analysis of the electrokinetic data has been developed more rigorously by means of a two-dimensional site-dissociation model of the outer cell wall-solution interface. Third, a description of the yeast cell-wall electrochemical properties in terms of a three-dimensional gel model incorporating site dissociation has been developed. The advantages and disadvantages of the three analyses are discussed. Only the three-dimensional gel model can account simultaneously for both the electrokinetic and pH surface titration data. It provides new insights into the changes that occur to the yeast cell wall during fermentation.

[Studies of viability and vitality after freezing of the probiotic yeast Saccharomyces boulardii: physiological preconditioning effect]

The aim of this study was to evaluate the vitality and viability of the probiotic yeast Saccharomyces boulardii after freezing/thawing and the physiological preconditioning effect on these properties. The results indicate that the specific growth rate (0.3/h(-1)) and biomass (2-3 x10(8)cells/ml) of S. boulardii obtained in flasks shaken at 28 degrees C and at 37 degrees C were similar. Batch cultures of the yeast in bioreactors using glucose or sugar-cane molasses as carbon sources, reached yields of 0.28 g biomass/g sugar consumed, after 10h incubation at 28 degrees C; the same results were obtained in fed batch fermentations. On the other hand, in batch cultures, the vitality of cells recovered during the exponential growth phase was greater than the vitality of cells from the stationary phase of growth. Vitality of cells from fed-batch fermentations was similar to that of stationary growing cells from batch fermentations. Survival to freezing at -20 degrees C and subsequent thawing of cells from batch cultures was 0.31% for cells in exponential phase of growth and 11.5% for cells in stationary phase. Pre-treatment of this yeast in media with water activity (a(w)) 0.98 increased the survival to freezing of S. boulardii cells stored at -20 degrees C for 2 months by 10 fold. Exposure of the yeast to media of reduced a(w) and/or freezing/thawing process negatively affected cell vitality. It was concluded that stress conditions studied herein decrease vitality of S. boulardii. Besides, the yeast strain studied presented good tolerance to bile salts even at low pH values.

Yeast vitality determination based on intracellular NAD(P)H fluorescence measurement during aerobic-anaerobic transition

This work presents a yeast-cell vitality-assessment method based on on-line intracellular fluorescence measurement. The intracellular NAD(P)H fluorescence of a cell suspension is recorded during transition from aerobic to anaerobic conditions and the output signal is evaluated as a measure of yeast vitality (quality). This fluorescence method showed a highly satisfactory correlation with even low dead cell numbers where the acidification power test could not be applied.

Net effect of wort osmotic pressure on fermentation course, yeast vitality, beer flavor, and haze

The net effect of increased wort osmolarity on fermentation time, bottom yeast vitality and sedimentation, beer flavor compounds, and haze was determined in fermentations with 12 degrees all-malt wort supplemented with sorbitol to reach osmolarity equal to 16 degrees and 20 degrees. Three pitchings were performed in 12 degrees/12 degrees/12 degrees, 16 degrees/16 degrees/12 degrees, and 20 degrees/20 degrees/12 degrees worts. Fermentations in 16 degrees and 20 degrees worts decreased yeast vitality measured as acidification power (AP) by a maximum of 10%, lowered yeast proliferation, and increased fermentation time. Repitching aggravated these effects. The 3rd "back to normal" pitching into 12 degrees wort restored the yeast AP and reproductive abilities while the extended fermentation time remained. Yeast sedimentation in 16 degrees and 20 degrees worts was delayed but increased about two times at fermentation end relative to that in 12 degrees wort. Third "back-to-normal" pitching abolished the delay in sedimentation and reduced its extent, which became nearly equal in all variants. Beer brewed at increased osmolarity was characterized by increased levels of diacetyl and pentanedione and lower levels of dimethylsulfide and acetaldehyde. Esters and higher alcohols displayed small variations irrespective of wort osmolarity or repitching. Increased wort osmolarity had no appreciable effect on the haze of green beer and accelerated beer clarification during maturation. In all variants, chill haze increased with repitching.

Control and prediction of the course of brewery fermentations by gravimetric analysis

A simple, fast and cheap test suitable for predicting the course of brewery fermentations based on mass analysis is described and its efficiency is evaluated. Compared to commonly used yeast vitality tests, this analysis takes into account wort composition and other factors that influence fermentation performance. It can be used to predict the shape of the fermentation curve in brewery fermentations and in research and development projects concerning yeast vitality, fermentation conditions and wort composition. It can also be a useful tool for homebrewers to control their fermentations.

A new method of optical detection of yeast acidification power

We describe here a newly developed method for a contact-free optical pH measurement in yeast suspensions supplemented with glucose, and containing the pH sensitive triphenylmethane dye bromocresol green. It is suitable for performing the acidification power test (based on measuring the rate of pH drop of yeast suspension caused by active extrusion of acidity from cells after glucose addition) used for assessing yeast vitality in fermentation industries. Using this methodology we monitored the pH in yeast suspensions in the course of acidification in the pH range of 3.5-5.3. Optical pH measurement allows simultaneous testing of several samples, minimizes the sample volume, simplifies sample handling and reduces the hands-on time in sample processing.

Factors affecting the outcome of the acidification power test of yeast quality: critical reappraisal

Brewery bottom yeast strain 95 from the Pilsner Urquell propagation unit was used to reappraise the efficiency of the acidification power (AP) test consisting in determining the spontaneous (oxygen-induced) and glucose-induced medium acidification caused by yeast and lactic acid bacteria under standard conditions, and used widely for assessing and predicting the vitality of industrial strains. AP was evaluated in yeast stored for different periods of time (0-28 d) at 4 degrees C, at different temperatures before and during the test (0-55 degrees C), and at different concentrations of cells and glucose and different cells-to-glucose ratios. All these factors had a strong effect on acidification kinetics and the AP value. By contrast, the duration of the lag period between yeast collection and the test (0-6 h) had no perceptible effect on the AP value. The best results were achieved at saturation concentrations of cells (> 10 g pressed yeast or approximately 14 g yeast slurry per 100 mL) and glucose (approximately 3 %) and at 25 degrees C. Since an exact evaluation of acidification characteristics depends strongly on the kinetics of the process, the AP test should include monitoring the time course of the acidification.

Rapid assessment of cell viability of Lactobacillus delbrueckii subsp. bulgaricus by measurement of intracellular pH in individual cells using fluorescence ratio imaging microscopy

The aim of this study was to investigate if the measurement of intracellular pH (pHi) of individual cells by fluorescence ratio imaging microscopy (FRIM) could be utilized as a rapid method for determining the bacterial viability, using Lactobacillus delbrueckii subsp. bulgaricus as a model organism. Five different standardized cultures with equal cell densities but varying viability were prepared on a trial-to-trial basis by combining aliquots of frozen and lyophilized cells with a 50-fold difference in viability, determined by the ability to form colonies on solid growth media. The acidification of milk and Acidification Power Test were used to determine the activity of these cultures. As expected, the cultures containing a higher proportion of viable cells acidified milk faster and performed better in the Acidification Power Test. All cells were fluorescent after staining with carboxyfluorescein diacetate succinimidyl ester but frozen cells reached higher fluorescence intensities than lyophilized cells. The number of strongly fluorescent cells determined by flow cytometry exceeded the number of viable cells determined by CFU. Analysis of pHi of individual cells by FRIM at an extracellular pH of 6.0 revealed two populations of cells with an average pHi of 6.9 +/- 0.1 and 6.1 +/- 0.1. As the number of cells maintaining a pH-gradient of 0.9 +/- 0.1 correlated well with CFU, we suggest that FRIM can be used as a rapid method for the determination of viability of L. delbrueckii subsp. bulgaricus. Measurement of pHi on a single cell basis is expected to provide accurate prediction of the fermentation performance in a wider range of industrial fermentation.

Influence of altered plasma membrane fatty acid composition on cesium transport characteristics and toxicity in Saccharomyces cerevisiae

The influence of altered plasma membrane fatty acid composition on cesium uptake and toxicity was investigated in Saccharomyces cerevisiae. Detailed kinetic studies revealed that both the Vmax and Km values for Cs+ transport increased (by approximately twofold in the latter case) when S. cerevisiae was grown in medium supplemented with the polyunsaturated fatty acid linoleate. In addition, Cs+ uptake by linoleate-enriched cells was considerably less sensitive to the competitive effects of other monovalent cations (K+, Rb+, and NH4+) than that by unsupplemented cells. Stimulation of Cs+ uptake in the presence of certain K+ and Rb+ concentrations was only evident in linoleate-enriched S. cerevisiae. At 100 mM CsCl, the initial rate of Cs+ uptake was greater in linoleate-supplemented cells than in unsupplemented cells and this was reflected in a more rapid displacement of cellular K+. However, little difference in net Cs+ accumulation between linoleate-supplemented and unsupplemented cells was evident during prolonged incubation in buffer or during growth. Thus, Cs+ toxicity was similar in linoleate-supplemented and unsupplemented cells. The results were consistent with the Cs+ (K+) transport mechanism adopting an altered conformational state in linoleate-enriched S. cerevisiae.